Semiconductors. Semiconductor devices

Solid State Electronics

1. Classify solids by conductivity. List the types of atomic bonds in solids.

2. Give energy diagrams of semiconductors and explain their features for n- and p-type semiconductors.

3. Explain the application features statistical methods when analyzing physical processes in semiconductors.

4. List the main types of currents in semiconductors and explain their features.

5. What is a nonequilibrium state of a semiconductor? Explain what is the lifetime of nonequilibrium charge carriers?

6. Explain the essence of the continuity equation.

7. Explain the structure of a pn junction. Give energy diagrams of the p-n junction.

8. Explain the features of the current-voltage characteristics of a p-n junction under various assumptions: a p-n junction with an infinitely wide base at low injection levels; pn junction taking into account recombination and generation of carriers in the space charge layer.

9. List the main types p-n breakdown transition and explain the features of the physical processes occurring during breakdown. What factors affect the breakdown voltage of a diode?

10. What are the barrier and diffusion capacitances of a pn junction?

11. How do transient processes occur in a diode when a voltage pulse and a high-amplitude current pulse are applied?

12. Explain the features of the structure and parameters of the Schottky rectifier diode.

13. Explain the device and principle of operation bipolar transistors.

14. Explain the features static characteristics bipolar transistor in a circuit with common emitter and in the scheme with common base.

15. List the main components of the emitter current transfer coefficient in a common base circuit. How does the indicated coefficient depend on the operating modes of the transistor?

16. Explain the features of breakdown of bipolar transistors.

17. How do transient processes occur in bipolar transistors?

18. Give the classification of semiconductor thyristors. Explain the design and principle of operation of triode and turn-off thyristors.

19. How do transient processes of turning on and off triode thyristors proceed? What is the du/dt effect?

20. Classify planar MOS transistors and give appropriate explanations.

21. Explain the structure and principle of operation of a planar MOS transistor. Draw and explain its static current-voltage characteristics and dynamic parameters.

22. Explain the design features of modern power MIS switches and give appropriate explanations.

23. Explain the design and principle of operation of a field-effect transistor with a control transition. Draw and explain its static current-voltage characteristics.

24. Explain the structure and principle of operation of an insulated gate bipolar transistor and indicate the features of its application.

25. Explain the device and principle of operation of a emitting diode and list its main parameters. List the main types of construction.

26. Explain the device, operating principle and main characteristics of semiconductor lasers. Explain the design features of heterolasers, distributed feedback lasers (DFBs), vertical cavity lasers (VCSELs).

27. Explain the structure and principle of operation of semiconductor photodiodes and list the main types of design. Explain the system of photodiode parameters.

Math modeling technological processes,
semiconductor devices and integrated circuits

Design and construction integrated circuits and semiconductor devices

1. Design n-p-n transistors. Fundamentals of calculating transistor geometry and electrophysical parameters (resistance of the base and collector regions, current transfer coefficients, breakdown voltages, etc.). Design and construction of multi-emitter n-p-n transistors.

2. Design of horizontal and vertical p-n-p transistors. Basics of calculating the geometric dimensions and transmission coefficients of vertical and horizontal transistors.

3. Design of integrated diodes based on p-n transitions. Structure and basic electrical parameters of integrated diodes. Calculation and design of diode geometry.

4. Design and construction of diodes and transistors with a Schottky barrier.

5. Designs of semiconductor resistors (diffusion, ion-implanted and MIS resistors). Calculation of volume and surface resistivity. Calculation electrical parameters resistors. Temperature coefficient of resistance. Frequency range of semiconductor resistors. Resistor topology design.

6. Designs of semiconductor capacitors. Equivalent circuits. Calculation of electrical parameters of semiconductor capacitors. Temperature coefficient of capacity.

8. Design of inter-element connections and metallization.

9. Stages of developing the topology of an IC on bipolar transistors (BT) and the basic principles of designing an IC on a BT.

10. Features of the development of the topology of digital ICs based on MOS transistors.

11. Designs and calculations of elements of hybrid integrated circuits.

12. Development of GIS topology. Functional and integral-group principles of GIS layout.

14. Features of LSI and VLSI design. Design limitations and challenges.

15. Main stages of calculation and design of LSI and VLSI. Definition functional composition microcircuits Topology design. Problems of machine design.

