NONRESIDENT TRAINING COURSE SEPTEMBER 1998 Navy Electricity and Electronics Training Series Module 7 — Introduction to Solid-State Devices and Power Supplies NAVEDTRA 14179 DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited. DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited. PREFACE By enrolling in this self-study course, you have demonstrated a desire to improve yourself and the Navy. Remember, however, this self-study course is only one pail of the total Navy training program. Practical experience, schools, selected reading, and your desire to succeed are also necessary to successfully round out a fully meaningful training program. COURSE OVERVIEW: To introduce the student to the subject of Solid-State Devices and Power Supplies who needs such a background in accomplishing daily work and/or in preparing for further study. THE COURSE: This self-study course is organized into subject matter areas, each containing learning objectives to help you determine what you should learn along with text and illustrations to help you understand the information. The subject matter reflects day-to-day requirements and experiences of personnel in the rating or skill area. It also reflects guidance provided by Enlisted Community Managers (ECMs) and other senior personnel, technical references, instructions, etc., and either the occupational or naval standards, which are listed in the Manual of Navy Enlisted Manpower Personnel Classifications and Occupational Standards, NAVPERS 18068. THE QUESTIONS: The questions that appear in this course are designed to help you understand the material in the text. VALUE: In completing this course, you will improve your military and professional knowledge. Importantly, it can also help you study for the Navy-wide advancement in rate examination. If you are studying and discover a reference in the text to another publication for further information, look it up. 1998 Edition Prepared by FCC(SW) R. Stephen Howard Published by NAVAL EDUCATION AND TRAINING PROFESSIONAL DEVELOPMENT AND TECHNOLOGY CENTER NAVSUP Logistics Tracking Number 0504-LP-026-8320 Sailor’s Creed “I am a United States Sailor. I will support and defend the Constitution of the United States of America and I will obey the orders of those appointed over me. I represent the fighting spirit of the Navy and those who have gone before me to defend freedom and democracy around the world. I proudly serve my country’s Navy combat team with honor, courage and commitment. I am committed to excellence and the fair treatment of all.” TABLE OF CONTENTS CHAPTER PAGE 1. Semiconductor Diodes 1-1 2. Transistors 2-1 3. Special Devices 3-1 4. Solid-State Power Supplies 4-1 APPENDIX I. Glossary Al-1 II. Periodic Table of Elements All- 1 INDEX INDEX- 1 NAVY ELECTRICITY AND ELECTRONICS TRAINING SERIES The Navy Electricity and Electronics Training Series (NEETS) was developed for use by personnel in many electrical- and electronic-related Navy ratings. Written by, and with the advice of, senior technicians in these ratings, this series provides beginners with fundamental electrical and electronic concepts through self-study. The presentation of this series is not oriented to any specific rating structure, but is divided into modules containing related information organized into traditional paths of instruction. The series is designed to give small amounts of information that can be easily digested before advancing further into the more complex material. For a student just becoming acquainted with electricity or electronics, it is highly recommended that the modules be studied in their suggested sequence. While there is a listing of NEETS by module title, the following brief descriptions give a quick overview of how the individual modules flow together. Module 1, Introduction to Mutter , Energy, and Direct Current . introduces the course with a short history of electricity and electronics and proceeds into the characteristics of matter, energy, and direct current (dc). It also describes some of the general safety precautions and first-aid procedures that should be common knowledge for a person working in the field of electricity. Related safety hints are located throughout the rest of the series, as well. Module 2, Introduction to Alternating Current and Transformers, is an introduction to alternating current (ac) and transformers, including basic ac theory and fundamentals of electromagnetism, inductance, capacitance, impedance, and transformers. Module 3, Introduction to Circuit Protection, Control, and Measurement, encompasses circuit breakers, fuses, and current limiters used in circuit protection, as well as the theory and use of meters as electrical measuring devices. Module 4, Introduction to Electrical Conductors, Wiring Techniques, and Schematic Reading, presents conductor usage, insulation used as wire covering, splicing, termination of wiring, soldering, and reading electrical wiring diagrams. Module 5, Introduction to Generators and Motors, is an introduction to generators and motors, and covers the uses of ac and dc generators and motors in the conversion of electrical and mechanical energies. Module 6, Introduction to Electronic Emission. Tubes, and Power Supplies, ties the first five modules together in an introduction to vacuum tubes and vacuum-tube power supplies. Module 7. Introduction to Solid-State Devices and Power Supplies, is similar to module 6, but it is in reference to solid-state devices. Module 8, Introduction to Amplifiers, covers amplifiers. Module 9, Introduction to Wave-Generation and Wave-Shaping Circuits, discusses wave generation and wave-shaping circuits. Module 10, Introduction to Wave Propagation. Transmission Lines, and Antennas, presents the characteristics of wave propagation, transmission lines, and antennas. IV Module 11, Microwave Principles, explains microwave oscillators, amplifiers, and waveguides. Module 12, Modulation Principles, discusses the principles of modulation. Module 13, Introduction to Number Systems and Logic Circuits, presents the fundamental concepts of number systems. Boolean algebra, and logic circuits, all of which pertain to digital computers. Module 14, Introduction to Microelectronics, covers microelectronics technology and miniature and microminiature circuit repair. Module 15, Principles of Synchros, Servos, and Gyros, provides the basic principles, operations, functions, and applications of synchro, servo, and gyro mechanisms. Module 16, Introduction to Test Equipment, is an introduction to some of the more commonly used test equipments and their applications. Module 17, Radio-Frequency Communications Principles, presents the fundamentals of a radio- frequency communications system. Module 18, Radar Principles, covers the fundamentals of a radar system. Module 19, The Technician's Handbook, is a handy reference of commonly used general information, such as electrical and electronic formulas, color coding, and naval supply system data. Module 20, Master Glossary, is the glossary of terms for the series. Module 21, Test Methods and Practices, describes basic test methods and practices. Module 22, Introduction to Digital Computers, is an introduction to digital computers. Module 23, Magnetic Recording, is an introduction to the use and maintenance of magnetic recorders and the concepts of recording on magnetic tape and disks. Module 24, Introduction to Fiber Optics, is an introduction to fiber optics. Embedded questions are inserted throughout each module, except for modules 19 and 20, which are reference books. If you have any difficulty in answering any of the questions, restudy the applicable section. Although an attempt has been made to use simple language, various technical words and phrases have necessarily been included. Specific terms are defined in Module 20. Master Glossary. Considerable emphasis has been placed on illustrations to provide a maximum amount of information. In some instances, a knowledge of basic algebra may be required. Assignments are provided for each module, with the exceptions of Module 19. The Technician's Handbook', and Module 20. Master Glossary. Course descriptions and ordering information are in NAVEDTRA 12061. Catalog of Nonresident Training Courses. v Throughout the text of this course and while using technical manuals associated with the equipment you will be working on. you will find the below notations at the end of some paragraphs. The notations are used to emphasize that safety hazards exist and care must be taken or observed. WARNING AN OPERATING PROCEDURE. PRACTICE, OR CONDITION. ETC., WHICH MAY RESULT IN INJURY OR DEATH IF NOT CAREFULLY OBSERVED OR FOLLOWED. CAUTION AN OPERATING PROCEDURE. PRACTICE, OR CONDITION. ETC., WHICH MAY RESULT IN DAMAGE TO EQUIPMENT IF NOT CAREFULLY OBSERVED OR FOLLOWED. NOTE An operating procedure, practice, or condition, etc., which is essential to emphasize. INSTRUCTIONS FOR TAKING THE COURSE ASSIGNMENTS The text pages that you are to study are listed at the beginning of each assignment. Study these pages carefully before attempting to answer the questions. Pay close attention to tables and illustrations and read the learning objectives. The learning objectives state what you should be able to do after studying the material. Answering the questions correctly helps you accomplish the objectives. SELECTING YOUR ANSWERS Read each question carefully, then select the BEST answer. You may refer freely to the text. The answers must be the result of your own work and decisions. You are prohibited from referring to or copying the answers of others and from giving answers to anyone else taking the course. SUBMITTING YOUR ASSIGNMENTS To have your assignments graded, you must be enrolled in the course with the Nonresident Training Course Administration Branch at the Naval Education and Training Professional Development and Technology Center (NETPDTC). Following enrollment, there are two ways of having your assignments graded: (1) use the Internet to submit your assignments as you complete them, or (2) send all the assignments at one time by mail to NETPDTC. Grading on the Internet: Advantages to Internet grading are: • you may submit your answers as soon as you complete an assignment, and • you get your results faster; usually by the next working day (approximately 24 hours). In addition to receiving grade results for each assignment, you will receive course completion confirmation once you have completed all the assignments. To submit your assignment answers via the Internet, go to: http://cour.ses.cnet.navy.mil Grading by Mail: When you submit answer sheets by mail, send all of your assignments at one time. Do NOT submit individual answer sheets for grading. Mail all of your assignments in an envelope, which you either provide yourself or obtain from your nearest Educational Services Officer (ESO). Submit answer sheets to: COMMANDING OFFICER NETPDTC N33 1 6490 SAUFLEY FIELD ROAD PENSACOLA FL 32559-5000 Answer Sheets: All courses include one “scannable" answer sheet for each assignment. These answer sheets are preprinted with your SSN, name, assignment number, and course number. Explanations for completing the answer sheets are on the answer sheet. Do not use answer sheet reproductions: Use only the original answer sheets that we provide — reproductions will not work with our scanning equipment and cannot be processed. Follow the instructions for marking your answers on the answer sheet. Be sure that blocks 1, 2, and 3 are filled in correctly. This information is necessary for your course to be properly processed and for you to receive credit for your work. COMPLETION TIME Courses must be completed within 12 months from the date of enrollment. This includes time required to resubmit failed assignments. Vll PASS/FAIL ASSIGNMENT PROCEDURES If your overall course score is 3.2 or higher, you will pass the course and will not be required to resubmit assignments. Once your assignments have been graded you will receive course completion confirmation. If you receive less than a 3.2 on any assignment and your overall course score is below 3.2, you will be given the opportunity to resubmit failed assignments. You may resubmit failed assignments only once. Internet students will receive notification when they have failed an assignment— they may then resubmit failed assignments on the web site. Internet students may view and print results for failed assignments from the web site. Students who submit by mail will receive a failing result letter and a new answer sheet for resubmission of each failed assignment. COMPLETION CONFIRMATION After successfully completing this course, you will receive a letter of completion. ERRATA Errata are used to correct minor errors or delete obsolete information in a course. Errata may also be used to provide instructions to the student. If a course has an errata, it will be included as the first page(s) after the front cover. 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Address: COMMANDING OFFICER NETPDTC N331 6490 SAUFLEY FIELD ROAD PENSACOLA FL 32559-5000 NAVAL RESERVE RETIREMENT CREDIT If you are a member of the Naval Reserve, you will receive retirement points if you are authorized to receive them under current directives governing retirement of Naval Reserve personnel. For Naval Reserve retirement, this course is evaluated at 6 points. (Refer to Administrative Procedures for Naval Reservists on Inactive Duty, BUPERSINST 1001.39. for more information about retirement points.) vm Student Comments NEETS Module 7 Course Title: Introduction to Solid-State Devices and Power Supplies NAVEDTRA: 14179 Date: We need some information about vou: Rate/Rank and Name: SSN: Command/Unit Street Address: City: Statc/FPO: Your comments, suggestions, etc.: Privacy Act Statement: Under authority of Title 5. USC 301. information regarding your military status is requested in processing your comments and in preparing a reply. This information will not be divulged without written authorization to anyone other than those within POD for official use in determining performance. NETPDTC 1550/41 (Rev 4-00) IX CHAPTER 1 SEMICONDUCTOR DIODES LEARNING OBJECTIVES Learning objectives are stated at the beginning of each chapter. These learning objectives serve as a preview of the information you are expected to learn in the chapter. The comprehensive check questions are based on the objectives. By successfully completing the NRTC. you indicate that you have met the objectives and have learned the information. The learning objective are listed below. Upon completion of this chapter, you should be able to do the following: 1. State, in terms of energy bands, the differences between a conductor, an insulator, and a semiconductor. 2. Explain the electron and the hole flow theory in semiconductors and how the semiconductor is affected by doping. 3. Define the term "diode" and give a brief description of its construction and operation. 4. Explain how the diode can be used as a half-wave rectifier and as a sw itch. 5. Identify the diode by its symbology, alphanumerical designation, and color code. 6. List the precautions that must be taken when working with diodes and describe the different ways to test them. INTRODUCTION TO SOLID-STATE DEVICES As you recall from previous studies in this series, semiconductors have electrical properties somewhere between those of insulators and conductors. The use of semiconductor materials in electronic components is not new; some devices are as old as the electron tube. Two of the most widely know n semiconductors in use today are the JUNCTION DIODE and TRANSISTOR. These semiconductors fall under a more general heading called solid-state devices. A SOLID-STATE DEVICE is nothing more than an electronic device, which operates by virtue of the movement of electrons within a solid piece of semiconductor material. Since the invention of the transistor, solid-state devices have been developed and improved at an unbelievable rate. Great strides have been made in the manufacturing techniques, and there is no foreseeable limit to the future of these devices. Solid-state devices made from semiconductor materials offer compactness, efficiency, ruggedness, and versatility. Consequently, these devices have invaded virtually every field of science and industry. In addition to the junction diode and transistor, a whole new family of related devices has been developed: the ZENER DIODE. LIGHT-EMITTING DIODE. FIELD EFFECT TRANSISTOR, etc. One development that has dominated solid-state technology, and probably has had a greater impact on the electronics industry than either the electron tube or transistor, is the INTEGRATED CIRCUIT. The integrated circuit is a minute piece of semiconductor material that can produce complete electronic circuit functions. 1-1 As the applications of solid-state devices mount, the need for knowledge of these devices becomes increasingly important. Personnel in the Navy today will have to understand solid-state devices if they are to become proficient in the repair and maintenance of electronic equipment. Therefore, our objective in this module is to provide a broad coverage of solid-state devices and. as a broad application, power supplies. We w ill begin our discussion with some background information on the development of the semiconductor. We will then proceed to the semiconductor diode, the transistor, special devices and. finally, solid-state power supplies. SEMICONDUCTOR DEVELOPMENT Although the semiconductor was late in reaching its present development, its story began long before the electron tube. Historically, we can go as far back as 1883 when Michael Faraday discovered that silver sulfide, a semiconductor, has a negative temperature coefficient . The term negative temperature coefficient is just another way of saying its resistance to electrical current flow decreases as temperature increases. The opposite is true of the conductor. It has a positive temperature coefficient . Because of this particular characteristic, semiconductors are used extensively in power-measuring equipment. Only 2 years later, another valuable characteristic was reported by Munk A. Rosenshold. He found that certain materials have rectifying properties. Strange as it may seem, his finding was given such little notice that it had to be rediscovered 39 years later by F. Braun. Toward the close of the 19th century, experimenters began to notice the peculiar characteristics of the chemical element SELENIUM. They discovered that in addition to its rectifying properties (the ability to convert ac into dc), selenium was also light sensitive-its resistance decreased with an increase in light intensity. This discovery eventually led to the invention of the photophone by Alexander Graham Bell. The photophone, which converted variations of light into sound, was a predecessor of the radio receiver; however, it wasn't until the actual birth of radio that selenium was used to any extent. Today, selenium is an important and widely used semiconductor. Many other materials were tried and tested for use in communications. SILICON was found to be the most stable of the materials tested while GALENA, a crystalline form of lead sulfide, was found the most sensitive for use in early radio receivers. By 1915. Carl Beredicks discovered that GERMANIUM, another metallic element, also had rectifying capabilities. Later, it became widely used in electronics for low-power, low-frequency applications. Although the semiconductor was known long before the electron tube was invented, the semiconductor devices of that time could not match the performance of the tube. Radio needed a device that could not only handle power and amplify but rectify and detect a signal as well. Since tubes could do all these things, whereas semiconductor devices of that day could not. the semiconductor soon lost out. It wasn't until the beginning of World War II that interest was renewed in the semiconductor. There was a dire need for a device that could work within the ultra-high frequencies of radar. Electron tubes had interelectrode capacitances that were too high to do the job. The point-contact semiconductor diode, on the other hand, had a very low internal capacitance. Consequently, it filled the bill; it could be designed to work within the ultra-high frequencies used in radar, whereas the electron tube could not. As radar took on greater importance and communication-electronic equipment became more sophisticated, the demands for better solid-state devices mounted. The limitations of the electron tube made necessary a quest for something new and different. An amplifying device was needed that was smaller, lighter, more efficient, and capable of handling extremely high frequencies. This was asking a 1-2 lot. but if progress was to be made, these requirements had to be met. A serious study of semiconductor materials began in the early 1940 s and has continued since. In June 1948, a significant breakthrough took place in semiconductor development. This was the discovery of POINT-CONTACT TRANSISTOR. Here at last was a semiconductor that could amplify. This discovery brought the semiconductor back into competition with the electron tube. A year later. JUNCTION DIODES and TRANSISTORS were developed. The junction transistor was found superior to the point-contact type in many respects. By comparison, the junction transistor was more reliable, generated less noise, and had higher power-handling ability than its point-contact brother. The junction transistor became a rival of the electron tube in many uses previously uncontested. Semiconductor diodes were not to be slighted. The initial work of Dr. Carl Zener led to the development of ZENER DIODE, which is frequently used today to regulate power supply voltages at precise levels. Considerably more interest in the solid-state diode was generated when Dr. Leo Esaki, a Japanese scientist, fabricated a diode that could amplify. The device, named the TUNNEL DIODE, has amazing gain and fast switching capabilities. Although it is used in the conventional amplifying and oscillating circuits, its primary use is in computer logic circuits. Another breakthrough came in the late 1950's when it was discovered that semiconductor materials could be combined and treated so that they functioned as an entire circuit or subassembly rather than as a circuit component. Many names have been given to this solid-circuit concept, such as INTEGRATED CIRCUITS. MICROELECTRONICS, and MICROCIRCUITRY. So as we see. in looking back, that the semiconductor is not something new. but it has come a long way in a short time. Ql. What is a solid-state device? Q2. Define the term negative temperature coefficient. SEMICONDUCTOR APPLICATIONS In the previous paragraphs, we mentioned just a few of the many different applications of semiconductor devices. The use of these devices has become so widespread that it would be impossible to list all their different applications. Instead, a broad coverage of their specific application is presented. Semiconductor devices are all around us. They can be found in just about every commercial product we touch, from the family car to the pocket calculator. Semiconductor devices are contained in television sets, portable radios, stereo equipment, and much more. Science and industry also rely heavily on semiconductor devices. Research laboratories use these devices in all sorts of electronic