16. Method of proportional microminiaturization in the design and construction of ICs.

17. Model of diffusion from a limited and unlimited source of impurities.

18. Model of diffusion of impurities from doped oxides.

19. Model of impurity diffusion in silicon with simultaneous thermal oxidation.

20. Model of diffusion at high concentrations of impurities.

21. Modeling of ion implantation processes. Model by Lindhart, Sciotte and Scharf.

22. Modeling of ion implantation processes. Peirce's model.

23. Mathematical model mobility of the main charge carriers.

Microcircuitry

1. Main characteristics of a bipolar transistor as an amplifying element.

2. Frequency dependences of the gain factors of bipolar transistors.

3. Basic properties of a common emitter amplifier.

4. Basic properties of amplifiers with a common collector and a common base.

5. Main characteristics of a field-effect transistor as an amplifying element.

6. Schemes for setting the operating point of the transistor in amplifiers.

7. Nonlinear distortion, noise properties and frequency response of amplifiers based on bipolar transistors.

8. Principle feedback in amplifiers.

9. Differential amplifiers.

10. Operational amplifiers.

11. Basic schemes on operational amplifiers(amplifiers, integrator, differentiator, adder, subtractor).

12. Active filters of the first and second order.

13. Generators harmonic vibrations, excitation conditions.

14. LC oscillator based on an operational amplifier.

15. RC oscillators.

16. Three-point self-generators.

17. Class A power amplifiers.

18. Push-pull power amplifiers in mode B and AB.

19. High-speed transistor switches.

20. Minimization of logical functions.

21. Basic circuits and principles of constructing logical elements (TTL, ESL, MDPTL, etc.).

22. Symmetrical trigger on discrete elements.

23. Asynchronous triggers on logical elements(RS, JK).

24. Synchronous triggers on logical elements (JK, D, T).

25. Functional units of combinational type (encoders, decoders, multiplexers, demultiplexers, adders).

26. Serial type functional units (counters, registers, memory).

27. Digital-to-analog and analog-to-digital conversion.

28. Multivibrators and monovibrators on operational amplifiers.

29. Multivibrators and monovibrators based on logical elements.

30. Multivibrators and monovibrators based on discrete elements.

BIBLIOGRAPHY

1. Gurtov, V.A. Solid-state electronics: textbook. allowance / V.A. Gurtov – Petrozavodsk: Petrozav. state univ., 2004.

2. Berezin, A.S. Technology and design of integrated circuits: Textbook. manual for universities. / A.S. Berezin, O.R. Mochalkina. – M.: Radio and Communications, 1992.

3. Efimov, I.E. Microelectronics. Design, types of microcircuits, new directions: Proc. manual for universities / I.E. Efimov, Yu.I. Gorbunov, I.Ya. Trump. – M.: Higher. school, – 1978. – 312 p.

4. Efimov, I.E. Microelectronics: design, types of microcircuits, functional microelectronics: textbook. manual for universities / I.E. Efimov, I.Ya. Kozyr, Yu.I. Gorbunov. – 2nd ed., revised. and additional – M.: Higher. school, –1987. – 420s.

5. Efimov, I.E. Microelectronics: physical and technological fundamentals, reliability / I.E. Efimov, I.Ya. Kozyr, Yu.I. Gorbunov. – 2nd ed., revised. and additional – M.: Higher. school, –1986. – 464s.

6. Kalnibolotsky, Yu.M. Calculation and design of microcircuits / Yu.M. Kalnibolotsky, Yu.V. Korolev, G.I. Bogdan, V.S. Rogoza - Kyiv: Vishcha School, 1983.

7. Bushminsky, I.P. Technological design of microwave microcircuits: textbook. allowance / I.P. Bushminsky, G.V. Morozov. - M.: publishing house of MSTU im. N.E. Bauman, 2001.-354 P.

8. Gusev A.I. Nanomaterials, nanostructures, nanotechnologies.-M.: Fizmatlit, 2005.-410 P.

9. Poole, Ch. Nanotechnologies: textbook. allowance / C. Poole, F. Owens; lane from English edited by Yu. I. Golovina; add. V.V. Luchinina. - 2nd ed., add. - M.: Tekhnosphere, 2005.-334 P.

10. Nevolin, V.K. Probe nanotechnologies in electronics / V.K. Nevolin - M.: Tekhnosphere, 2005.-147 P.

11. Tairov, Yu.M. Technology of semiconductor and dielectric materials / Yu.M. Tairov, V.F. Tsvetkov – M.: Higher School. 1990. – 423 S.

12. Krapukhin, V.V. Materials technology electronic technology/ V.V. Krapukhin, I.A. Sokolov, G.D. Kuznetsov – M.: MISIS, 1995. – 492 P.

13. Popkov, V.I. Methods of operational quality control of semiconductor materials: monograph / Popkov V.I., Kazakov O.G., Radkova N.O. - BSTU. Bryansk: BSTU publishing house, 2001. – 50 P.

14. Bystrov, Yu.A. Electronic circuits and microcircuitry: textbook. for universities / Yu.A. Bystrov, I.G. Mironenko – M.: Higher. school, 2002. – 383 S.

15. Pavlov, V.N. analog circuitry electronic devices: textbook for universities / V.N. Pavlov, V.N. Nogin - M.: Hotline-Telecom, 2001. – 320 C.

16. Ugryumov, E.P. Digital circuitry: textbook. allowance / E.P. Ugryumov - St. Petersburg. And others: Bkhv-Petersburg, 2001. – 517 P.

17. Opadchiy, Yu.F. Analog and digital electronics: full course: textbook for universities / Yu.F. Opadchiy, O.P. Gludkin, A.I. Gurov; edited by O.P. Gludkina. –M.: Hot Line-Telecom, 2000. – 768 pp.

18. Brandon D., Kaplan U. Microstructure of materials. Methods of research and control: textbook. allowance / D. Brandon, W. Kaplan; lane from English edited by S.L. Bazhenova, with additional O. V. Egorova. – M.: Tekhnosphere, 2004.-377 P.

19. Pavlov, L.I. Methods for measuring parameters of semiconductor materials / L.I. Pavlov – M.: Higher School, 1987. – 239 P.

20. Physical methods for controlling the structure and quality of materials: textbook. manual / Bataev A.A., Bataev V.A., Tushinsky L.I., . – Novosibirsk: NSTU Publishing House, 2000. – 154 p.

UDC 621.382 BBK 32.852 L 33

The physical principles of operation of the most important classes of modern semiconductor devices are considered: diodes, bipolar and field-effect transistors, thyristors, microwave devices with negative differential resistance (Gunn diodes, avalanche-flight and injection-flight diodes), charge-coupled devices, optoelectronic devices (photodetectors, LEDs, injection lasers, etc.). The basic theoretical relationships that determine the characteristics of these devices are derived. Much attention is paid to describing the features of modern high-speed devices with submicron and nanometer dimensions, including devices that use heterojunctions, quantum wells and quantum dots. In addition, the book examines the basics of planar technology, describes the technological problems that have arisen recently and indicates promising ways to solve them.

For senior students, graduate students and researchers working in the field of semiconductor physics.

Recommended by the Educational Institution for Classical University Education of the Russian Federation as teaching aid for university students studying in specialties 010701 - “Physics”, 010704 - “Physics of Condensed Matter”, 010803 - “Microelectronics and Semiconductor Devices”.

ISBN 978-5-9221-0995-6

© FIZMATLIT, 2008 ® A. I. Lebedev, 2008

2.1. A little history. Bipolar transistor designs. . . 140

2.2. Parameters that determine the transient gain

2.4. High-frequency properties and performance of transistors. . 165 2.4.1. Cutoff frequency and maximum generation frequency. . 165

5 Contents

4.1.2.

4.2.3. 4.2.2. CMOS structures

Introduction

This book is written based on lectures that the author has given at the Physics Faculty of Moscow State University over the past several years. More than 10 years have passed since the publication of M. Schur’s last two-volume monograph on the physics of semiconductor devices. Due to the extremely rapid pace of development of applied developments in the field of semiconductor devices - and the pace of development of microelectronics is perhaps one of the fastest - a lot has changed in this field. The parameters of the devices have improved significantly; several generations of computer processor chips have changed. New physical ideas have emerged, new operating principles have been proposed, and new device designs have been implemented. Technological techniques used in the production of semiconductor devices have reached the limits of their capabilities. Some directions that were just emerging 10 years ago have become mainstream, while others that seemed promising have faded into the background. So, it's time to think about publishing a new book on the physics of semiconductor devices that would reflect these changes.