instruments to perform tests, measurements, and numerous other experimental tasks. Industrial control systems (such as those used to manufacture automobiles) and automatic telephone exchanges also use semiconductors. Even today heavy-duty versions of the solid- state rectifier diode are being use to convert large amounts of power for electric railroads. Of the many different applications for solid-state devices, space systems, computers, and data processing equipment are some of the largest consumers. The various types of modem military equipment are literally loaded with semiconductor devices. Many radars, communication, and airborne equipment are transistorized. Data display systems, data processing units, computers, and aircraft guidance-control assemblies are also good examples of 1-3 electronic equipments that use semiconductor devices. All of the specific applications of semiconductor devices would make a long impressive list. The fact is, semiconductors are being used extensively in commercial products, industry, and the military. SEMICONDUCTOR COMPETITION It should not be difficult to conclude, from what you already know, that semiconductor devices can and do perform all the conventional functions of rectification, amplification, oscillation, timing, switching, and sensing. Simply stated, these devices perform the same basic functions as the electron tube: but they perform more efficiently, economically, and for a longer period of time. Therefore, it should be no surprise to you to see these devices used in place of electron tubes. Keeping this in mind, we see that it is only natural and logical to compare semiconductor devices with electron tubes. Physically, semiconductor devices are much smaller than tubes. You can see in figure 1-1 that the difference is quite evident. This illustration shows some commonly used tube sizes alongside semiconductor devices of similar capabilities. The reduction in size can be as great as 100:1 by weight and 1000:1 by volume. It is easy to see that size reduction favors the semiconductor device. Therefore, whenever miniaturization is required or is convenient, transistors are favored over tubes. Bear in mind, however, that the extent of practical size reduction is a big factor: many things must be considered. Miniature electron tubes, for example, may be preferred in certain applications to transistors, thus keeping size reduction to a competitive area. Figure 1-1. — Size comparisons of electron tubes and semiconductors. Power is also a two-sided story. For low-power applications, where efficiency is a significant factor, semiconductors have a decided advantage. This is true mainly because semiconductor devices perform very well with an extremely small amount of power: in addition, they require no filaments or heaters as in the case of the electron tube. For example, a computer operating with over 4000 solid-state devices may require no more than 20 watts of power. However, the same number of tubes would require several kilow atts of power. For high-power applications, it is a different story — tubes have the upper hand. The high-power tube has no equivalent in any semiconductor device. This is because a tube can be designed to operate 1-4 with over a thousand volts applied to its plate whereas the maximum allowable voltage for a transistor is limited to about 2(H) volts (usually 50 volts or less). A tube can also handle thousands of watts of power. The maximum power output for transistor generally ranges from 30 milliwatts to slightly over 100 watts. When it comes to ruggedness and life expectancy, the tube is still in competition. Design and functional requirements usually dictate the choice of device. However, semiconductor devices are rugged and long-lived. They can be constructed to withstand extreme vibration and mechanical shock. They have been known to withstand impacts that would completely shatter an ordinary electron tube. Although some specially designed tubes render extensive service, the life expectancy of transistors is better than three to four times that of ordinary electronic tubes. There is no known failure mechanism (such as an open filament in a tube) to limit the semiconductor's life. However, semiconductor devices do have some limitations. They are usually affected more by temperature, humidity, and radiation than tubes are. Q3. Name three of the largest users of semiconductor devices. Q4. State one requirement of an electron tube, which does not exist for semiconductors, that makes the tube less efficient than the semiconductor. SEMICONDUCTOR THEORY To understand why solid-state devices function as they do. we will have to examine closely the composition and nature of semiconductors. This entails theory that is fundamental to the study of solid- state devices. Rather than beginning with theory, let’s first become reacquainted with some of the basic information you studied earlier concerning matter and energy (NEETS. Module 1). ATOMIC STRUCTURE The universe, as we know it today, is divided into two parts: matter and energy. Matter, which is our main concern at this time, is anything that occupies space and has weight. Rocks, water, air. automobiles, clothing, and even our own bodies are good examples of matter. From this, we can conclude that matter may be found in any one of three states: SOLIDS. LIQUIDS, and GASES. All matter is composed of either an element or combination of elements. As you know, an element is a substance that cannot be reduced to a simpler form by chemical means. Examples of elements with which you are in contact everyday are iron. gold, silver, copper, and oxygen. At present, there are over 100 known elements of which all matter is composed. As we work our way dow n the size scale, we come to the atom, the smallest particle into which an element can be broken down and still retain all its original properties. The atoms of one element, however, differ from the atoms of all other elements. Since there are over 100 known elements, there must be over 100 different atoms, or a different atom for each element. Now let us consider more than one element at a time. This brings us to the term "compound." A compound is a chemical combination of two or more elements. Water, table salt, ethyl alcohol, and ammonia are all examples of compounds. The smallest part of a compound, which has all the characteristics of the compound, is the molecule. Each molecule contains some of the atoms of each of the elements forming the compound. Consider sugar, for example. Sugar in general terms is matter, since it occupies space and has weight. It is also a compound because it consists of two or more elements. Take a lump of sugar and crush 1-5 it into small particles; each of the particles still retains its original identifying properties of sugar. The only thing that changed was the physical size of the sugar. If we continue this subdividing process by grinding the sugar into a line power, the results are the same. Even dissolving sugar in water does not change its identifying properties, in spite of the fact that the particles of sugar are now too small to be seen even with a microscope. Eventually, we end up with a quantity of sugar that cannot be further divided without its ceasing to be sugar. This quantity is known as a molecule of sugar. If the molecule is further divided, it is found to consist of three simpler kinds of matter: carbon, hydrogen, and oxygen. These simpler forms are called elements. Therefore, since elements consist of atoms, then a molecule of sugar is made up of atoms of carbon, hydrogen, and oxygen. As we investigate the atom, we find that it is basically composed of electrons, protons, and neutrons. Furthermore, the electrons, protons, and neutrons of one element are identical to those of any other element. There are different kinds of elements because the number and the arrangement of electrons and protons are different for each element. The electron carries a small negative charge o f electricity. The proton carries a positive charge of electricity equal and opposite to the charge of the electron. Scientists have measured the mass and size of the electron and proton, and they know” how much charge each possesses. Both the electron and proton have the same quantity of charge, although the mass of the proton is approximately 1.827 times that of the electron. In some atoms there exists a neutral particle called a neutron. The neutron has a mass approximately equal to that of a proton, but it has no electrical charge . According to theory, the electrons, protons, and neutrons of the atoms are thought to be arranged in a manner similar to a miniature solar system. Notice the helium atom in figure 1-2. Two protons and two neutrons form the heavy nucleus with a positive charge around which two very light electrons revolve. The path each electron takes around the nucleus is called an orbit. The electrons are continuously being acted upon in their orbits by the force of attraction of the nucleus. To maintain an orbit around the nucleus, the electrons travel at a speed that produces a counterforce equal to the attraction force of the nucleus. Just as energy is required to move a space vehicle away from the earth, energy is also required to move an electron away from the nucleus. Like a space vehicle, the electron is said to be at a higher energy level when it travels a larger orbit. Scientific experiments have shown that the electron requires a certain amount of energy to stay in orbit. This quantity is called the electron s energy level. By virtue of just its motion alone, the electron contains kinetic energy. Because of its position, it also contains potential energy. The total energy contained by an electron (kinetic energy plus potential energy) is the main factor that determines the radius of the electron’s orbit. For an electron to remain in this orbit, it must neither gain nor lose energy. Figure 1-2. — The composition of a simple helium atom. 1-6 The orbiting electrons do not follow random paths, instead they are confined to definite energy levels. Visualize these levels as shells with each successive shell being spaced a greater distance from the nucleus. The shells, and the number of electrons required to fill them, may be predicted by using Pauli’s exclusion principle. Simply stated, this principle specifies that each shell will contain a maximum of 2n' electrons, where n corresponds to the shell number starting with the one closest to the nucleus. By this principle, the second shell, for example, would contain 2(2) " or 8 electrons when full. In addition to being numbered, the shells are also given letter designations starting with the shell closest to the nucleus and progressing outward as shown in figure 1-3. The shells are considered to be full, or complete, when they contain the following quantities of electrons: 2 in the K(lst) shell, 8 in the L(2nd) shell. 18 in the M(3rd) shell, and so on. in accordance w ith the exclusion principle. Each of these shells is a major shell and can be divided into subshells, of which there are four, labeled s, p. d. and f. Like the major shells, the subshells are also limited as to the number of electrons they contain. Thus, the "s" subshell is complete when it contains 2 electrons, the "p" subshell when it contains 6. the "d" subshell when it contains 10. and the "f" subshell when it contains 14 electrons. Figure 1-3. — Shell designation. Inasmuch as the K shell can contain no more than 2 electrons, it must have only one subshell, the s subshell. The M shell is composed of three subshells: s, p. and d. If the electrons in the s, p. and d subshells are added together, their total is found to be 18. the exact number required to fill the M shell. Notice the electron configuration of copper illustrated in figure 1-4. The copper atom contains 29 electrons, which completely fill the first three shells and subshells, leaving one electron in the "s" subshell of the N shell. A list of all the other known elements, with the number of electrons in each atom, is contained in the PERIODIC TABLE OF ELEMENTS. The periodic table of elements is included in appendix 2. 1-7 Figure 1-4. — Copper atom. Valence is an atom’s ability to combine with other atoms. The number of electrons in the outermost shell of an atom determines its valence. For this reason, the outer shell of an atom is called VALENCE SHELL, and the electrons contained in this shell are called VALENCE ELECTRONS. The valence of an atom determines its ability to gain or lose an electron, which in turn determines the chemical and electrical properties of the atom. An atom that is lacking only one or two electrons from its outer shell will easily gain electrons to complete its shell, but a large amount of energy is required to free any of its electrons. An atom having a relatively small number of electrons in its outer shell in comparison to the number of electrons required to fill the shell will easily lose these valence electrons. The valence shell always refers to the outermost shell. Q5. Define mailer and list ils three different states. Q6. What is the smallest particle into which an element can he broken down and still retain all its original properties ? Q7. What are the three particles that comprise an atom and state the type of charge they hold ? Q8. What is the outer shell of an atom called? ENERGY BANDS Now that you have become reacquainted with matter and energy, we will continue our discussion with electron behavior. As stated earlier, orbiting electrons contain energy and are confined to definite energy levels. The various shells in an atom represent these levels. Therefore, to move an electron from a lower shell to a higher shell a certain amount of energy is required. This energy can be in the form of electric fields, heat, light, and even bombardment by other particles. Failure to provide enough energy to the electron, even if the energy supplied is just short of the required amount, will cause it to remain at its present energy level. Supplying more energy than is needed will only cause the electron to move to the next higher shell and the remaining energy will be wasted. In simple terms, energy is required in definite units to move electrons from one shell to the next higher shell. These units are called QUANTA (for example 1. 2. or 3 quanta). 1-8 Electrons can also lose energy as well as receive it. When an electron loses energy, it moves to a lower shell. The lost energy, in some cases, appears as heat. If a sufficient amount of energy is absorbed by an electron, it is possible for that electron to be completely removed from the influence of the atom. This is called IONIZATION. When an atom loses electrons or gains electrons in this process of electron exchange, it is said to be ionized. For ionization to take place, there must be a transfer of energy that results in a change in the internal energy of the atom. An atom having more than its normal amount of electrons acquires a negative charge, and is called a NEGATIVE ION. The atom that gives up some of its normal electrons is left with fewer negative charges than positive charges and is called a POSITIVE ION. Thus, we can define ionization as the process by which an atom loses or gains electrons. Up to this point in our discussion, we have spoken only of isolated atoms. When atoms are spaced far enough apart, as in a gas, they have very little influence upon each other, and are very much like lone atoms. But atoms within a solid have a marked effect upon each other. The forces that bind these atoms together greatly modify the behavior of the other electrons. One consequence of this close proximity of atoms is to cause the individual energy levels of an atom to break up and form bands of energy . Discrete (separate and complete) energy levels still exist within these energy bands, but there are many more energy levels than there were with the isolated atom. In some cases, energy levels will have disappeared. Figure 1 -5 shows the difference in the energy arrangement between an isolated atom and the atom in a solid. Notice that the isolated atom (such as in gas) has energy levels, whereas the atom in a solid has energy levels grouped into ENERGY BANDS. FI FCTROHS ENERGY LEVELS ELECTRON'S ENERGY Figure 1-5. — The energy arrangement in atoms. The upper band in the solid lines in figure 1-5 is called the CONDUCTION BAND because electrons in this band are easily removed by the application of external electric fields. Materials that have a large number of electrons in the conduction band act as good conductors of electricity. Below the conduction band is the FORBIDDEN BAND or energy gap. Electrons are never found in this band, but may travel back and forth through it. provided they do not come to rest in the band. The last band or VALENCE BAND is composed of a series of energy levels containing valence electrons. Electrons in this band are more tightly bound to the individual atom than the electrons in the conduction band. However, the electrons in the valence band can still be moved to the conduction band with the application of energy, usually thermal energy. There are more bands below the valence band, but they are not important to the understanding of semiconductor theory and will not be discussed. 1-9 The concept of energy bands is particularly important in classifying materials as conductors, semiconductors, and insulators. An electron can exist in either of two energy bands, the conduction band or the valence band. All that is necessary to move an electron from the valence band to the conduction band so it can be used for electric current, is enough energy to carry the electron through the forbidden band. The width of the forbidden band or the separation betw een the conduction and valence bands determines whether a substance is an insulator, semiconductor, or conductor. Figure 1-6 uses energy level diagrams to show the difference between insulators, semiconductors, and conductors. CONDUCTION BAND FORBIDDEN BAND CONDUCTION BAND FORBIDDEN BAND CONDUCTION BAND INSULATOR SEMICONDUCTOR CONDUCTOR Figure 1-6. — Energy level diagram. The energy diagram for the insulator shows the insulator with a very w ide energy gap. The wider this gap. the greater the amount of energy required to move the electron from the valence band to the conduction band. Therefore, an insulator requires a large amount of energy to obtain a small amount of current. The insulator "insulates" because of the w ide forbidden band or energy gap. The semiconductor, on the other hand, has a smaller forbidden band and requires less energy to move an electron from the valence band to the conduction band. Therefore, for a certain amount of applied voltage, more current will How in the semiconductor than in the insulator. The last energy level diagram in figure 1-6 is that of a conductor. Notice, there is no forbidden band or energy gap and the valence and conduction bands overlap. With no energy gap. it takes a small amount of energy to move electrons into the conduction band; consequently, conductors pass electrons very easily. Q9. What term is used to describe the definite discrete amounts of energy required to move an electron from a low shell to a higher shell ? QIO. What is a negative ion? QI I . What is the main difference in the energy arrangement between an isolated atom and the atom in a solid? Q12. What determines, in terms of energy bands, whether a substance is a good insulator, semiconductor, or conductor? COVALENT BONDING The chemical activity of an atom is determined by the number of electrons in its valence shell. When the valence shell is complete, the atom is stable and shows little tendency to combine with other atoms to form solids. Only atoms that possess eight valence electrons have a complete outer shell. These atoms are 1-10 referred to as inert or inactive atoms. However, if the valence shell of an atom lacks the required number of electrons to complete the shell, then the activity of the atom increases. Silicon and germanium, for example, are the most frequently used semiconductors. Both are quite similar in their structure and chemical behavior. Each has four electrons in the valence shell. Consider just silicon. Since it has fewer than the required number of eight electrons needed in the outer shell, its atoms will unite with other atoms until eight electrons are shared. This gives each atom a total of eight electrons in its valence shell; four of its own and four that it borrowed from the surrounding atoms. The sharing of valence electrons between two or more atoms produces a COVALENT BOND between the atoms. It is this bond that holds the atoms together in an orderly structure called a CRYSTAL. A crystal is just another name for a solid whose atoms or molecules are arranged in a three-dimensional geometrical pattern commonly referred to as a lattice. Figure 1-7 shows a typical crystal structure. Each sphere in the figure represents the nucleus of an atom, and the arms that join the atoms and support the structure are the covalent bonds. Figure 1-7. — A typical crystal structure. As a result of this sharing process, the valence electrons are held tightly together. This can best be illustrated by the two-dimensional view of the silicon lattice in figure 1-8. The circles in the figure represent the nuclei of the atoms. The +4 in the circles is the net charge of the nucleus plus the inner shells (minus the valence shell). The short lines indicate valence electrons. Because every atom in this pattern is bonded to four other atoms, the electrons are not free to move within the crystal. As a result of this bonding, pure silicon and germanium are poor conductors of electricity. The reason they are not insulators but semiconductors is that w ith the proper application of heat or electrical pressure, electrons can be caused to break free of their bonds and move into the conduction band. Once in this band, they wander aimlessly through the crystal. 1-1 1 = ELECIRON- PAIR (C OVALE N I BOND) ^4) NUCLEUS AND INNER SHELLS Figure 1-8. — A two-dimensional view of a silicon cubic lattice. Q 13. What determines the chemical activity of an atom ? Q14. What is the term used to describe the sharing of valence electrons between two or more atoms ? CONDUCTION PROCESS As staled earlier, energy can be added to electrons by applying heat. When enough energy is absorbed by the valence electrons, it is possible for them to break some of their covalent bonds. Once the bonds are broken, the electrons move to the conduction band where they are capable of supporting electric current. When a voltage is applied to a crystal containing these conduction band electrons, the electrons move through the crystal toward the applied voltage. This movement of electrons in a semiconductor is referred to as electron current flow . There is still another type of current in a pure semiconductor. This current occurs when a covalent bond is broken and a vacancy is left in the atom by the missing valence electron. This vacancy is commonly referred to as a "hole." The hole is considered to have a positive charge because its atom is deficient by one electron, which causes the protons to outnumber the electrons. As a result of this hole, a chain reaction begins when a nearby electron breaks its own covalent bond to fill the hole, leaving another hole. Then another electron breaks its bond to fill the previous hole, leaving still another hole. Each time an electron in this process fills a hole, it enters into a covalent bond. Even though an electron has moved from one covalent bond to another, the most important thing to remember is that the hole is also moving . Therefore, since this process of conduction resembles the movement of holes rather than electrons, it is termed hole tlow (short for hole current How or conduction by holes). Hole How is very similar to electron How except that the holes move toward a negative potential and in an opposite direction to that of the electron . Since hole flow results from the breaking of covalent bonds, which are at the valence band level, the electrons associated with this type of conduction contain only valence band energy and must remain in the valence band. However, the electrons associated with electron How have conduction band energy and can. therefore, move throughout the crystal. A good analogy of hole flow is the movement of a hole through a tube filled with balls (figure 1-9). 1-12 SFACE MCW BA BIT }► y SFACE LEFT Bf* BALL HQ 3 SFACE MOVBIB-JT SFACE HOVBIBJT- ► \ SPACE LEFT BY BALL HQ 8 1 Figure 1-9. — Analogy of hole How. When ball number 1 is removed from the tube, a hole is left. This hole is then filled by ball number 2, which leaves still another hole. Ball number 3 then moves into the hole left by ball number 2. This causes still another hole to appear where ball 3 was. Notice the holes are moving to the right side of the tube. This action continues until all the balls have moved one space to the left in which time the hole moved eight spaces to the right and came to rest at the right-hand end of the tube. In the theory just described, two current carriers were created by the breaking of covalent bonds: the negative electron and the positive hole. These carriers are referred to as electron-hole pairs. Since the semiconductor we have been discussing contains no impurities, the number of holes in the electron-hole pairs is always equal to the number of conduction electrons. Another way of describing this condition where no impurities exist is by saying the semiconductor is INTRINSIC. The term intrinsic is also used to distinguish the pure semiconductor that we have been working with from one containing impurities. Q15. Name the two types of current flow in a semiconductor. QI6. What is the name given to a piece of pure semiconductor material that has an equal number of electrons and holes? DOPING PROCESS The pure semiconductor mentioned earlier is basically neutral. It contains no free electrons in its conduction bands. Even with the application of thermal energy, only a few covalent bonds are broken, yielding a relatively small current flow. A much more efficient method of increasing current flow in semiconductors is by adding very small amounts of selected additives to them, generally no more than a few parts per million. These additives are called impurities and the process of adding them to crystals is referred to as DOPING. The purpose of semiconductor doping is to increase the number of free charges that can be moved by an external applied voltage. When an impurity increases the number of free electrons, the doped semiconductor is NEGATIVE or N TYPE, and the impurity that is added is known as an N-type impurity. However, an impurity that reduces the number of free electrons, causing more 1-13 holes, creates a POSITIVE or P-TYPE semiconductor, and the impurity that was added to it is known as a P-type impurity. Semiconductors which are doped in this manner — either with N- or P-type impurities — are referred to as EXTRINSIC semiconductors. N-Tvpe Semiconductor The N-type impurity loses its extra valence electron easily when added to a semiconductor material, and in so doing, increases the conductivity of the material by contributing a free electron. This type of impurity has 5 valence electrons and is called a PENTAVALENT impurity. Arsenic, antimony, bismuth, and phosphorous are pentavalent impurities. Because these materials give or donate one electron to the doped material, they are also called DONOR impurities. When a pentavalent (donor) impurity, like arsenic, is added to germanium, it will form covalent bonds with the germanium atoms. Figure 1-10 illustrates this by showing an arsenic atom (AS) in a germanium (GE) lattice structure. Notice the arsenic atom in the center of the lattice. It has 5 valence electrons in its outer shell but uses only 4 of them to form covalent bonds with the germanium atoms, leaving 1 electron relatively free in the crystal structure. Pure germanium may be converted into an N-type semiconductor by "doping" it with any donor impurity having 5 valence electrons in its outer shell. Since this type of semiconductor (N-type) has a surplus of electrons, the electrons are considered MAJORITY carriers, while the holes, being few in number, are the MINORITY carriers. EXCESS ELECTRON Figure 1-10. — Germanium crystal doped with arsenic. P-Type Semiconductor The second type of impurity, when added to a semiconductor material, tends to compensate for its deficiency of 1 valence electron by acquiring an electron from its neighbor. Impurities of this type have only 3 valence electrons and are called TRIVALENT impurities. Aluminum, indium, gallium, and boron are trivalent impurities. Because these materials accept 1 electron from the doped material, they are also called ACCEPTOR impurities. A trivalent (acceptor) impurity element can also be used to dope germanium. In this case, the impurity is 1 electron short of the required amount of electrons needed to establish covalent bonds with 4 neighboring atoms. Thus, in a single covalent bond, there will be only 1 electron instead of 2. This arrangement leaves a hole in that covalent bond. Figure 1-11 illustrates this theory by showing what happens when germanium is doped with an indium (In) atom. Notice, the indium atom in the figure is 1 1-14 electron short of the required amount of electrons needed to form covalent bonds with 4 neighboring atoms and, therefore, creates a hole in the structure. Gallium and boron, which are also trivalent impurities, exhibit these same characteristics when added to germanium. The holes can only be present in this type semiconductor when a trivalent impurity is used. Note that a hole carrier is not created by the removal of an electron from a neutral atom, but is created when a trivalent impurity enters into covalent bonds with a tetravalent (4 valence electrons) crystal structure. The holes in this type of semiconductor (P-type) are considered the MAJORITY carriers since they are present in the material in the greatest quantity. The electrons, on the other hand, are the MINORITY carriers. ikssssi RS JKSxmDu KSffl n Figure 1-11. — Germanium crystal doped with indium. Q17. What is the name given to a doped germanium crystal with an excess of free holes? Q18. What are the majority carriers in an N-type semiconductor? SEMICONDUCTOR DIODE If we join a section of N-type semiconductor material with a similar section of P-type semiconductor material, we obtain a device known as a PN JUNCTION. (The area where the N and P regions meet is appropriately called the junction.) The usual characteristics of this device make it extremely useful in electronics as a diode rectifier. The diode rectifier or PN junction diode performs the same function as its counterpart in electron tubes but in a different way. The diode is nothing more than a two-element semiconductor device that makes use of the rectifying properties of a PN junction to convert alternating current into direct current by permitting current How in only one direction. The schematic symbol of a PN junction diode is shown in figure 1-12. The vertical bar represents the cathode (N-type material) since it is the source of electrons and the arrow represents the anode . (P-type material) since it is the destination of the electrons. The label "CR1" is an alphanumerical code used to identify the diode. In this figure, we have only one diode so it is labeled CR1 (crystal rectifier number one). If there were four diodes shown in the diagram, the last diode would be labeled CR4. The heavy dark line shows electron flow. Notice it is against the arrow. For further clarification, a pictorial diagram of a PN junction and an actual semiconductor (one of many types) are also illustrated. 1-15 CR 1 _ ELECTRON "► FLOW CATHODE 0 ANODE PN JUNCTION n-type| p -typeV / SCHEMATIC! \ VIEW t ( PICTORIAL! I VIEW / /ACTUAL! \ VIEW | Figure 1-12. — The PN junction diode. CONSTRUCTION Merely pressing together a section of P material and a section of N material, however, is not sufficient to produce a rectifying junction. The semiconductor should be in one piece to form a proper PN junction, but divided into a P-type impurity region and an N-type impurity region. This can be done in various ways. One way is to mix P-type and N-type impurities into a single crystal during the manufacturing process. By so doing, a P-region is grown over part of a semiconductors length and N- region is grown over the other part. This is called a GROWN junction and is illustrated in view A of figure 1-13. Another way to produce a PN junction is to melt one type of impurity into a semiconductor of the opposite type impurity. For example, a pellet of acceptor impurity is placed on a wafer of N-type germanium and heated. Under controlled temperature conditions, the acceptor impurity fuses into the wafer to form a P-region within it. as shown in view B of figure 1-13. This type of junction is known as an ALLOY or FUSED-ALLOY junction, and is one of the most commonly used junctions. In figure 1-14, a POINT-CONTACT type of construction is shown. It consists of a fine metal wire, called a cat whisker, that makes contact with a small area on the surface of an N-type semiconductor as shown in view A of the figure. The PN union is formed in this process by momentarily applying a high-surge current to the wire and the N-type semiconductor. The heat generated by this current converts the material nearest the point of contact to a P-type material (view B). CUT BAR Figure 1-13. — Grown and fused I'N junctions from which bars are cut. 1-16 CAT N (A) Figure 1-14A. — The point-contact type of diode construction. Figure 1-14B. — The point-contact type of diode construction. Still another process is to heat a section of semiconductor material to near melting and then diffuse impurity atoms into a surface layer. Regardless of the process, the objective is to have a perfect bond everywhere along the union (interface) between P and N materials. Proper contact along the union is important because, as we will see later, the union (junction or interface) is the rectifying agent in the diode. QI9. What is the purpose of a PN junction diode ? Q20. In reference to the schematic symbol for a diode, do electrons flow toward or away from the arrow? 1-17 Q2I. What type of PN diode is formed by using a fine metal wire and a section ofN-type semiconductor material? PN JUNCTION OPERATION Now that you Lire familiar with P- and N-type materials, how these materials are joined together to form a diode, and the function of the diode, let us continue our discussion with the operation of the PN junction. But before we can understand how the PN junction works, we must first consider current flow in the materials that make up the junction and what happens initially within the junction when these two materials are joined together. Current Flow in the N-Type Material Conduction in the N-type semiconductor, or crystal, is similar to conduction in a copper wire. That is, with voltage applied across the material, electrons will move through the crystal just as current would How in a copper wire. This is shown in figure 1-15. The positive potential of the battery will attract the free electrons in the crystal. These electrons will leave the crystal and flow into the positive terminal of the battery. As an electron leaves the crystal, an electron from the negative terminal of the battery will enter the crystal, thus completing the current path. Therefore, the majority current carriers in the N-type material (electrons) are repelled by the negative side of the battery and move through the crystal toward the positive side of the battery. Figure 1-15. — Current flow In the N-type material. Current Flow in the I'- Type Material Current flow through the P-type material is illustrated in figure 1-16. Conduction in the P material is by positive holes, instead of negative electrons. A hole moves from the positive terminal of the P material to the negative terminal. Electrons from the external circuit enter the negative terminal of the material and fill holes in the vicinity of this terminal. At the positive terminal, electrons are removed from the covalent bonds, thus creating new holes. This process continues as the steady stream of holes (hole current) moves toward the negative terminal. 1-18 ELECTRON FLOW Figure 1-16. — Current How In the P-type material. Notice in both N-type and P-type materials, current How in the external circuit consists of electrons moving out of the negative terminal of the battery and into the positive terminal of the batter)'. Hole flow, on the other hand, only exists within the material itself. Q22. What are the majority carriers in a P-type semiconductor? Q23. Conduction in which type of semiconductor material is similar to conduction in a copper wire ? Junction Barrier Although the N-type material has an excess of free electrons, it is still electrically neutral. This is because the donor atoms in the N material were left with positive charges after free electrons became available by covalent bonding (the protons outnumbered the electrons). Therefore, for every free electron in the N material, there is a corresponding positively charge atom to balance it. The end result is that the N material has an overall charge of zero. By the same reasoning, the P-type material is also electrically neutral because the excess of holes in this material is exactly balanced by the number of electrons. Keep in mind that the holes and electrons are still free to move in the material because they are only loosely bound to their parent atoms. It would seem that if we joined the N and P materials together by one of the processes mentioned earlier, all the holes and electrons would pair up. On the contrary, this does not happen. Instead the electrons in the N material diffuse (move or spread out) across the junction into the P material and fill some of the holes. At the same time, the holes in the P material diffuse across the junction into the N material and are filled by N material electrons. This process, called JUNCTION RECOMBINATION, reduces the number of free electrons and holes in the vicinity of the junction. Because there is a depletion, or lack of free electrons and holes in this area, it is known as the DEPLETION REGION. The loss of an electron from the N-type material created a positive ion in the N material, while the loss of a hole from the P material created a negative ion in that material. These ions are fixed in place in the crystal lattice structure and cannot move. Thus, they make up a layer of fixed charges on the two sides of the junction as shown in figure 1-17. On the N side of the junction, there is a layer of positively charged ions; on the P side of the junction, there is a layer of negatively charged ions. An electrostatic field, represented by a small battery in the figure, is established across the junction between the oppositely 1-19 charged ions. The diffusion of electrons and holes across the junction will continue until the magnitude of the electrostatic field is increased to the point where the electrons and holes no longer have enough energy to overcome it, and are repelled by the negative and positive ions respectively. At this point equilibrium is established and. for all practical purposes, the movement of carriers across the junction ceases. For this reason, the electrostatic field created by the positive and negative ions in the depletion region is called a barrier. JUNCTION e P ooo o ooo o ooo O Ooo OHOLE - FREE ELECTRON © NEGATIVE ION ©POSITIVE ION N e , — DEPLETION REGION ELECTROSTATIC FIELD Figure 1-17. — The PN junction harrier formation. The action just described occurs almost instantly when the junction is formed. Only the carriers in the immediate vicinity of the junction are affected. The carriers throughout the remainder of the N and P material are relatively undisturbed and remain in a balanced condition. FORWARD BIAS. — An external voltage applied to a PN junction is call BIAS. If. for example, a battery is used to supply bias to a PN junction and is connected so that its voltage opposes the junction field, it will reduce the junction barrier and. therefore, aid current flow through the junction. This type of bias is known as forward bias, and it causes the junction to offer only minimum resistance to the flow of current. Forward bias is illustrated in figure 1-18. Notice the positive terminal of the bias battery is connected to the P-type material and the negative terminal of the battery is connected to the N-type material. The positive potential repels holes toward the junction where they neutralize some of the negative ions. At the same time the negative potential repels electrons toward the junction where they neutralize some of the positive ions. Since ions on both sides of the barrier are being neutralized, the width of the barrier decreases. Thus, the effect of the battery voltage in the forward-bias direction is to reduce the barrier potential across the junction and to allow majority carriers to cross the junction. Current flow in the forward-biased PN junction is relatively simple. An electron leaves the negative terminal of the battery and moves to the terminal of the N-type material. It enters the N material, where it is the majority carrier and moves to the edge of the junction barrier. Because of forward bias, the barrier offers less opposition to the electron and it will pass through the depletion region into the P-type material. The electron loses energy in overcoming the opposition of the junction barrier, and upon entering the P material, combines with a hole. The hole was produced when an electron was extracted from the P material by the positive potential of the battery. The created hole moves through the P material toward the junction where it combines with an electron. 1-20 ELECTRON FLOW Figure 1-18. — Forward-biased PN junction. It is important lo remember that in the forward biased condition, conduction is by MAJORITY current carriers (holes in the P-type material and electrons in the N-type material). Increasing the battery voltage will increase the number of majority carriers arriving at the junction and will therefore increase the current flow. If the battery voltage is increased to the point where the barrier is greatly reduced, a heavy current will flow and the junction may be damaged from the resulting heat. REVERSE BIAS. — If the batter)’ mentioned earlier is connected across the junction so that its voltage aids the junction, it will increase the junction barrier and thereby offer a high resistance to the current flow through the junction. This type of bias is known as reverse bias. To reverse bias a junction diode, the negative battery terminal is connected to the P-type material, and the positive battery terminal to the N-type material as shown in figure 1-19. The negative potential attracts the holes away from the edge of the junction barrier on the P side, while the positive potential attracts the electrons away from the edge of the barrier on the N side. This action increases the barrier width because there are more negative ions on the P side of the junction, and more positive ions on the N side of the junction. Notice in the figure the width of the barrier has increased. This increase in the number of ions prevents current flow across the junction by majority carriers . However, the current flow across the barrier is not quite zero because of the minority carriers crossing the junction. As you recall, when the crystal is subjected to an external source of energy (light, heat, etc.), electron-hole pairs are generated. The electron-hole pairs produce minority current carriers. There are minority current carriers in both regions: holes in the N material and electrons in the P material. With reverse bias, the electrons in the P-type material are repelled toward the junction by the negative terminal of the battery. As the electron moves across the junction, it will neutralize a positive ion in the N-type material. Similarly, the holes in the N-type material will be repelled by the positive terminal of the battery toward the junction. As the hole crosses the junction, it will neutralize a negative ion in the P-type material. This movement of minority carriers is called MINORITY CURRENT FLOW, because the holes and electrons involved come from the electron-hole pairs that are generated in the crystal lattice structure, and not from the addition of impurity atoms. 1-21 NO ELECTRON FLOW Figure 1-19. — Reverse-biased PN junction. Therefore, when a PN junction is reverse biased, there will be no current tlow because of majority carriers but a very small amount of current because of minority carriers crossing the junction. However, at normal operating temperatures, this small current may be neglected. In summary, the most important point to remember about the PN junction diode is its ability to offer very little resistance to current flow in the forward-bias direction but maximum resistance to current flow when reverse biased. A good way of illustrating this point is by plotting a graph of the applied voltage versus the measured current. Figure 1-20 shows a plot of this voltage-current relationship (characteristic curve) for a typical PN junction diode. Figure 1-20. — PN junction diode characteristic curve. To determine the resistance from the curve in this figure we can use Ohm’s law: 1-22 For example at point A the forward-bias voltage is 1 volt and the forward-bias current is 5 milliamperes. This represents 200 ohms of resistance ( 1 volt/5 m A = 200 ohms). However, at point B the voltage is 3 volts and the current is 50 milliamperes. This results in 60 ohms of resistance for the diode. Notice that when the forward-bias voltage was tripled ( 1 volt to 3 volts), the current increased 10 times (5mA to 50 mA). At the same time the forward-bias voltage increased, the resistance decreased from 200 ohms to 60 ohms. In other words, when forward bias increases, the junction barrier gets smaller and its resistance to current How decreases. On the other hand, the diode conducts very little when reverse biased. Notice at point C the reverse bias voltage is 80 volts and the current is only 100 microamperes. This results in 800 k ohms of resistance, which is considerably larger than the resistance of the junction with forward bias. Because of these unusual features, the FN junction diode is often used to convert alternating current into direct current (rectification). Q24. What is the name of the area in a PN junction that has a shortage of electrons and holes ? Q25. In order to reverse bias in a PN junction, what terminal of a battery' is connected to the P material? Q26. What ty pe of bias opposes the PN junction barrier ? FN JUNCTION APPLICATION Until now, we have mentioned only one application for the diode-rectification, but there are many more applications that we have not yet discussed. Variations in doping agents, semiconductor materials, and manufacturing techniques have made it possible to produce diodes that can be used in many different applications. Examples of these types of diodes are signal diodes, rectifying diodes. Zener diodes (voltage protection diodes for power supplies), varactors (amplifying and switching diodes), and many more. Only applications for two of the most commonly used diodes, the signal diode and rectifier diode, will be presented in this chapter. The other diodes will be explained later on in this module. Half-Wave Rectifier One of the most important uses of a diode is rectification. The normal PN junction diode is well-suited for this purpose as it conducts very heavily when forward biased (low-resistance direction) and only slightly when reverse biased (high-resistance direction). If we place this diode in series with a source of ac power, the diode will be forward and reverse biased every cycle. Since in this situation current flows more easily in one direction than the other, rectification is accomplished. The simplest rectifier circuit is a half-wave rectifier (fig. 1-21 view A and view B) which consists of a diode, an ac power source, and a load resister. 