The development of semiconductor device physics is inextricably linked with fundamental research in semiconductor physics. Some of the discoveries made in this area, which have found wide practical application, have been recognized

for the discovery of the tunnel effect in semiconductors and superconductors), the work of our compatriot Zh. I. Alferov together with J. Kilby and G. Kremer (2000 prize for fundamental work in the field information technology and communications, contribution to the invention of the integrated circuit and the creation of semiconductor heterostructures for high-speed electronics and optoelectronics).

Developing a modern semiconductor device is an expensive task that requires a lot of labor and time. For example, the manufacturing cycle of a modern complex

microchips can take up to three months. Therefore, at present, the task of preliminary modeling of the device comes to the fore, only after solving which can we begin to create it. This requires the ability to quantify everything required parameters devices. For this reason, the physics of semiconductor devices, as a field of science, is required to be able to not only explain qualitatively, but also quantitatively predict the behavior of the semiconductor structure under consideration. This determines the abundance of formulas in the book, discussions of the validity of certain physical approximations - all this is necessary to provide the required quantitative result.

An additional complexity when solving the problem of developing semiconductor devices is created by the fact that these devices are made from specific semiconductors, the real properties of which are often far from idealized concepts. Therefore, the author considered it necessary to add to this book some features of semiconductors (for example, concerning the behavior of specific dopant impurities), without knowledge of which the creation of perfect devices is simply impossible. In addition, the book includes a description of the basic techniques of planar technology and discusses directions for their improvement, since without understanding the relationship between the physical principles of operation of devices and the technology of their manufacture, it is impossible to fully understand the “spirit” of modern microelectronics.

Understanding the material presented in this book requires prior familiarity with the course of semiconductor physics, the fundamentals of quantum mechanics and radio engineering.

The book examines the main classes of modern semiconductor devices and physical basis their work. The first and largest chapter discusses the physical phenomena that arise at the contact of two semiconductors different types conductivity - in the so-called p-n junction. Here we also consider phenomena that manifest themselves in more complex structures with potential barriers: metal-semiconductor contacts (Schottky barriers), heterojunctions, single quantum wells and superlattices. This chapter lays the foundation needed to understand the material in subsequent chapters. The second chapter is devoted to the study of bipolar transistors and ways to further improve their characteristics. In particular, approaches to the creation of high-speed transistors (heterojunction transistors, hot-wire transistors) are considered.

9 Introduction

electrons). This chapter also outlines the basics of plenary technology, which is currently the basis for the production of almost all types of semiconductor devices, as well as some circuit design techniques that, through functional integration, can significantly increase the packing density of elements in integrated circuits and approach the creation of very large scale integrated circuits (VLSI). The operating principles and properties of four-layer and even more complex bipolar structures, from which thyristors and triacs, which are extremely necessary for modern power engineering, are made, are discussed in Chapter 3. The fourth chapter is devoted to field-effect transistors - the most common semiconductor devices today. The fact that we are now surrounded by high-performance computers, the speed of which is increasing at a dizzying speed, we owe to the development of this particular class of semiconductor devices. Particular attention in this chapter is occupied by modern ideas and solutions that make it possible to create field-effect transistors that are capable of operating at frequencies related to the submillimeter region of the spectrum (above 300 GHz). This chapter also discusses relevant modern electronics hybrid (bipolar + field) structures such as IGBT and BiCMOS, as well as the main types of field-effect transistor ICs (n-MOS, KM.OP, static, dynamic and reprogrammable memories, flash memory). The fifth chapter discusses the operating principles of an important class of functionally integrated devices based on the field effect - charge-coupled devices. The most interesting direction in the development of these devices is, apparently, the creation of image receivers, which are widely used in such household appliances, How digital cameras and video cameras. Chapter 6 of the book discusses a completely different class of devices - semiconductor microwave devices. This chapter describes methods for obtaining negative differential resistance in semiconductors and creating, based on this phenomenon, generators of electromagnetic oscillations in the ranges of centimeter, millimeter and submillimeter wavelengths. Finally, the seventh chapter of the book is devoted to the physical principles of operation of a wide class of optoelectronic devices. These are radiation receivers used to register electromagnetic oscillations ranging from the far infrared region of the spectrum (BIB and HIWIP detectors) to the range of X-ray and gamma radiation (nuclear radiation detectors), and semiconductor sources

radiation (LEDs, lasers). Particular attention in this chapter is paid to the physical phenomena in new semiconductor objects (quantum wells, threads and dots) and the use of these phenomena to significantly improve the parameters of optoelectronic devices.