1-23 Tl - TRANSFORMER RL-LOAD RESISTOR CRI - DIODE Figure 1-21A. — Simple half-wave rectifier. (B) Figure I -2 IB. — Simple half-wave rectifier. The transformer (Tl) in the figure provides the ac input to the circuit: the diode (CRI) provides the rectification; and the load resistor (Rl) serves two purposes: it limits the amount of current flow in the circuit to a safe level, and it also develops the output signal because of the current How through it. Before describing how this circuit operates, the definition of the word "load” as it applies to power supplies must be understood. Load is defined as any device that draws current . A device that draws little current is considered a light load, whereas a device that draws a large amount of current is a heavy load. Remember that when we speak of "load." we are speaking about the device that draws current from the power source. This device may be a simple resistor, or one or more complicated electronic circuits. During the positive half-cycle of the input signal (solid line) in figure 1-21 view A. the top of the transformer is positive with respect to ground. The dots on the transformer indicate points of the same polarity. With this condition the diode is forward biased, the depletion region is narrow, the resistance of the diode is low. and current flows through the circuit in the direction of the solid lines. When this current flows through the load resistor, it develops a negative to positive voltage drop across it. which appears as a positive voltage at the output terminal. When the ac input goes in a negative direction (fig. 1-21 view A), the top of the transformer becomes negative and the diode becomes reverse biased. With reverse bias applied to the diode, the depletion region increases, the resistance of the diode is high, and minimum current flows through the diode. For all 1-24 practical purposes, there is no output developed across the load resistor during the negative alternation of the input signal as indicated by the broken lines in the figure. Although only one cycle of input is shown, it should be realized that the action described above continually repeats itself, as long as there is an input. Therefore, since only the positive half-cycles appear at the output this circuit converted the ac input into a positive pulsating dc voltage. The frequency of the output voltage is equal to the frequency of the applied ac signal since there is one pulse out for each cycle of the ac input. For example, if the input frequency is 60 hertz (60 cycles per second), the output frequency is 60 pulses per second (pps). However, if the diode is reversed as shown in view B of figure 1-21. a negative output voltage would be obtained. This is because the current would be flowing from the top of Rl toward the bottom, making the output at the top of Rl negative with respect to the bottom or ground. Because current flows in this circuit only during half of the input cycle, it is called a half-wave rectifier . The semiconductor diode shown in the figure can be replaced by a metallic rectifier and still achieve the same results. The metallic rectifier, sometimes referred to as a dry-disc rectifier, is a metal-to- semiconductor. large-area contact device. Its construction is distinctive; a semiconductor is sandwiched between two metal plates, or electrodes, as shown in figure 1-22. Note in the figure that a barrier, with a resistance many times greater than that of the semiconductor material, is constructed on one of the metal electrodes. The contact having the barrier is a rectifying contact: the other contact is nonrectifying. Metallic rectifiers act just like the diodes previously discussed in that they permit current to flow” more readily in one direction than the other. However, the metallic rectifier is fairly large compared to the crystal diode as can be seen in figure 1-23. The reason for this is: metallic rectifier units are stacked (to prevent inverse voltage breakdown), have large area plates (to handle high currents), and usually have cooling fins (to prevent overheating). ELECTRODE METAL ELECTRODE Figure 1-22. — A metallic rectifier. 1-25 -^=4 CFRAMIC CARTRIDGE TYPF PIG I AIL TV PCS Figure 1-23. — Different types of crystal and metallic rectifiers. There are many known metal-semiconductor combinations that can be used for contact rectification. Copper oxide and selenium devices are by far the most popular. Copper oxide and selenium are frequently used over other types of metallic rectifiers because they have a large forward current per unit contact area, low forward voltage drop, good stability, and a lower aging rate. In practical application, the selenium rectifier is used where a relatively large amount of power is required. On the other hand, copper-oxide rectifiers are generally used in small-current applications such as ac meter movements or for delivering direct current to circuits requiring not more than 10 amperes. Since metallic rectifiers are affected by temperature, atmospheric conditions, and aging (in the case of copper oxide and selenium), they are being replaced by the improved silicon crystal rectifier. The silicon rectifier replaces the bulky selenium rectifier as to current and voltage rating, and can operate at higher ambient (surrounding) temperatures. Diode Switch In addition to their use as simple rectifiers, diodes are also used in circuits that mix signals together (mixers), detect the presence of a signal (detector), and act as a switch "to open or close a circuit." Diodes used in these applications are commonly referred to as "signal diodes." The simplest application of a signal diode is the basic diode switch shown in figure 1-24. 1-26 +v Figure 1-24. — Basic diode switch. When the input to this circuit is at zero potential, the diode is forw ard biased because of the zero potential on the cathode and the positive voltage on the anode. In this condition, the diode conducts and acts as a straight piece of wire because of its very low forward resistance. In effect, the input is directly coupled to the output resulting in zero volts across the output terminals. Therefore, the diode, acts as a closed switch when its anode is positive with respect to its cathode. If we apply a positive input voltage (equal to or greater than the positive voltage supplied to the anode) to the diode's cathode, the diode will be reverse biased. In this situation, the diode is cut off and acts as an open switch between the input and output terminals. Consequently, with no current flow in the circuit, the positive voltage on the diode's anode will be felt at the output terminal. Therefore, the diode acts as an open switch when it is reverse biased. Q27. What is a load? Q2H. What is the output of a half-wave rectifier ? Q29. What type of rectifier is constructed by sandwiching a section of semiconductor material between two metal plates? Q30. What type of bias makes a diode act as a dosed switch? DIODE CHARACTERISTICS Semiconductor diodes have properties that enable them to perform many different electronic functions. To do their jobs, engineers and technicians must be supplied with data on these different types of diodes. The information presented for this purpose is called DIODE CHARACTERISTICS. These characteristics are supplied by manufacturers either in their manuals or on specification sheets (data sheets). Because of the scores of manufacturers and numerous diode types, it is not practical to put before you a specification sheet and call it typical. Aside from the difference between manufacturers, a single manufacturer may even supply specification sheets that differ both in format and content. Despite these differences, certain performance and design information is normally required. We will discuss this information in the next few paragraphs. 1-27 A standard specification sheet usually has a brief description of the diode. Included in this description is the type of diode, the major area of application, and any special features. Of particular interest is the specific application for which the diode is suited. The manufacturer also provides a drawing of the diode which gives dimension, weight, and. if appropriate, any identification marks. In addition to the above data, the following information is also provided: a static operating table (giving spot values of parameters under fixed conditions), sometimes a characteristic curve similar to the one in figure 1-20 (showing how parameters vary over the full operating range), and diode ratings (which are the limiting values of operating conditions outside which could cause diode damage). Manufacturers specify these various diode operating parameters and characteristics with "letter symbols" in accordance with fixed definitions. The following is a list, by letter symbol, of the major electrical characteristics for the rectifier and signal diodes. RECTIFIER DIODES DC BLOCKING VOLTAGE |V R J — the maximum reverse dc voltage that will not cause breakdown. AVERAGE FORWARD VOLTAGE DROP | V F iav.I — the average forward voltage drop across the rectifier given at a specified forward current and temperature. AVERAGE RECTIFIER FORWARD CURRENT |I FIAV ,]— the average rectified forward current at a specified temperature, usually at 60 Hz with a resistive load. AVERAGE REVERSE CURRENT [Ik, a viI — the average reverse current at a specified temperature, usually at 60 Hz. PEAK SURGE CURRENT IIsurgeI — the peak current specified for a given number of cycles or portion of a cycle. SIGNAL DIODES PEAK REVERSE VOLTAGE |PRV| — the maximum reverse voltage that can be applied before reaching the breakdown point. (PRV also applies to the rectifier diode.) REVERSE CURRENT (IrJ — the small value of direct current that flows when a semiconductor diode has reverse bias. MAXIMUM FORWARD VOLTAGE DROP AT INDICATED FORW'ARD CURRENT (V F @I F |— the maximum forward voltage drop across the diode at the indicated forward current. REVERSE RECOVERY TIME |t„| — the maximum time taken for the forward-bias diode to recover its reverse bias. The ratings of a diode (as stated earlier) are the limiting values of operating conditions, w hich if exceeded could cause damage to a diode by either voltage breakdown or overheating. The PN junction diodes are generally rated for: MAXIMUM AVERAGE FORWARD CURRENT. PEAK RECURRENT FORWARD CURRENT. MAXIMUM SURGE CURRENT, and PEAK REVERSE VOLTAGE. Maximum average forward current is usually given at a special temperature, usually 25° C, (77° F) and refers to the maximum amount of average current that can be permitted to How in the forward direction. If this rating is exceeded, structure breakdown can occur. Peak recurrent forward current is the maximum peak current that can be permitted to flow in the forward direction in the form of recurring pulses. Maximum surge current is the maximum current permitted to How in the forward direction in the form of nonrecurring pulses. Current should not equal this value for more than a few milliseconds. Peak reverse voltage (PRV) is one of the most important ratings. PRV indicates the maximum reverse-bias voltage that may be applied to a diode without causing junction breakdown. All of the above ratings are subject to change with temperature variations. If. for example, the operating temperature is above that stated for the ratings, the ratings must be decreased. Q3 1. What is used to show how diode parameters vary over a full operating range ? Q32. What is meant by diode ratings? DIODE IDENTIFICATION There are many types of diodes varying in size from the size of a pinhead (used in subminiature circuitry) to large 250-ampere diodes (used in high-power circuits). Because there are so many different types of diodes, some system of identification is needed to distinguish one diode from another. This is accomplished with the semiconductor identification system shown in figure 1-25. This system is not only used for diodes but transistors and many other special semiconductor devices as well. As illustrated in this figure, the system uses numbers and letters to identify different types of semiconductor devices. The first number in the system indicates the number of junctions in the semiconductor device and is a number, one less than the number of active elements. Thus 1 designates a diode; 2 designates a transistor (which may be considered as made up of two diodes); and 3 designates a tetrode (a four-element transistor). The letter "N" following the first number indicates a semiconductor. The 2- or 3-digit number following the letter "N" is a serialized identification number. If needed, this number may contain a suffix letter after the last digit. For example, the suffix letter "M" may be used to describe matching pairs of separate semiconductor devices or the letter "R" may be used to indicate reverse polarity. Other letters are used to indicate modified versions of the device which can be substituted for the basic numbered unit. For example, a semiconductor diode designated as type 1N345A signifies a two-element diode ( 1) of semiconductor material (N) that is an improved version (A) of type 345. 1-29 XNYYY XN YYY COMPONENT IDENTIFICATION NUMBER X- NUMBER OF SEMICONDUCTOR JUNCTIONS N-A SEMICONDUCTOR YYY - IDENTIFICATION NUMBER (ORDER OR REGISTRATION NUMBER) ALSO INCLUDES SUFFIX LETTER (IF APPLICABLE) TO INDICATE 1. MATCHING DEVICES 2. REVERSE POLARITY 3. MODIFICATION EXAMPLE -1N345A (AN IMPROVED VERSION OF THE SEMICONDUCTOR DIODE TYPE 345) Figure 1-25. — Semiconductor identification codes. When working with these different types of diodes, it is also necessary to distinguish one end of the diode from the other (anode from cathode). For this reason, manufacturers generally code the cathode end of the diode with a "k," "cath.” a color dot or band, or by an unusual shape (raised edge or taper) as shown in figure 1-26. In some cases, standard color code bands are placed on the cathode end of the diode. This serves two purposes: ( 1 ) it identifies the cathode end of the diode, and (2) it also serves to identify the diode by number. ANODES cQo BAND MARKED MARKED COLOR SPOT /■CATHODES GLASS COLOR BANDS $ MARKED Figure 1-26. — Semiconductor diode markings. The standard diode color code system is shown in figure 1-27. Take, for example, a diode with brown, orange, and white bands at one terminal and figure out its identification number. With brown 1-30 being a "I," orange a "3," and white "9," the device would be identified as a type 139 semiconductor diode, or specifically IN 139. s 1ST 2ND 3RD4TH, DIGfTS COLOR © DIGIT © DIODE SUFFIX LETTER BLACK 0 - BROWN 1 A RED 2 B ORANGE 3 C YELLOW 4 D GREEN 5 E BLUE 6 F VIOLET 7 G GRAY 8 H WHITE 9 J SILVER • - GOLD - • NONE - Figure 1-27. — Semiconductor diode color code system. Keep in mind, whether the diode is a small crystal type or a large power rectifier type, both are still represented schematically, as explained earlier, by the schematic symbol shown in figure 1-12. Q33. What does the letter "N" indicate in the semiconductor identification system ? Q34. What type of diode has orange, blue, and gray bands ? DIODE MAINTENANCE Diodes are rugged and efficient. They are also expected to be relatively trouble free. Protective encapsulation processes and special coating techniques have even further increased their life expectancies. In theory, a diode should last indefinitely. However, if diodes are subjected to current overloads, their junctions will be damaged or destroyed. In addition, the application of excessively high operating voltages can damage or destroy junctions through arc-over, or excessive reverse currents. One of the greatest dangers to the diode is heat. Heat causes more electron-hole pairs to be generated, which in turn increases current flow. This increase in current generates more heat and the cycle repeats itself until 1-31 the diode draws excessive current. This action is referred to as THERMAL RUNAWAY and eventually causes diode destruction. Extreme caution should be used when working with equipment containing diodes to ensure that these problems do not occur and cause irreparable diode damage. The following is a list of some of the special safety precautions that should be observed w hen working with diodes: • Never remove or insert a diode into a circuit with voltage applied. • Never pry diodes to loosen them from their circuits. • Always be careful when soldering to ensure that excessive heat is not applied to the diode. • When testing a diode, ensure that the test voltage does not exceed the diode’s maximum allowable voltage. • Never put your fingers across a signal diode because the static charge from your body could short it out. • Always replace a diode with a direct replacement, or with one of the same type. • Ensure a replacement diode is put into a circuit in the correct direction. If a diode has been subjected to excessive voltage or temperature and is suspected of being defective, it can be checked in various ways. The most convenient and quickest way of testing a diode is with an ohmmeter (fig. 1-28). To make the check, simply disconnect one of the diode leads from the circuit wiring, and make resistance measurements across the leads of the diode. The resistance measurements obtained depend upon the test-lead polarity of the ohmmeter: therefore, two measurements must be taken. The first measurement is taken with the test leads connected to either end of the diode and the second measurement is taken with the test leads reversed on the diode. The larger resistance value is assumed to be the reverse (back) resistance of the diode, and the smaller resistance (front) value is assumed to be the forward resistance. Measurement can be made for comparison purposes using another identical-type diode (known to be good) as a standard. Two high-value resistance measurements indicate that the diode is open or has a high forward resistance. Two low-value resistance measurements indicate that the diode is shorted or has a low reverse resistance. A normal set of measurements w ill show” a high resistance in the reverse direction and a low resistance in the forw ard direction. The diode’s efficiency is determined by how low the forward resistance is compared with the reverse resistance. That is, it is desirable to have as great a ratio (often known as the front-to-back ratio or the back-to-front ratio) as possible between the reverse and forward resistance measurements. However, as a rule of thumb, a small signal diode w ill have a ratio of several hundred to one. while a power rectifier can operate satisfactorily with a ratio of 10 to 1 . 1-32 OHMMETER REVERSE CONDITION - HIGH RESISTANCE MEASUREMENT OHMMETER FORWARD CONDITION - LOW RESISTANCE MEASUREMENT Figure 1 - 28 . — Checking a diode with an ohinmeter. One tiling you should keep in mind about the ohmmeter check-it is not conclusive. It is still possible for a diode to check good under this test, but break down when placed back in the circuit. The problem is that the meter used to check the diode uses a lower voltage than the diode usually operates at in the circuit. Another important point to remember is that a diode should not be condemned because two ohmmeters give different readings on the diode. This occurs because of the different internal resistances of the ohmmeters and the different states of charge on the ohmmeter batteries. Because each ohmmeter sends a different current through the diode, the two resistance values read on the meters will not be the same. Another way of checking a diode is with the substitution method . In this method, a good diode is substituted for a questionable diode. This technique should be used only after you have made voltage and resistance measurements to make certain that there is no circuit defect that might damage the substitution diode. If more than one defective diode is present in the equipment section where trouble has been localized, this method becomes cumbersome, since several diodes may have to be replaced before the trouble is corrected. To determine which stages failed and which diodes are not defective, all of the removed diodes must be tested. This can be accomplished by observing whether the equipment operates correctly as each of the removed diodes is reinserted into the equipment. In conclusion, the only valid check of a diode is a dynamic electrical test that determines the diodes forward current (resistance) and reverse current (resistance) parameters. This test can be accomplished using various crystal diode test sets that are readily available from many manufacturers. 1-33 Q35. What is the greatest threat to a diode'/ Q36. When checking a diode with an ohmmeter, what is indicated by two high resistance measurements ? SUMMARY Now that we have completed this chapter, a short review of the more important points covered in the chapter will follow. You should be thoroughly familiar with these points before continuing on to chapter 2 . The UNIVERSE consists of two main parts-matter and energy. MATTER is anything that occupies space and has weight. Rocks, water, and air are examples of matter. Matter may be found in any one of three states: solid, liquid and gaseous. It can also be composed of either an element or a combination of elements. An ELEMENT is a substance that cannot be reduced to a simpler form by chemical means. Iron, gold, silver, copper, and oxygen are all good examples of elements. A COMPOUND is a chemical combination of two or more elements. Water, table salt, ethyl alcohol, and ammonia are all examples of compounds. A MOLECULE is the smallest part of a compound that has all the characteristics of the compound. Each molecule contains some of the atoms of each of the elements forming the compound. The ATOM is the smallest particle into which an element can be broken down and still retain all its original properties. An atom is made up of electrons, protons, and neutrons. The number and arrangement of these particles determine the kind of element. An ELECTRON carries a small negative charge of electricity. The PROTON carries a positive charge of electricity that is equal and opposite to the charge of the electron. However, the mass of the proton is approximately 1.837 times that of the electron. 1-34 The NEUTRON is a neutral particle in that it has no electrical charge. The mass of the neutron is approximately equal to that of the proton. An ELECTRON'S ENERGY LEVEL is the amount of energy required by an electron to stay in orbit. Just by the electron’s motion alone, it has kinetic energy. The electron's position in reference to the nucleus gives it potential energy. An energy balance keeps the electron in orbit and as it gains or loses energy, it assumes an orbit further from or closer to the center of the atom. SHELLS and SL’BSHELLS are the orbits of the electrons in an atom. Each shell can contain a maximum number of electrons, which can be determined by the formula 2n Shells are lettered K through Q. starting with K. which is the closest to the nucleus. The shell can also be split into four subshells labeled s, p. d. and f. which can contain 2. 6. 10. and 14 electrons, respectively. VALENCE is the ability of an atom to combine with other atoms. The valence of an atom is determined by the number of electrons in the atom’s outermost shell. This shell is referred to as the VALENCE SHELL. The electrons in the outermost shell are called VALENCE ELECTRONS. IONIZATION is the process by which an atom loses or gains electrons. An atom that loses some of its electrons in the process becomes positively charged and is called a POSITIVE ION. An atom that has an excess number of electrons is negatively charged and is called a NEGATIVE ION. ENERGY BANDS are groups of energy levels that result from the close proximity of atoms in a solid. The three most important energy bands are the CONDUCTION BAND. EORBIDDEN BAND, and VALENCE BAND. 1-35 ELECTRON'S ELECTRON'S ATOM IN A SOLID CONDUCTORS, SEMICONDUCTORS, and INSULATORS are categorized as such by using the energy band concept. It is the width of the forbidden band that determines whether a material is an insulator, a semiconductor, or a conductor. A CONDUCTOR has a very narrow forbidden band or none at all. A SEMICONDUCTOR has a medium width forbidden band. An INSULATOR has a wide forbidden band. INSULATOR SEMICONDUCTOR CONDUCTOR COVALENT BONDING is the sharing of valence electrons between two or more atoms. It is this bonding that holds the atoms together in an orderly structure called a CRYSTAL. 1-36 The CONDUCTION PROCESS in a SEMICONDUCTOR is accomplished by two different types of current flow: HOLE FLOW and ELECTRON FLOW. Hole flow is very similar to electron flow except that holes (positive charges) move toward a negative potential and in an opposite direction to that of the electrons. In an INTRINSIC semiconductor (one which does not contain any impurities), the number of holes always equals the number of conducting electrons. IFFT FY RILL 1-0 1 •It) 1 R BIO/ EC .9 FACE LEFT BY EKLI (O 2 SFRCE U3VRIFW >■ »SFftCE LEFT EV‘ EftLL (O 3 iehSzzxxd 5FBCE M3VSIB4T ' ■ srice wen - a an 1 p» \ 3 CSCE LEFT Ui B‘LL IO 6 * DOPING is the process by which small amounts of selected additives, called impurities, are added to semiconductors to increase their current flow. Semiconductors that undergo this treatment are referred to as EXTRINSIC SEMICONDUCTORS. An N-TYPE SEMICONDUCTOR is one that is doped with an N-TYPE or donor impurity (an impurity that easily loses its extra electron to the semiconductor causing it to have an excess number of 1-37 free electrons). Since this type of semiconductor has a surplus of electrons, the electrons are considered the majority current carriers, while the holes are the minority current carriers. EXCESS ELECTRON A P-TYPE SEMICONDUCTOR is one which is doped with a P-TYPE or acceptor impurity (an impurity that reduces the number of free electrons causing more holes). The holes in this type semiconductor are the majority current carriers since they are present in the greatest quantity while the electrons are the minority current carriers. The SEMICONDUCTOR DIODE, also known as a PN JUNCTION DIODE, is a two-element semiconductor device that makes use of the rectifying properties of a PN junction to convert alternating current into direct current by permitting current How in only one direction. CR 1 , ELECTRON FLOW CATHODE 0 ANODE PN JUNCTION n-type|p-typeV # SCHEMATIC! \ VIEW I ( PICTORIAL! I VIEW I \P /ACTUAL! \ VIEW I A PN JUNCTION CONSTRUCTION varies from one manufacturer to the next. Some of the more commonly used manufacturing techniques are: GROWN. ALLOY or FUSED-ALLOY. DIFFUSED, and POINT-CONTACT. CUT BAR CURRENT FLOW in an N-TYPE MATERIAL is similar to conduction in a copper wire. That is, with voltage applied across the material, electrons will move through the crystal toward the positive terminal just like current Hows in a copper wire. 1-39 N-TYPE MATERIAL ELECTRON FLOW CURRENT FLOW in a P-TYPE MATERIAL is by positive holes, instead of negative electrons. Unlike the electron, the hole moves from the positive terminal of the P material to the negative terminal. ELECTRON FLOW JUNCTION BARRIER is an electrostatic field that has been created by the joining of a section of N material with a section of P material. Since holes and electrons must overcome this field to cross the junction, the electrostatic field is commonly called a BARRIER. Because there is a lack or depletion of free electrons and holes in the area around the barrier, this area has become known as the DEPLETION REGION. 1-40 JUNCTION - FREE ELECTRON © NEGATIVE ION ©POSITIVE ION FORWARD BIAS is an external voltage that is applied to a PN junction to reduce its barrier and. therefore, aid current How through the junction. To accomplish this function, the external voltage is connected so that it opposes the electrostatic field of the junction. ELECTRON FLOW REVERSE BIAS is an external voltage that is connected across a PN junction so that its voltage aids the junction and. thereby, offers a high resistance to the current flow through the junction. 1-41 DEPLETION REGION The I*N JUNCTION has a unique ability to offer very little resistance to current How in the forward-bias direction, but maximum resistance to current flow when reverse biased. For this reason, the PN junction is commonly used as a diode to convert ac to dc. The PN JUNCTION'S APPLICATION expands many different areas-from a simple voltage protection device to an amplifying diode. Two of the most commonly used applications for the PN junction are the SIGNAL DIODE (mixing, detecting, and switching signals) and the RECTIFYING DIODE (converting ac to dc). 1-42 CRI Rl OUTPUT Tl - TRANSFORMER RL-LOAD RESISTOR CRI - DIODE The METALLIC RECTIFIER or dry-disc rectifier is a metal-to-semiconductor device that acts just like a diode in that it permits current to flow more readily in one direction than the other. Metallic rectifiers are used in many applications where a relatively large amount of power is required. BARRIER RECTIFYING METAL ELECTRODE SEMICONDUCTOR NONRECTIFYING METAL ELECTRODE DIODE CHARACTERISTICS is the information supplied by manufacturers on different types of diodes, either in their manuals or on specification sheets. DIODE RATINGS are the limiting value of operating conditions of a diode. Operation of the diode outside of its operating limits could damage the diode. Diodes are generally rated for: MAXIMUM AVERAGE FORWARD CURRENT. REAR RECURRENT FORWARD CURRENT. MAXIMUM SURGE CURRENT, and PEAK REVERSE VOLTAGE. 1-43 The SEMICONDUCTOR IDENTIFICATION SYSTEM is an alphanumerical code used to distinguish one semiconductor from another. It is used for diodes, transistors, and many other special semiconductor devices. XNYYY XN YYY COMPONENT IDENTIFICATION NUMBER X- NUMBER OF SEMICONDUCTOR JUNCTIONS N - A SEMICONDUCTOR YYY - IDENTIFICATION NUMBER (ORDER OR REGISTRATION NUMBER) ALSO INCLUDES SUFFIX LETTER (IF APPLICABLE) TO INDICATE 1. MATCHING DEVICES 2. REVERSE POLARITY 3. MODIFICATION EXAMPLE -1N345A (AN IMPROVED VERSION OF THE SEMICONDUCTOR DIODE TYPE 345) DIODE MARKINGS are letters and symbols placed on the diode by manufacturers to distinguish one end of the diode from the other. In some cases, an unusual shape or the addition of color code bands is used to distinguish the cathode from the anode. GLASS * COIOR BANDS MARKED 1-44 The STANDARD DIODE COLOR CODE SYSTEM serves two purposes when it is used: ( I ) it identifies the cathode end of the diode, and (2) it also serves to identify the diode by number. 2 DIGIT TYPE (BLACK BAND)' 3 DIGIT TYPE 1 1 1 1 1 1 DIGITS (?) SUFFIX LETTER (IF USED) © SUFFIX LETTER _ (BLACK IF NO ( 2 ) LETTER) © © DIODE COLOR DIGIT SUFFIX LETTER BLACK 0 - BROWN 1 A RED 2 B ORANGE 3 C YELLOW 4 D GREEN 5 E BLUE 6 F VIOLET 7 G GRAY 8 H WHITE 3 J SILVER - GOLD NONE DIODE MAINTENANCE is the procedures or methods used to keep a diode in good operating condition. To prevent diode damage, you should observe standard diode safety precautions and ensure that diodes are not subjected to heat, current overloads, and excessively high operating voltages. TESTING A DIODE can be accomplished by using an ohmmeter. the substitution method, or a dynamic diode tester. The most convenient and quickest way of testing a diode is with an ohmmeter. 1-45 OHMMEIER CIEVtflit COK01CN- hgh resistance measurement QHWEIER FOPVARD CCCOTOU- 10V RESISTANCE r/EOSlBEr/EMT ANSWERS TO QUESTIONS Ql. THROUGH Q36. Al. An electronic device that operates by virtue of the movement of electrons within a solid piece of semiconductor material. A2. It is the decrease in a semiconductor's resistance as temperature rises. A3. Space systems, computers, and data processing equipment. A4. The electron tube requires filament or heater voltage, whereas the semiconductor device does not: consequently, no power input is spent by the semiconductor for conduction. A5. Anything that occupies space and has weight. Solid, liquid, and gas. A6. The atom. A7. Electrons-negative, protons-positive, and neutrons-neutral AX. The valence shell. A9. Quanta. A 10. A negatively charged atom having more than its normal amount of electrons. All. The energy levels of an atom in a solid group together to form energy bands , whereas the isolated atom does not. All. The width of the forbidden band. A 1 3. The number of electrons in the valence shell. A 14. Covalent bonding. 1-46 A 15. Electron flow and hole flow. A 1 6. Intrinsic. A 1 7. P-type crystal. A 18. Electrons. APE To convert alternating current into direct current. A20. Toward the arrow. A21. Point-contact. A22. Holes. A23. N-type material. A24. Depletion region. A25. Negative. A26. Forward. A27. Any device that draws current. A28. A pulsating dc voltage. A29. Metallic rectifier. A30. Forward bias. A31. A characteristic curve. A32. They are the limiting values of operating conditions outside which operations could cause diode damage. A33. A semiconductor. A34. IN 368. A3 5. Heat. A36. The diode is open or has a high-forward resistance. 1-47 CHAPTER 2 TRANSISTORS LEARNING OBJECTIVES Upon completion of this chapter, you should be able to do the following: 1 . Define the term transistor and give a brief description of its construction and operation. 2. Explain how the transistor can be used to amplify a signal. 3. Name the four classes of amplifiers and give an explanation for each. 4. List the three different transistor circuit configurations and explain their operation. 5. Identify the different types of transistors by their symbology and alphanumerical designations. 6. List the precautions to be taken when working with transistors and describe ways to test them. 7. Explain the meaning of the expression "integrated circuits." 8. Give a brief description on how integrated circuits are constructed and the advantages they offer over conventional transistor circuits. 9. Name the two types of circuit boards. 1 0. State the purpose and function of modular circuitry. INTRODUCTION TO TRANSISTORS The discovery of the first transistor in 1948 by a team of physicists at the Bell Telephone Laboratories sparked an interest in solid-state research that spread rapidly. The transistor, which began as a simple laboratory oddity, was rapidly developed into a semiconductor device of major importance. The transistor demonstrated for the first time in history that amplification in solids was possible. Before the transistor, amplification was achieved only with electron tubes. Transistors now perform numerous electronic tasks with new and improved transistor designs being continually put on the market. In many cases, transistors are more desirable than tubes because they are small, rugged, require no filament power, and operate at low voltages with comparatively high efficiency. The development of a family of transistors has even made possible the miniaturization of electronic circuits. Figure 2-1 shows a sample of the many different types of transistors you may encounter when working with electronic equipment. 2-1 Figure 2-1. — An assortment of different types of transistors. Transistors have infiltrated virtually every area of science and industry, from the family car to satellites. Even the military depends heavily on transistors. The ever increasing uses for transistors have created an urgent need for sound and basic information regarding their operation. From your study of the PN-junction diode in the preceding chapter, you now have the basic knowledge to grasp the principles of transistor operation. In this chapter you will first become acquainted with the basic types of transistors, their construction, and their theory of operation. You will also find out just how and why transistors amplify. Once this basic information is understood, transistor terminology, capabilities, limitations, and identification will be discussed. Last, we will talk about transistor maintenance, integrated circuits, circuit boards, and modular circuitry. TRANSISTOR FUNDAMENTALS The first solid-state device discussed was the two-element semiconductor diode. The next device on our list is even more unique. It not only has one more element than the diode but it can amplify as well. Semiconductor devices that have-three or more elements are called TRANSISTORS. The term transistor was derived from the words TRANS fer and resISTOR. This term was adopted because it best describes the operation of the transistor - the transfer of an input signal current from a low-resistance circuit to a high- resistance circuit. Basically, the transistor is a solid-state device that amplifies by controlling the How of current carriers through its semiconductor materials. There are many different types of transistors, but their basic theory of operation is all the same. As a matter of fact, the theory we will be using to explain the operation of a transistor is the same theory used earlier with the PN-junction diode except that now two such junctions are required to form the three elements of a transistor. The three elements of the two-junction transistor are (1) the EMITTER, which gives off, or emits," current carriers (electrons or holes); (2) the BASE, which controls the flow of current carriers; and (3) the COLLECTOR, which collects the current carriers. CLASSIFICATION Transistors are classified as either NPN or PNP according to the arrangement of their N and P materials. Their basic construction and chemical treatment is implied by their names, "NPN" or "PNP." That 2-2 is, an NPN transistor is formed by introducing a thin region of P-type material between two regions of N-type material. On the other hand, a PNP transistor is formed by introducing a thin region of N-type material between two regions of P-type material. Transistors constructed in this manner have two PN junctions, as shown in figure 2-2. One PN junction is between the emitter and the base; the other PN junction is between the collector and the base. The two junctions share one section of semiconductor material so that the transistor actually consists of three elements. PN JUNCTIONS EMITTER i N P N BASE NPN TYPE COLLECTOR EMITTER PN JUNCTIONS l 1 N BASE PNP TYPE COLLECTOR Figure 2-2. — Transistor block diagrams. Since the majority and minority current carriers are different for N and P materials, it stands to reason that the internal operation of the NPN and PNP transistors will also be different. The theory of operation of the NPN and PNP transistors will be discussed separately in the next few paragraphs. Any additional information about the PN junction will be given as the theory of transistor operation is developed. To prepare you for the forthcoming information, the two basic types of transistors along with their circuit symbols are shown in figure 2-3. It should be noted that the two symbols are different. The horizontal line represents the base, the angular line with the arrow on it represents the emitter, and the other angular line represents the collector. The direction of the arrow on the emitter distinguishes the NPN from the PNP transistor. If the arrow points in, (Points iN) the transistor is a PNP . On the other hand if the arrow points out, the transistor is an NPN ( Not Pointing iN). 2-3 PNP "POINTS IN" NPN /t\ "NOT POINTING IN" EMITTER |N|P|N EMITTER n EMITTER EMITTER I P I N I P A © COLLECTOR (A) COLLECTOR ' NPN (B) (C) COLLECTOR (A) COLLECTOR ' PNP (B) (C) Figure 2-3. — Transistor representations. Another point you should keep in mind is that the arrow always points in the direction of hole tlow . or from the P to N sections, no matter whether the P section is the emitter or base. On the other hand, electron flow is always toward or against the arrow, just like in the junction diode. CONSTRUCTION The very first transistors were known as point-contact transistors. Their construction is similar to the construction of the point-contact diode covered in chapter 1 . The difference, of course, is that the point-contact transistor has two P or N regions formed instead of one. Each of the two regions constitutes an electrode (element) of the transistor. One is named the emitter and the other is named the collector, as shown in figure 2-4, view A. 2-4 PONT CONTACT (A) GROWN JUNCTION OR RATE-GROVM JUNCTION (B) ALLOV OR FUSED JUNCTON (C) DIFFUSED JUNCTION (D) EMITTER A 1 BASE P OP 1 h N0 Rp ^ N OR p COLLECTOR L y- EMITTER I COLLECTOR BASE COLLECTOR BASE * EMITTER BASE COLLECTOR EMITTER Figure 2-4. — Transistor constructions. Point-conlact transistors are now practically obsolete. They have been replaced by junction transistors, which are superior to point-contact transistors in nearly all respects. The junction transistor generates less noise, handles more power, provides higher current and voltage gains, and can be mass-produced more cheaply than the point-contact transistor. Junction transistors arc manufactured in much the same manner as the PN junction diode discussed earlier. However, when the PNP or NPN material is grown (view B), the impurity mixing process must be reversed twice to obtain the two junctions required in a transistor. Likewise, when the alloy-junction (view C) or the diffused-junction (view D) process is used, two junctions must also be created within the crystal. Although there are numerous ways to manufacture transistors, one of the most important parts of any manufacturing process is quality control. Without good quality control, many transistors would prove unreliable because the construction and processing of a transistor govern its thermal ratings, stability, and electrical characteristics. Even though there are many variations in the transistor manufacturing processes, certain structural techniques, which yield good reliability and lone life , are common to all processes: (1) Wire leads are connected to each semiconductor electrode; (2) the crystal is specially mounted to protect it against mechanical damage; and (3) the unit is sealed to prevent harmful contamination of the crystal. QI. What is the name given to the semiconductor device that has three or more elements? Q2. What electronic function made the transistor famous? Q3. In which direction does the arrow point on an NPN transistor? Q4. What was the name of the very first transistor? Q5. What is one of the most important parts of any transistor manufacturing process? 2-5 TRANSISTOR THEORY You should recall from an earlier discussion that a forward-biased PN junction is comparable to a low- resistance circuit element because it passes a high current for a given voltage. In turn, a reverse-biased PN junction is comparable to a high-resistance circuit element. By using the Ohm's law formula for power (P = PR) and assuming current is held constant, you can conclude that the power developed across a high resistance is greater than that developed across a low resistance. Thus, if a crystal were to contain two PN junctions (one forward-biased and the other reverse-biased), a low-power signal could be injected into the forward-biased junction and produce a high-power signal at the reverse-biased junction. In this manner, a power gain would be obtained across the crystal. This concept, which is merely an extension of the material covered in chapter 1, is the basic theory behind how the transistor amplifies. With this information fresh in your mind, let's proceed directly to the NPN transistor. NPN Transistor Operation Just as in the case of the PN junction diode, the N material comprising the two end sections of the NP N transistor contains a number of free electrons, while the center P section contains an excess number of holes. The action at each junction between these sections is the same as that previously described for the diode; that is, depletion regions develop and the junction barrier appears. To use the transistor as an amplifier, each of these junctions must be modified by some external bias voltage. For the transistor to function in this capacity, the first PN junction (emitter-base junction) is biased in the forward, or low-resistance, direction. At the same time the second PN junction (base-collector junction) is biased in the reverse, or high- resistance, direction. A simple way to remember how to properly bias a transistor is to observe the NPN or PNP elements that make up the transistor. The letters of these elements indicate what polarity voltage to use for correct bias. For instance, notice the NPN transistor below: 1 . The emitter, which is the first letter in the NPN sequence, is connected to the negative side of the battery while the base, which is the second letter (NPN), is connected to the positive side. 2. However, since the second PN junction is required to be reverse biased for proper transistor operation, the collector must be connected to an opposite polarity voltage (positive) than that indicated by its letter designation NPN). The voltage on the collector must also be more positive than the base, as shown below: 2-6 We now have a properly biased NPN transistor. In summary, the base of the NPN transistor must be positive with respect to the emitter, and the collector must be more positive than the base. NPN FORWARD-BIASED JUNCTION. — An important point to bring out at this time, which was not necessarily mentioned during the explanation of the diode, is the fact that the N material on one side of the forward-biased junction is more heavily doped than the P material. This results in more current being carried across the junction by the majority carrier electrons from the N material than the majority carrier holes from the P material. Therefore, conduction through the forward-biased junction, as shown in figure 2-5, is mainly by majority carrier electrons from the N material (emitter). 2-7 -4- HOLE FLOW Figure 2-5. — The forward-biased j unction in an NPN transistor. Willi the emitter-to-base junction in the figure biased in the forward direction, electrons leave the negative terminal of the battery and enter the N material (emitter). Since electrons are majority current carriers in the N material, they pass easily through the emitter, cross over the junction, and combine with holes in the P material (base). For each electron that fills a hole in the P material, another electron will leave the P material (creating a new hole) and enter the positive terminal of the battery. NPN REVERSE-BIASED JUNCTION. — The second PN junction (base-to-collector), or reverse- biased junction as it is called (fig. 2-6), blocks the majority current carriers from crossing the junction. However, there is a very small current, mentioned earlier, that does pass through this junction. This current is called minority current , or reverse current . As you recall, this current was produced by the electron-hole pairs. The minority carriers for the reverse-biased PN junction are the electrons in the P material and the holes in the N material. These minority carriers actually conduct the current for the reverse-biased junction when electrons from the P material enter the N material, and the holes from the N material enter the P material. However, the minority current electrons (as you will see later) play the most important part in the operation of the NPN transistor. 2-8 Al this point you may wonder why the second PN junction (base-to-collector) is not forward biased like the first PN junction (emitter-to-base). If both junctions were forward biased, the electrons would have a tendency to flow from each end section of the N PN transistor (emitter and collector) to the center P section (base). In essence, we would have two junction diodes possessing a common base, thus eliminating any amplification and defeating the purpose of the transistor. A word of caution is in order at this time. If you should mistakenly bias the second PN junction in the forward direction, the excessive current could develop enough heat to destroy the junctions, making the transistor useless. Therefore, be sure your bias voltage polarities are correct before making any electrical connections. NPN JUNCTION INTERACTION. — We are now ready to see what happens when we place the two junctions of the NPN transistor in operation at the same time. For a better understanding of just how the two junctions work together, refer to figure 2-7 during the discussion. 2-9 ELECTRON FLOW " HOLE FLOW Figure 2-7. — NPN transistor operation. The bias batteries in this figure have been labeled Vcc for the collector voltage supply, and Vbb for the base voltage supply. Also notice the base supply battery is quite small, as indicated by the number of cells in the batter)’, usually 1 volt or less. However, the collector supply is generally much higher than the base supply, normally around 6 volts. As you will see later, this difference in supply voltages is necessary to have current flow from the emitter to the collector. As stated earlier, the current flow in the external circuit is always due to the movement of free electrons. Therefore, electrons flow from the negative terminals of the supply batteries to the N-type emitter. This combined movement of electrons is known as emitter current (IQ. Since electrons are the majority carriers in the N material, they will move through the N material emitter to the emitter-base junction. With this junction forward biased, electrons continue on into the base region. Once the electrons are in the base, which is a P-type material, they become minority carriers . Some of the electrons that move into the base recombine with available holes. For each electron that recombines, another electron moves out through the base lead as base current 1 B (creating a new hole for eventual combination) and returns to the base supply battery V bb- The electrons that recombine are lost as far as the collector is concerned. Therefore, to make the transistor more efficient, the base region is made very thin and lightly doped. This reduces the opportunity for an electron to recombine with a hole and be lost. Thus, most of the electrons that move into the base region come under the influence of the large collector reverse bias. This bias acts as forward bias for the minority carriers (electrons) in the base and, as such, accelerates them through the base-collector junction and on into the collector region. Since the collector is made of an N-type material, the electrons that reach the collector again become majority current carriers . Once in the collector, the electrons move easily through the N material and return to the positive terminal of the collector supply batter)'’ Vcc as collector current (I< ). 2-10 To further improve on the efficiency of the transistor, the collector is made physically larger than the base for two reasons: (1) to increase the chance of collecting carriers that diffuse to the side as well as directly across the base region, and (2) to enable the collector to handle more heat without damage. In summary, total current flow in the NPN transistor is through the emitter lead. Therefore, in terms of percentage, I E is 100 percent. On the other hand, since the base is very thin and lightly doped, a smaller percentage of the total current (emitter current) will flow in the base circuit than in the collector circuit. Usually no more than 2 to 5 percent of the total current is base current (I«) while the remaining 95 to 98 percent is collector current <1 ( ). A very basic relationship exists between these two currents: Ie = Ib + ic In simple terms this means that the emitter current is separated into base and collector current. Since the amount of current leaving the emitter is solely a function of the emitter-base bias, and because the collector receives most of this current, a small change in emitter-base bias will have a far greater effect on the magnitude of collector current than it will have on base current. In conclusion, the relatively small emitter- base bias controls the relatively large emitter-to-collcctor current. Q6. To properly bias an NPN transistor, what polarity voltage is applied to the collector, and what is its relationship to the base voltage? Q7. Why is conduction through the forward-biased junction of an NPN transistor primarily in one direction, namely from the emitter to base? Q8. In the NPN transistor, what section is made very thin compared with the other two sections? Q9. What percentage of current in an NPN transistor reaches the collector? PNP Transistor Operation The PNP transistor works essentially the same as the NPN transistor. However, since the emitter, base, and collector in the PNP transistor are made of materials that are different from those used in the NPN transistor, different current carriers flow in the PNP unit. The majority current carriers in the PNP transistor are holes. This is in contrast to the NPN transistor where the majority current carriers are electrons. To support this different type of current (hole flow), the bias batteries are reversed for the PNP transistor. A typical bias setup for the PNP transistor is shown in figure 2-8. Notice that the procedure used earlier to properly bias the NPN transistor also applies here to the PNP transistor. The first letter (P) in the J]NP sequence indicates the polarity of the voltage required for the emitter (positive), and the second letter (N) indicates the polarity of the base voltage (negative). Since the base-collector junction is always reverse biased, then the opposite polarity voltage ( negative ) must be used for the collector. Thus, the base of the PNP transistor must be negative with respect to the emitter, and the collector must be more negative than the base. Remember, just as in the case of the NPN transistor, this difference in supply voltage is necessary to have current flow (hole flow in the case of the PNP transistor) from the emitter to the collector. Although hole flow is the predominant type of current flow in the PNP transistor, hole flow only takes place within the transistor itself, while electrons flow in the external circuit. I lowever, it is the internal hole flow that leads to electron flow in the external wires connected to the transistor. 2-11 Figure 2-8. — A properly biased PNP transistor. PNP FORWARD-BIASED JUNCTION. — Now let us consider what happens when the emitter-base junction in figure 2-9 is forward biased. With the bias setup shown, the positive terminal of the battery repels the emitter holes toward the base, while the negative terminal drives the base electrons toward the emitter. When an emitter hole and a base electron meet, they combine. For each electron that combines with a hole, another electron leaves the negative terminal of the battery, and enters the base. At the same time, an electron leaves the emitter, creating a new hole, and enters the positive terminal of the battery. This movement of electrons into the base and out of the emitter constitutes base current flow (Ig), and the path these electrons take is referred to as the emitter-base circuit. 2-12 ELECTRON FLOW HOLE FLOW Figure 2-9. — The forward-biased junction in a PNP transistor. PNP REVERSE-BIASED JUNCTION. — In the reverse-biased junction (fig. 2-10), the negative voltage on the collector and the positive voltage on the base block the majority current carriers from crossing the junction. However, this same negative collector voltage acts as forward bias lor the minority current holes in the base, which cross the junction and enter the collector. The minority current electrons in the collector also sense forward bias-the positive base voltage-and move into the base. The holes in the collector are filled by electrons that flow from the negative terminal of the battery. At the same time the electrons leave the negative terminal of the battery, other electrons in the base break their covalent bonds and enter the positive terminal of the battery. Although there is only minority current flow in the reverse-biased junction, it is still very' small because of the limited number of minority current carriers. 2-13 Figure 2-10. — The reverse-biased junction in a PNP transistor. PNP JUNCTION INTERACTION. — The interaction between the forward- and reverse-biased junctions in a PNP transistor is very similar to that in an NPN transistor, except that in the PNP transistor, the majority current carriers are holes. In the PNP transistor shown in figure 2-11, the positive voltage on the emitter repels the holes toward the base. Once in the base, the holes combine with base electrons. But again, remember that the base region is made very thin to prevent the recombination of holes with electrons. Therefore, well over 90 percent of the holes that enter the base become attracted to the large negative collector voltage and pass right through the base. However, lor each electron and hole that combine in the base region, another electron leaves the negative terminal of the base battery (V bb) and enters the base as base current (1b). At the same time an electron leaves the negative terminal of the battery, another electron leaves the emitter as IE (creating a new hole) and enters the positive terminal of Vbb- Meanwhile, in the collector circuit, electrons from the collector battery (Vcc) enter the collector as Ic and combine with the excess holes from the base. For each hole that is neutralized in the collector by an electron, another electron leaves the emitter and starts its way back to the positive terminal of Vcc- 2-14 Figure 2-11. — PNP transistor operation. Although current flow in the external circuit of the PNP transistor is opposite in direction to that of the NPN transistor, the majority carriers always flow from the emitter to the collector. This flow of majority carriers also results in the formation of two individual current loops within each transistor. One loop is the base-current path, and the other loop is the collector-current path. The combination of the current in both of these loops (In + I t ) results in total transistor current (Ie). The most important thing to remember about the two different types of transistors is that the emitter-base voltage of the PNP transistor has the same controlling effect on collector current as that of the NPN transistor. In simple terms, increasing the forward- bias voltage of a transistor reduces the emitter-base junction barrier. This action allows more carriers to reach the collector, causing an increase in current flow from the emitter to the collector and through the external circuit. Conversely, a decrease in the forward-bias voltage reduces collector current. QIO. What are the majority current carriers in a PNP transistor? QI I. What is the relationship between the polarity of the voltage applied to the PNP transistor and that applied to the NPN transistor? QI2. What is the letter designation for base current? QI 3. Name the two current loops in a transistor THE BASIC TRANSISTOR AMPLIFIER In the preceding pages we explained the internal workings of the transistor and introduced new terms, such as emitter, base, and collector. Since you should be familiar by now with all of the new terms 2-15 mentioned earlier and with the internal operation of the transistor, we will move on to the basic transistor amplifier. To understand the overall operation of the transistor amplifier, you must only consider the current in and out of the transistor and through the various components in the circuit. Therefore, from this point on, only the schematic symbol for the transistor will be used in the illustrations, and rather than thinking about majority and minority carriers, we will now stall thinking in terms of emitter, base, and collector current. Before going into the basic transistor amplifier, there are two terms you should be familiar with: AMPLIFICATION and AMPLIFIER. Amplification is the process of increasing the strength of a SIGNAL. A signal is just a general term used to refer to any particular current, voltage, or power in a circuit. An amplifier is the device that provides amplification (the increase in current, voltage, or power of a signal) without appreciably altering the original signal. Transistors are frequently used as amplifiers. Some transistor circuits are CURRENT amplifiers, with a small load resistance; other circuits are designed for VOLTAGE amplification and have a high load resistance; others amplify POWER. Now take a look at the NPN version of the basic transistor amplifier in figure 2-12 and let's see just how it works. So far in this discussion, a separate batter)' has been used to provide the necessary forward-bias voltage. Although a separate batter)' has been used in the past for convenience, it is not practical to use a battery for emitter-base bias. For instance, it would take a battery slightly over .2 volts to properly forward bias a germanium transistor, while a similar silicon transistor would require a voltage slightly over .6 volts. However, common batteries do not have such voltage values. Also, since bias voltages are quite critical and must be held within a few tenths of one volt, it is easier to work with bias currents flowing through resistors of high ohmic values than with batteries. By inserting one or more resistors in a circuit, different methods of biasing may be achieved and the emitter-base batter)' eliminated. In addition to eliminating the battery, some of these biasing methods compensate for slight variations in transistor characteristics and changes in transistor conduction resulting from temperature irregularities. Notice in figure 2-12 that the emitter-base batter)' has been eliminated and the bias resistor R b has been inserted between the collector and the base. Resistor Rb provides the necessary forward bias for the emitter-base junction. Current flows in the emitter-base bias circuit from ground to the emitter, out the base lead, and through R B to V cc- Since the current in the base circuit is very small (a few hundred microamperes) and the forward resistance of the transistor is low, only a few tenths of a volt of positive bias will be felt on the base of the transistor. However, this is enough voltage on the base, along with ground on the emitter and the large positive voltage on the collector, to properly bias the transistor. 2-16 OUTPUT --‘'OUTPUT Figure 2-12. — The basic transistor amplifier. With Q1 properly biased, direct current flows continuously, with or without an input signal, throughout the entire circuit. The direct current flowing through the circuit develops more than just base bias; it also develops the collector voltage (Vc) as it flows through Q1 and R l. Notice the collector voltage on the output graph. Since it is present in the circuit without an input signal, the output signal starts at the Vc level and either increases or decreases. These dc voltages and currents that exist in the circuit before the application of a signal are known as QUIESCENT voltages and currents (the quiescent state of the circuit). Resistor R L , the collector load resistor, is placed in the circuit to keep the full effect of the collector supply voltage off the collector. This permits the collector voltage (Vc ) to change with an input signal, which in turn allows the transistor to amplify voltage. Without R, in the circuit, the voltage on the collector would always be equal to Vcc- The coupling capacitor (Cc) is another new addition to the transistor circuit. It is used to pass the ac input signal and block the dc voltage from the preceding circuit. This prevents dc in the circuitry on the left of the coupling capacitor from affecting the bias on QI. The coupling capacitor also blocks the bias of Q1 from reaching the input signal source. The input to the amplifier is a sine wave that varies a few millivolts above and below zero. It is introduced into the circuit by the coupling capacitor and is applied between the base and emitter. As the input signal goes positive, the voltage across the emitter-base junction becomes more positive. This in effect increases forward bias, which causes base current to increase at the same rate as that of the input sine wave. Emitter and collector currents also increase but much more than the base current. With an increase in collector current, more voltage is developed across R L . Since the voltage across R, and the voltage across Ql (collector to emitter) must add up to Vcc, *111 increase in voltage across Rl results in an equal decrease in 2-17 voltage across Q1 . Therefore, the output voltage from the amplifier, taken at the collector of Q1 with respect to the emitter, is a negative alternation of voltage that is larger than the input, but has the same sine wave characteristics. During the negative alternation of the input, the input signal opposes the forward bias. This action decreases base current, which results in a decrease in both emitter and collector currents. The decrease in current through R l decreases its voltage drop and causes the voltage across the transistor to rise along with the output voltage. Therefore, the output for the negative alternation of the input is a positive alternation of voltage that is larger than the input but has the same sine wave characteristics. By examining both input and output signals for one complete alternation of the input, we can see that the output of the amplifier is an exact reproduction of the input except for the reversal in polarity and the increased amplitude (a few millivolts as compared to a few volts). The PNP version of this amplifier is shown in the upper pail of the figure. The primary difference between the NPN and PNP amplifier is the polarity of the source voltage. With a negative Vcc, the PNP base voltage is slightly negative with respect to ground, which provides the necessary forward bias condition between the emitter and base. When the PNP input signal goes positive, it opposes the forward bias of the transistor. This action cancels some of the negative voltage across the emitter-base junction, which reduces the current through the transistor. Therefore, the voltage across the load resistor decreases, and the voltage across the transistor increases. Since V t c is negative, the voltage on the collector (V() goes in a negative direction (as shown on the output graph) toward -Vcc (for example, from -5 volts to -7 volts). Thus, the output is a negative alternation of voltage that varies at the same rate as the sine wave input, but it is opposite in polarity and has a much larger amplitude . During the negative alternation of the input signal, the transistor current increases because the input voltage aids the forward bias. Therefore, the voltage across R L increases, and consequently, the voltage across the transistor decreases or goes in a positive direction (for example: from -5 volts to -3 volts). This action results in a positive output voltage, which has the same characteristics as the input except that it has been amplified and the polarity is reversed. In summary, the input signals in the preceding circuits were amplified because the small change in base current caused a large change in collector current. And, by placing resistor Rl in series with the collector, voltage amplification was achieved. Q14. What is the name of the device that provides an increase in current, voltage, or power of a signal without appreciably altering the original signal? QI5. Besides eliminating the emitter-base battery, what other advantages can different biasing methods offer? Q16. In the basic transistor amplifier discussed earlier, what is the relationship between the polarity of the input and output signals? QI 7. What is the primary difference between the NPN and PNP amplifiers? TYPES OF BIAS One of the basic problems with transistor amplifiers is establishing and maintaining the proper values of quiescent current and voltage in the circuit. This is accomplished by selecting the proper circuit-biasing conditions and ensuring these conditions are maintained despite variations in ambient (surrounding) 2-18 temperature, which cause changes in amplification and even distortion (an unwanted change in a signal). Thus a need arises for a method to properly bias the transistor amplifier and at the same time stabilize its dc operating point (the no signal values of collector voltage and collector current). As mentioned earlier, various biasing methods can be used to accomplish both of these functions. Although there are numerous biasing methods, only three basic types will be considered. Base-Current Bias (Fixed Bias) The first biasing method, called BASE CURRENT BIAS or sometimes FIXED BIAS, was used in figure 2-12. As you recall, it consisted basically of a resistor (Rb) connected between the collector supply voltage and the base. Unfortunately, this simple arrangement is quite thermally unstable. If the temperature of the transistor rises for any reason (due to a rise in ambient temperature or due to current flow through it), collector current will increase. This increase in current also causes the dc operating point, sometimes called the quiescent or static point, to move away from its desired position (level). This reaction to temperature is undesirable because it affects amplifier gain (the number of times of amplification) and could result in distortion, as you will see later in this discussion. Self-Bias A better method of biasing is obtained by inserting the bias resistor directly between the base and collector, as shown in figure 2-13. By tying the collector to the base in this manner, feedback voltage can be fed from the collector to the base to develop forward bias. This arrangement is called SELF-BIAS. Now, if an increase of temperature causes an increase in collector current, the collector voltage (Vc) w ill fall because of the increase of voltage produced across the load resistor (Rl). This drop in Vc will be fed back to the base and will result in a decrease in the base current. The decrease in base current will oppose the original increase in collector current and tend to stabilize it. The exact opposite effect is produced when the collector current decreases. * y cc Figure 2-13. — A basic transistor amplifier with self-bias. Self-bias has two small drawbacks: (1) It is only partially effective and, therefore, is only used where moderate changes in ambient temperature are expected; (2) it reduces amplification since the signal on the collector also affects the base voltage. This is because the collector and base signals for this particular amplifier configuration are 180 degrees out of phase (opposite in polarity) and the part of the collector signal that is fed back to the base cancels some of the input signal. This process of returning a part of the output back to its input is known as DEGENERATION or NEGATIVE FEEDBACK. Sometimes degeneration is 2-19 desired to prevent amplitude distortion (an output signal that fails to follow the input exactly) and self-bias may be used for this purpose. Combination Bias A combination of fixed and self-bias can be used to improve stability and at the same time overcome some of the disadvantages of the other two biasing methods. One of the most widely used combination-bias systems is the voltage-divider type shown in figure 2-14. Fixed bias is provided in this circuit by the voltage- divider network consisting of Rl, R2, and the collector supply voltage (V cc ). The dc current flowing through the voltage-divider network biases the base positive with respect to the emitter. Resistor R3, which is connected in series with the emitter, provides the emitter with self-bias. Should 1 e increase, the voltage drop across R3 would also increase, reducing Vc. This reaction to an increase in 4. by R3 is another form of degeneration, which results in less output from the amplifier. However, to provide long-term or dc thermal stability, and at the same time, allow minimal ac signal degeneration, the bypass capacitor (Cb P ) is placed across R3. If Cbp is large enough, rapid signal variations will not change its charge materially and no degeneration of the signal will occur. +V CC Figure 2-14. — A basic transistor amplifier with combination bias. In summary, the fixed-bias resistors, Rl and R2, tend to keep the base bias constant while the emitter bias changes with emitter conduction. This action greatly improves thermal stability and at the same time maintains the correct operating point for the transistor. QI8. Which biasing method is the most unstable? QI9. What type of bias is used where only moderate changes in ambient temperature are expected? Q20. When is degeneration tolerable in an amplifier? Q2I. What is the most widely used combination-bias system? 2-20 AMPLIFIER CLASSES OF OPERATION In the previous discussions, we assumed that for every portion of the input signal there was an output from the amplifier. This is not always the case with amplifiers. It may be desirable to have the transistor conducting for only a portion of the input signal. The portion of the input for which there is an output determines the class of operation of the amplifier. There are four classes of amplifier operations. They are class A, class AB, class B, and class C. Class A Amplifier Operation Class A amplifiers are biased so that variations in input signal polarities occur within the limits of CUTOFF and SATURATION. In a PNP transistor, for example, if the base becomes positive with respect to the emitter, holes will be repelled at the PN junction and no current can flow in the collector circuit. This condition is known as cutoff . Saturation occurs when the base becomes so negative with respect to the emitter that changes in the signal are not reflected in collector-current flow. Biasing an amplifier in this manner places the dc operating point between cutoff and saturation and allows collector current to tlow during the complete cycle (360 degrees) of the input signal, thus providing an output which is a replica of the input. Figure 2-12 is an example of a class A amplifier. Although the output from this amplifier is 180 degrees out of phase with the input, the output current still flows for the complete duration of the input. The class A operated amplifier is used as an audio- and radio-frequency amplifier in radio, radar, and sound systems, just to mention a few examples. For a comparison of output signals for the different amplifier classes of operation, refer to figure 2-15 during the following discussion. Figure 2-15. — A comparison of output signals for the different amplifier classes of operation. 2-21 Class AB Amplifier Operation Amplifiers designed for class AB operation are biased so that collector current is zero (cutoff) for a portion of one alternation of the input signal. This is accomplished by making the forward-bias voltage less than the peak value of the input signal. By doing this, the base-emitter junction will be reverse biased during one alternation for the amount of time that the input signal voltage opposes and exceeds the value of forward-bias voltage. Therefore, collector current will How for more than 180 degrees but less than 360 degrees of the input signal, as shown in figure 2-15 view B. As compared to the class A amplifier, the dc operating point for the class AB amplifier is closer to cutoff. The class AB operated amplifier is commonly used as a push-pull amplifier to overcome a side effect of class B operation called crossover distortion. Class B Amplifier Operation Amplifiers biased so that collector current is cut off during one-half of the input signal are classified class B. The dc operating point for this class of amplifier is set up so that base current is zero with no input signal. When a signal is applied, one half cycle will forward bias the base-emitter junction and 1 ( will flow. The other half cycle will reverse bias the base-emitter junction and I<- will be cut off. Thus, for class B operation, collector current will flow for approximately 180 degrees (halt) of the input signal, as shown in figure 2-15 view C. The class B operated amplifier is used extensively for audio amplifiers that require high-power outputs. It is also used as the driver- and power-amplifier stages of transmitters. Class C Amplifier Operation In class C operation, collector current tlows for less than one half cycle of the input signal, as shown in figure 2-15 view D. The class C operation is achieved by reverse biasing the emitter-base junction, which sets the dc operating point below cutoff and allows only the portion of the input signal that overcomes the reverse bias to cause collector current flow. The class C operated amplifier is used as a radio- frequency amplifier in transmitters. From the previous discussion, you can conclude that two primary items determine the class of operation of an amplifier — ( 1 ) the amount of bias and (2) the amplitude of the input signal. With a given input signal and bias level, you can change the operation of an amplifier from class A to class B just by removing forward bias. Also, a class A amplifier can be changed to class AB by increasing the input signal amplitude. However, if an input signal amplitude is increased to the point that the transistor goes into saturation and cutoff, it is then called an OVHRDR1VEN amplifier. You should be familiar with two terms used in conjunction with amplifiers — FIDELITY and EFFICIENCY. Fidelity is the faithful reproduction of a signal. In other words, if the output of an amplifier is just like the input except in amplitude, the amplifier has a high degree of fidelity. The opposite of fidelity is a term we mentioned earlier — distortion. Therefore, a circuit that has high fidelity has low distortion. In conclusion, a class A amplifier has a high degree of fidelity. A class AB amplifier has less fidelity, and class B and class C amplifiers have low or "poor" fidelity. The efficiency of an amplifier refers to the ratio of output-signal power compared to the total input power. An amplifier has two input power sources: one from the signal, and one from the power supply. Since every device takes power to operate, an amplifier that operates for 360 degrees of the input signal uses more power than if operated for 180 degrees of the input signal. By using more power, an amplifier has less power available for the output signal; thus the efficiency of the amplifier is low. This is the case 2-22 with the class A amplifier. It operates for 360 degrees of the input signal and requires a relatively large input from the power supply. Even with no input signal, the class A amplifier still uses power from the power supply. Therefore, the output from the class A amplifier is relatively small compared to the total input power. This results in low efficiency, which is acceptable in class A amplifiers because they are used where efficiency is not as important as fidelity. Class AB amplifiers are biased so that collector current is cut off for a portion of one alternation of the input, which results in less total input power than the class A amplifier. This leads to better efficiency. Class B amplifiers are biased with little or no collector current at the dc operating point. With no input signal, there is little wasted power. Therefore, the efficiency of class B amplifiers is higher still. The efficiency of class C is the highest of the four classes of amplifier operations. Q22. What amplifier class of operation allows collector current to flow during the complete cycle of the input? Q23. What is the name of the term used to describe the condition in a transistor when the emitter-base junction has zero bias or is reverse biased and there is no collector current? Q24. IVhat two primary items determine the class of operation of an amplifier? Q25. IVhat amplifier class of operation is the most inefficient but has the least distortion? TRANSISTOR CONFIGURATIONS A transistor may be connected in any one of three basic configurations (fig. 2-16): common emitter (CE), common base (CB), and common collector (CC). The term common is used to denote the element that is common to both input and output circuits. Because the common element is often grounded, these configurations are frequently referred to as grounded emitter, grounded base, and grounded collector. 2-23 Figure 2-16. — Transistor configurations. Each configuration, as you will see later, has particular characteristics that make it suitable for specific applications. An easy way to identify a specific transistor configuration is to follow three simple steps: 1. Identify the element (emitter, base, or collector) to which the input signal is applied. 2. Identify the element (emitter, base, or collector) from which the output signal is taken. 3. The remaining element is the common element, and gives the configuration its name. Therefore, by applying these three simple steps to the circuit in figure 2-12, we can conclude that this circuit is more than just a basic transistor amplifier. It is a common-emitter amplifier. Common Emitter The common-emitter configuration (CE) shown in figure 2-16 view A is the arrangement most frequently used in practical amplifier circuits, since it provides good voltage, current, and power gain. The common emitter also has a somewhat low input resistance (500 ohms- 1500 ohms), because the input is applied to the forward-biased junction, and a moderately high output resistance (30 kilohms-50 kilohms or more), because the output is taken off the reverse-biased junction. Since the input signal is applied to the base-emitter circuit and the output is taken from the collector-emitter circuit, the emitter is the element common to both input and output. 2-24 Since you have already covered what you now know to be a common-emitter amplifier (fig. 2-12), let's take a few minutes and review its operation, using the PNP common-emitter configuration shown in figure 2-16 view A. When a transistor is connected in a common-emitter configuration, the input signal is injected between the base and emitter, which is a low resistance, low-current circuit. As the input signal swings positive, it also causes the base to swing positive with respect to the emitter. This action decreases forward bias which reduces collector current (If) and increases collector voltage (making Vc more negative). During the negative alternation of the input signal, the base is driven more negative with respect to the emitter. This increases forward bias and allows more current carriers to be released from the emitter, which results in an increase in collector current and a decrease in collector voltage (making Vc less negative or swing in a positive direction). The collector current that flows through the high resistance reverse-biased junction also flows through a high resistance load (not shown), resulting in a high level of amplification. Since the input signal to the common emitter goes positive when the output goes negative, the two signals (input and output) are 180 degrees out of phase. The common-emitter circuit is the only configuration that provides a phase reversal. The common-emitter is the most popular of the three transistor configurations because it has the best combination of current and voltage gain. The term GAIN is used to describe the amplification capabilities of the amplifier. It is basically a ratio of output versus input. Each transistor configuration gives a different value of gain even though the same transistor is used. The transistor configuration used is a matter of design consideration. However, as a technician you will become interested in this output versus input ratio (gain) to determine whether or not the transistor is working properly in the circuit. The current gain in the common-emitter circuit is called BETA (p). Beta is the relationship of collector current (output current) to base current (input current). To calculate beta, use the following formula: (A is the Greek letter delta, it is used to indicate a small change) For example, if the input current (I») in a common emitter changes from 75 uA to 100 uA and the output current (I< ) changes from 1.5 mA to 2.6 mA, the current gain
ersal between the input and output signals?
Q28. What is the input current in the common-emitter circuit?
Q29. What is the current gain in a common-base circuit called?
Q30. Which transistor configuration has a current gain of less than I?
Q31. IVhat is the output current in the common-collector circuit?
Q32. Which transistor configuration has the highest input resistance?
Q33. What is the formula for GAMMA (y)?
2-29
TRANSISTOR SPECIFICATIONS
Transistors are available in a large variety of shapes and sizes, each with its own unique
characteristics. The characteristics for each of these transistors are usually presented on
SPECIFICATION SHEETS or they may be included in transistor manuals. Although many properties of
a transistor could be specified on these sheets, manufacturers list only some of them. The specifications
listed vary with different manufacturers, the type of transistor, and the application of the transistor. The
specifications usually cover the following items.
1 . A general description of the transistor that includes the following information:
a. The kind of transistor. This covers the material used, such as germanium or silicon; the type
of transistor (NPN or PNP); and the construction of the transistor whether alloy-junction,
grown, or diffused junction, etc.).
b. Some of the common applications for the transistor, such as audio amplifier, oscillator, rf
amplifier, etc.
c. General sales features, such as size and packaging mechanical data).
2. The "Absolute Maximum Ratings" of the transistor are the direct voltage and current values that
if exceeded in operation may result in transistor failure. Maximum ratings usually include
collector-to-base voltage, emitter-to-base voltage, collector current, emitter current, and collector
power dissipation.
3. The typical operating values of the transistor. These values are presented only as a guide. The
values vary widely, are dependent upon operating voltages, and also upon which element is
common in the circuit. The values listed may include collector-emitter voltage, collector current,
input resistance, load resistance, current-transfer ratio (another name for alpha or beta), and
collector cutoff current, which is leakage current from collector to base when no emitter current
is applied. Transistor characteristic curves may also be included in this section. A transistor
characteristic curve is a graph plotting the relationship between currents and voltages in a circuit.
More than one curve on a graph is called a "family of curves."
4. Additional information for engineering-design purposes.
So far, many letter symbols, abbreviations, and terms have been introduced, some frequently used
and others only rarely used. For a complete list of all semiconductor letter symbols and terms, refer to
EIMB series 000-0140, Section III.
TRANSISTOR IDENTIFICATION
Transistors can be identified by a Joint Army-Navy (JAN) designation printed directly on the case of
the transistor. The marking scheme explained earlier for diodes is also used for transistor identification.
The first number indicates the number of junctions. The letter "N" following the first number tells us that
the component is a semiconductor. And, the 2- or 3-digit number following the N is the manufacturer's
identification number. If the last number is followed by a letter, it indicates a later, improved version of
the device. For example, a semiconductor designated as type 2N130A signifies a three-element transistor
of semiconductor material that is an improved version of type 130:
2-30
2
N
130
A
NUMBER OF JUNCTIONS
SEMI-
IDENTIFICATION
FIRST
(TRANSISTOR)
CONDUCTOR
NUMBER
MODIFICATION
You may also find other markings on transistors that do not relate to the JAN marking system. These
markings are manufacturers' identifications and may not conform to a standardized system. If in doubt,
always replace a transistor with one having identical markings. To ensure that an identical replacement or
a correct substitute is used, consult an equipment or transistor manual for specifications on the transistor.
TRANSISTOR MAINTENANCE
Transistors are very rugged and are expected to be relatively trouble free. Encapsulation and
conformal coating techniques now in use promise extremely long life expectancies. In theory, a transistor
should last indefinitely. However, if transistors are subjected to current overloads, the junctions will be
damaged or even destroyed. In addition, the application of excessively high operating voltages can
damage or destroy the junctions through arc-over or excessive reverse currents. One of the greatest
dangers to the transistor is heat, which will cause excessive current flow and eventual destruction of the
transistor.
To determine if a transistor is good or bad, you can check it with an ohmmeter or a transistor tester.
In many cases, you can substitute a transistor known to be good for one that is questionable and thus
determine the condition of a suspected transistor. This method of testing is highly accurate and sometimes
the quickest, but it should be used only after you make certain that there are no circuit defects that might
damage the replacement transistor. If more than one defective transistor is present in the equipment where
the trouble has been localized, this testing method becomes cumbersome, as several transistors may have
to be replaced before the trouble is corrected. To determine which stages failed and which transistors are
not defective, all the removed transistors must be tested. This test can be made by using a standard Navy
ohmmeter, transistor tester, or by observing whether the equipment operates correctly as each of the
removed transistors is reinserted into the equipment. A word of caution-indiscriminate substitution of
transistors in critical circuits should be avoided.
When transistors are soldered into equipment, substitution is not practicable; it is generally desirable
to test these transistors in their circuits.
Q34. List three items of information normally included in the general description section of a
specification sheet for a transistor.
Q35. What does the number "2" (before the letter "N") indicate in the JAN marking scheme?
Q36. What is the greatest danger to a transistor?
Q37. What method for checking transistors is cumbersome when more than one transistor is bad in a
circuit?
PRECAUTIONS
Transistors, although generally more rugged mechanically than electron tubes, are susceptible to
damage by electrical overloads, heat, humidity, and radiation. Damage of this nature often occurs during
transistor servicing by applying the incorrect polarity voltage to the collector circuit or excessive voltage
to the input circuit. Careless soldering techniques that overheat the transistor have also been known to
cause considerable damage. One of the most frequent causes of damage to a transistor is the electrostatic
2-31
discharge from the human body when the device is handled. You may avoid such damage before starting
repairs by discharging the static electricity from your body to the chassis containing the transistor. You
can do this by simply touching the chassis. Thus, the electricity will be transferred from your body to the
chassis before you handle the transistor.
To prevent transistor damage and avoid electrical shock, you should observe the following
precautions when you are working with transistorized equipment:
1 . Test equipment and soldering irons should be checked to make certain there is no leakage current
from the power source. If leakage current is detected, isolation transformers should be used.
2. Always connect a ground between test equipment and circuit before attempting to inject or
monitor a signal.
3. Ensure test voltages do not exceed maximum allowable voltage for circuit components and
transistors. Also, never connect test equipment outputs directly to a transistor circuit.
4. Ohmmeter ranges that require a current of more than one milliampere in the test circuit should not
be used for testing transistors.
5. Battery eliminators should not be used to furnish power for transistor equipment because they
have poor voltage regulation and, possibly, high-ripple voltage.
6. The heat applied to a transistor, when soldered connections are required, should be kept to a
minimum by using a low-wattage soldering iron and heat shunts, such as long-nose pliers, on the
transistor leads.
7. When it becomes necessary to replace transistors, never pry transistors to loosen them from
printed circuit boards.
8. All circuits should be checked for defects before replacing a transistor.
9. The power must be removed from the equipment before replacing a transistor.
10. Using conventional test probes on equipment with closely spaced parts often causes accidental
shorts between adjacent terminals. These shorts rarely cause damage to an electron tube but may
ruin a transistor. To prevent these shorts, the probes can be covered with insulation, except for a
very short length of the tips.
LEAD IDENTIFICATION
Transistor lead identification plays an important part in transistor maintenance; because, before a
transistor can be tested or replaced, its leads or terminals must be identified. Since there is no standard
method of identifying transistor leads, it is quite possible to mistake one lead for another. Therefore, when
you are replacing a transistor, you should pay close attention to how the transistor is mounted, particularly
to those transistors that are soldered in, so that you do not make a mistake when you are installing the new
transistor. When you are testing or replacing a transistor, if you have any doubts about which lead is
which, consult the equipment manual or a transistor manual that shows the specifications for the transistor
being used.
There are, however, some typical lead identification schemes that will be very helpful in transistor
troubleshooting. These schemes are shown in figure 2-17. In the case of the oval-shaped transistor shown
in view A, the collector lead is identified by a wide space between it and the base lead. The lead farthest
from the collector, in line, is the emitter lead. When the leads are evenly spaced and in line, as shown in
2-32
view B, a colored dot, usually red, indicates the collector. If the transistor is round, as in view C, a red
line indicates the collector, and the emitter lead is the shortest lead. In view D the leads are in a triangular
arrangement that is offset from the center of the transistor. The lead opposite the blank quadrant in this
scheme is the base lead. When viewed from the bottom, the collector is the first lead clockwise from the
base. The leads in view E are arranged in the same manner as those is view D except that a tap is used to
identify the leads. When viewed from the bottom in a clockwise direction, the first lead following the tab
is the emitter, followed by the base and collector.
A 1
EMITTERvJ 1 ^COLLECTOR
3 21
BASE-*
3 2 1
B ■
■§*- COLOR DOT
em.tter47L collector
BASE— 1
COLOR DOT
'—S
3 2 1
BR- RED LIME
EMITTER J1 LcOLLECTOR
*21
BASE RED LINE
2
° 1
EMITTER - JTLcOLLECTOR
Bass— 1 l 3
E #
1 2 3
BASZ
EMITTER jJjj
COLLECTOR
1 (^COLLECTOR
MM CONNECTED TO
HI MOUNTING BASE
EMITTER JlL BASE
T . C (INDICATED BY
GREEN
^ SLEEVING)
31 2
G
£ COLLECTOR -g^* E
BASE 11' EMITTE ; vf.
Figure 2-17. — Transistor lead identification.
In a conventional power transistor as shown in views F and G, the collector lead is usually connected
to the mounting base. For further identification, the base lead in view F is covered with green sleeving.
While the leads in view G are identified by viewing the transistor from the bottom in a clockwise
direction (with mounting holes occupying 3 o'clock and 9 o'clock positions), the emitter lead will be
either at the 5 o'clock or 1 1 o'clock position. The other lead is the base lead.
TRANSISTOR TESTING
There are several different ways of testing transistors. They can be tested while in the circuit, by the
substitution method mentioned, or with a transistor tester or ohmmeter.
2-33
Transistor testers are nothing more than the solid-state equivalent of electron-tube testers (although
they do not operate on the same principle). With most transistor testers, it is possible to test the transistor
in or out of the circuit.
There are four basic tests required for transistors in practical troubleshooting: gain, leakage,
breakdown, and switching time. For maintenance and repair, however, a check of two or three parameters
is usually sufficient to determine whether a transistor needs to be replaced.
Since it is impractical to cover all the different types of transistor testers and since each tester comes
with its own operator's manual, we will move on to something you will use more frequently for testing
transistors-the ohmmeter.
Testing Transistors with an Ohmmeter
Two tests that can be done with an ohmmeter are gain, and junction resistance. Tests of a transistor's
junction resistance will reveal leakage, shorts, and opens.
TRANSISTOR GAIN TEST. — A basic transistor gain test can be made using an ohmmeter and a
simple test circuit. The test circuit can be made with just a couple of resistors and a switch, as shown in
figure 2-18. The principle behind the test lies in the fact that little or no current will flow in a transistor
between emitter and collector until the emitter-base junction is forward biased. The only precaution you
should observe is with the ohmmeter. Any internal battery may be used in the meter provided that it does
not exceed the maximum collector-emitter breakdown voltage.
Figure 2-18. — Testing a transistor's gain with an ohmmeter.
With the switch in figure 2-18 in the open position as shown, no voltage is applied to the PNP
transistor's base, and the emitter-base junction is not forward biased. Therefore, the ohmmeter should read
a high resistance, as indicated on the meter. When the switch is closed, the emitter-base circuit is forward
biased by the voltage across R1 and R2. Current now flows in the emitter-collector circuit, which causes a
lower resistance reading on the ohmmeter. A 10-to-l resistance ratio in this test between meter readings
indicates a normal gain for an audio-frequency transistor.
To test an NPN transistor using this circuit, simply reverse the ohmmeter leads and carry out the
procedure described earlier.
2-34
TRANSISTOR JUNCTION RESISTANCE TEST. — An ohmmeter can be used to test a transistor
for leakage (an undesirable flow of current) by measuring the base-emitter, base-collector, and collector-
emitter forward and reverse resistances.
For simplicity, consider the transistor under test in each view of figure 2-19 (view A, view B and
view C) as two diodes connected back to back. Therefore, each diode will have a low forward resistance
and a high reverse resistance. By measuring these resistances with an ohmmeter as shown in the Figure,
you can determine if the transistor is leaking current through its junctions. When making these
measurements, avoid using the R1 scale on the meter or a meter with a high internal battery voltage.
Either of these conditions can damage a low-power transistor.
Figure 2-19A. — Testing a transistor's leakage with an ohmmeter. COLLECTOR-TO-EMITTER TEST
Figure 2-19B. — Testing a transistor's leakage with an ohmmeter. BASE-TO-COLLECTOR TEST
2-35
OHM METER
NOTE: Reversing the meter leads will
give a low reading.
Figure 2-19C. — Testing a transistor's leakage with an ohmmeter. BASE-TO-EMITTER TEST
Now consider the possible transistor problems that could exist if the indicated readings in figure 2-19
are not obtained. A list of these problems is provided in table 2-2.
Table 2-2. — Possible Transistor Problems from Ohmmeter Readings
RESISTANCE READINGS
PROBLEMS
FORWARD
REVERSE
The transistor is:
LOW (NOT SHORTED)
LOW (NOT SHORTED)
LEAKING
LOW (SHORTED)
LOW (SHORTED)
SHORTED
HIGH
HIGH
OPEN*
’Except collector-to-emitter test.
By now. you should recognize that the transistor used in figure 2-19 (view A. view B and view C) is
a PNP transistor. If you wish to test an NPN transistor for leakage, the procedure is identical to that used
for testing the PNP except the readings obtained are reversed.
When testing transistors (PNP or NPN), you should remember that the actual resistance values
depend on the ohmmeter scale and the batter}' voltage. Typical forward and reverse resistances are
insignificant. The best indicator for showing whether a transistor is good or bad is the ratio of forward-to-
reverse resistance . If the transistor you are testing shows a ratio of at least 30 to 1, it is probably good.
Many transistors show ratios of 100 to 1 or greater.
Q38. IVhat safety precaution must be taken before replacing a transistor?
Q39. How is the collector lead identified on an oval-shaped transistor?
Q40. What are two transistor tests that can be done with an ohmmeter?
Q4 1. When you are testing the gain of an audio-frequency transistor with an ohmmeter, what is
indicated by a 10-to-l resistance ratio?
2-36
Q42. When you are using an ohmmeter to test a transistor for leakage, what is indicated by a low, but
not shorted, reverse resistance reading?
MICROELECTRONICS
Up to now the various semiconductors, resistors, capacitors, etc., in our discussions have been
considered as separately packaged components, called DISCRETE COMPONENTS. In this section we
will introduce some of the more complex devices that contain complete circuits packaged as a single
component. These devices are referred to as INTEGRATED CIRCUITS and the broad term used to
describe the use of these devices to miniaturize electronic equipment is called MICROELECTRONICS.
With the advent of the transistor and the demand by the military for smaller equipment, design
engineers set out to miniaturize electronic equipment. In the beginning, their efforts were frustrated
because most of the other components in a circuit such as resistors, capacitors, and coils were larger than
the transistor. Soon these other circuit components were miniaturized, thereby pushing ahead the
development of smaller electronic equipment. Along with miniature resistors, capacitors, and other circuit
elements, the production of components that were actually smaller than the space required for the
interconnecting wiring and cabling became possible. The next step in the research process was to
eliminate these bulky wiring components. This was accomplished with the PRINTED CIRCUIT BOARD
(PCB).
A printed circuit board is a Bat insulating surface upon which printed wiring and miniaturized
components are connected in a predetermined design, and attached to a common base. Figure 2-20 (view
A and view B) shows a typical printed circuit board. Notice that various components are connected to the
board and the printed wiring is on the reverse side. With this technique, all interconnecting wiring in a
piece of equipment, except for the highest power leads and cabling, is reduced to lines of conducting
material (copper, silver, gold, etc.) deposited directly on the surface of an insulating "circuit board." Since
printed circuit boards are readily adapted as plug-in units, the elimination of terminal boards, fittings and
tie points, not to mention wires, results in a substantial reduction in the overall size of electronic
equipment.
2-37
Figure 2-20B. — A typical printed circuit board (PCB). REV'ERSE SIDE
2-38
After the printed circuit boards were perfected, efforts to miniaturize electronic equipment were then
shifted to assembly techniques, which led to MODULAR CIRCUITRY. In this technique, printed circuit
boards are stacked and connected together to form a module. This increases the packaging density of
circuit components and results in a considerable reduction in the size of electronic equipment. Since the
module can be designed to perform any electronic function, it is also a very versatile unit.
However, the drawback to this approach was that the modules required a considerable number of
connections that took up too much space and increased costs. In addition, tests showed the reliability was
adversely affected by the increase in the number of connections.
A new technique was required to improve reliability and further increase packaging density. The
solution was INTEGRATED CIRCUITS.
An integrated circuit is a device that integrates (combines) both active components (transistors,
diodes, etc. ) and passive components (resistors, capacitors, etc.) of a complete electronic circuit in a
single chip (a tiny slice or wafer of semiconductor crystal or insulator).
Integrated circuits (ICs) have almost eliminated the use of individual electronic components
(resistors, capacitors, transistors, etc.) as the building blocks of electronic circuits. Instead, tiny CHIPS
have been developed whose functions are not that of a single part, but of dozens of transistors, resistors,
capacitors, and other electronic elements, all interconnected to perform the task of a complex circuit.
Often these comprise a number of complete conventional circuit stages, such as a multistage amplifier (in
one extremely small component). These chips are frequently mounted on a printed circuit board, as shown
in figure 2-21, which plugs into an electronic unit.
Figure 2-21. — ICs on a printed circuit board.
Integrated circuits have several advantages over conventional wired circuits of discrete components.
These advantages include (1) a drastic reduction in size and weight, (2) a large increase in reliability, (3)
lower cost, and (4) possible improvement in circuit performance. However, integrated circuits are
2-39
composed of parts so closely associated with one another that repair becomes almost impossible. In case
of trouble, the entire circuit is replaced as a single component.
Basically, there are two general classifications of integrated circuits: HYBRID and MONOLITHIC.
In the monolithic integrated circuit, all elements (resistors, transistors, etc.) associated with the circuit are
fabricated inseparably within a continuous piece of material (called the SUBSTRATE), usually silicon.
The monolithic integrated circuit is made very much like a single transistor. While one part of the crystal
is being doped to form a transistor, other parts of the crystal are being acted upon to form the associated
resistors and capacitors. Thus, all the elements of the complete circuit are created in the crystal by the
same processes and in the same time required to make a single transistor. This produces a considerable
cost savings over the same circuit made with discrete components by lowering assembly costs.
Hybrid integrated circuits are constructed somewhat differently from the monolithic devices. The
PASSIVE components (resistors, capacitors) are deposited onto a substrate (foundation) made of glass,
ceramic, or other insulating material. Then the ACTIVE components (diodes, transistors) are attached to
the substrate and connected to the passive circuit components on the substrate using very fine (.001 inch)
wire. The term hybrid refers to the fact that different processes are used to form the passive and active
components of the device.
Hybrid circuits are of two general types: (1) thin film and (2) thick film. "Thin" and "thick" film
refer to the relative thickness of the deposited material used to form the resistors and other passive
components. Thick film devices are capable of dissipating more power, but are somewhat more bulky.
Integrated circuits are being used in an ever increasing variety of applications. Small size and weight
and high reliability make them ideally suited for use in airborne equipment, missile systems, computers,
spacecraft, and portable equipment. They are often easily recognized because of the unusual packages that
contain the integrated circuit. A typical packaging sequence is shown in figure 2-22. These tiny packages
protect and help dissipate heat generated in the device. One of these packages may contain one or several
stages, often having several hundred components. Some of the most common package styles are shown in
figure 2-23.
2-40
BONDING ISLAND
IC CHIP
GOLD BACKING
SOLDER
ALUMINUM
BONDING
BASE
WIRE LEADS -v
- GLASS
LEADS
GOLD-PLATED
METAL
EXTERNAL LEADS
METAL
ADHESIVE
CERAMIC
MOUNTING BASE
Figure 2-22. — A typical integrated circuit packaging sequence.
Figure 2-23. — Common IC packaging styles.
The preceding information was presented to give you a brief introduction into integrated circuits. If
you wish to pursue this subject further, additional information is available in your ship's or station's
library'.
2-41
SUMMARY
Now that you have completed this chapter, a short review of the more important points covered in
the chapter will follow. This review should refresh your memory of transistors, their theory of operation,
and how they are tested with an ohmmeter.
A TRANSISTOR is a three or more clement solid-state device that amplifies by controlling the How
of current carriers through its semiconductor materials.
The THREE ELEMENTS OF A TRANSISTOR are ( 1 ) the EMITTER, which gives off current
carriers, (2) the BASE, which controls the carriers, and (3) the COLLECTOR, which collects the carriers.
m
COLLECTOR
EMITTER
BASE
The two BASIC TYPES OF TRANSISTORS are the NPN and PNP. The only difference in
symbology between the two transistors is the direction of the arrow on the emitter. If the arrow points in .
it is a PNP transistor and if it points outward , it is an NPN transistor.
2-42
EMITTER
iNlPlNl
A
COLLECTOR
EMITTER ■ \ COLLECTOR
NPN
EMITTER
EMITTER
COLLECTOR
COLLECTOR
PNP
The four TRANSISTOR MANUFACTURING PROCESSES are the (1) point contact, (2) grown
or rate-grown junction, (3) alloy or fused junction, and (4) diffused junction.
POINT CONTACT
(A)
EMITTER
COLLECTOR
X X
BASE
GROWN JUNCTION
OR
RATE-GROWN
JUNCTION
P OR N
- |N OR P^N OR P| —
EMITTER I COLLECTOR
BASE
COLLECTOR
ALLOY
OR
FUSED JUNCTION
BASE
*
(C)
EMITTER
DIFFUSED JUNCTION
BASE
(D)
COLLECTOR
T 3
EMITTER
2-43
The PROPER BIASING OF A TRANSISTOR enables the transistor to be used as an amplifier. To
function in this capacity, the emitter-to-base junction of the transistor is forward biased, while the base-to-
collector junction is reverse biased.
NPN TRANSISTOR OPERATION is basically the action of a relatively small emitter-base bias
voltage controlling a relatively large emitter-to-collector current.
PNP TRANSISTOR OPERATION is essentially the same as the NPN operation except the
majority current carriers are holes and the bias batteries are reversed.
AMPLIFICATION is the process of increasing the strength of a signal.
An AMPLIFIER is the device that provides amplification without appreciably altering the original
signal.
2-44
The BASIC TRANSISTOR AMPLIFIER amplifies by producing a large change in collector
current for a small change in base current. This action results in voltage amplification because the load
resistor placed in series with the collector reacts to these large changes in collector current which, in turn,
results in large variations in the output voltage.
V cc 10V
(NPN)
The three types of BIAS used to properly bias a transistor are base-current bias (fixed bias), self-bias,
and combination bias.
Combination bias is the one most widely used because it improves circuit stability and at the same
time overcomes some of the disadvantages of base-current bias and self-bias.
+ Vcc
COMBINATION
3 IAS
THE CLASS OF AMPLIFIER OPERATION is determined by the portion of the input signal for
which there is an output.
2-45
There are four classes of amplifier operations: class A, class AB, class B, and class C.
CUTOFF occurs when the base-to-emitter bias prevents current from flowing in the emitter circuit.
For example, in the PNP transistor, if the base becomes positive with respect to the emitter, holes are
repelled at the emitter-base junction. This prevents current from flowing in the collector circuit.
SATURATION occurs in a PNP transistor when the base becomes so negative, with respect to the
emitter, that changes in the signal are not reflected in collector-current flow.
CLASS A AMPLIFIERS are biased so that variations in input signal polarities occur within the
limits of cutoff and saturation. Biasing an amplifier in this manner allows collector current to flow during
the complete cycle (360 degrees) of the input signal, thus providing an output which is a replica of the
input but 1 80 degrees out of phase.
Class A operated amplifiers are used as audio- and radio-frequency amplifiers in radio, radar, and
sound systems.
CLASS AB AMPLIFIERS are biased so that collector current is zero (cutoff) for a portion of one
alternation of the input signal. Therefore, collector current will flow for more than 180 degrees but less
than 360 degrees of the input signal. The class AB amplifier is commonly used as a push-pull amplifier to
overcome a side effect of class B operations.
2-46
CLASS B AMPLIFIERS are biased so that collector current is cut off during one-half of the input
signal. Thus, for a class B operation, collector current will flow for approximately 180 degrees (halt) of
the input signal.
The class B operated amplifier is used as an audio amplifier and sometimes as the driver- and power-
amplifier stage of transmitters.
CLASS C AMPLIFIERS are biased so that collector current flows for less than one-half cycle of
the input signal.
The class C operated amplifier is used as a radio-frequency amplifier in transmitters.
FIDELITY and EFFICIENCY are two terms used in conjunction with amplifiers. Fidelity is the
faithful reproduction of a signal, while efficiency is the ratio of output signal power compared to the total
input power.
The class A amplifier has the highest degree of fidelity, but the class C amplifier has the highest
efficiency.
A TRANSISTOR CONFIGURATION is the particular way a transistor is connected in a circuit. A
transistor may be connected in any one of three different configurations: common emitter (CE), common
base (CB), and common collector