A distinctive feature of this book is that most of the information about the most important ideas, developments and achievements of recent years in the field of semiconductor device physics is gleaned not from journal articles, but from the Internet. The Internet has made analytical reviews written by specialists from leading development companies around the world publicly available; it allows you to quickly monitor the latest achievements in the field of fundamental and applied semiconductor research and identify the main development trends in this field of knowledge.

The author considers it his pleasant duty to express gratitude to his colleagues, Professor A.E. Yunovich, associate professor M.V. Chukichev, Art. scientific co-workers I.A. Kurova and I.A. Sluchinskaya, Ph.D. S.G. Dorofeev and V.M. Shakhparonov, who read individual sections of the manuscript and made a number of valuable comments that contributed to improving the content of the book as a whole.

Chapter 1

SEMICONDUCTOR DIODES

The operation of most semiconductor devices is based on the use of specific contact properties of semiconductors of different types of conductivity - the so-called pn junction. These properties are due to a number of physical phenomena occurring in such a contact: injection, tunneling, impact ionization of carriers, etc. In this chapter we will consider these physical phenomena, establish their role in the specific operating conditions of semiconductor diodes, and calculate the characteristics of the pn junction in these conditions and discuss how they can be controlled by changing the geometry of the device and the parameters of the semiconductor.

1.1. Potential barrier in pn junction

ity is the emergence of an energy barrier and an area

The reason for the appearance of this barrier is the diffusion of free charge carriers (electrons and holes). Let's consider these phenomena in more detail.

From the general course of semiconductor physics it is known that in a non-degenerate semiconductor at any point the concentrations of electrons n and holes p are related by the relation

The basic semiconductor devices of modern microelectronics and physical processes that ensure their work. The static, frequency and impulse characteristics of devices are analyzed, methods of circuit modeling of devices are considered and their equivalent circuits are given. The limiting parameters of modern microelectronic devices are considered. For each device it is done short review modern methods their structural implementation in integrated circuits. For students studying in the direction 210100 "Electronics and microelectronics" (210100.62 - bachelor, 210100.68 - master) and in engineering specialties 210104.65 "Microelectronics and solid-state electronics", 210108.65 "Microsystem technology", 010803.65 "Microelectronics and semiconductor nic devices", 210601.65 "Nanotechnologies in electronics." The material in the book can also be useful to scientists, engineers and graduate students seeking to gain the necessary professional knowledge

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2.1. 2. Semiconductors.

Semiconductors are substances whose conductivity is intermediate between the conductivities of metals and dielectrics. Semiconductors are both poor conductors and poor dielectrics. The boundary between semiconductors and dielectrics is arbitrary, since dielectrics high temperatures can behave like semiconductors, and pure semiconductors when low temperatures behave like dielectrics. In metals, the electron concentration is practically independent of temperature, and in semiconductors, charge carriers appear only when the temperature increases or when energy is absorbed from another source.

Typical semiconductors are carbon (C), germanium (Ge), and silicon (Si). Germanium is a brittle, grayish-white element discovered in 1886. The source of powdered germanium dioxide, from which solid pure germanium is obtained, is the ash of certain types of coal.

Silicon was discovered in 1823. It is widely distributed in the earth's crust in the form of silica (silicon dioxide), silicates and aluminosilicates. Sand, quartz, agate and flint are rich in silicon dioxide. Pure silicon is obtained from silicon dioxide chemically. Silicon is the most widely used semiconductor material.

Let us consider in more detail the formation of conduction electrons in semiconductors using silicon as an example. The silicon atom has the serial number Z=14 in the periodic table of D.I. Mendeleev. Therefore, its atom contains 14 electrons. However, only 4 of them are on empty outer shell and are weakly coupled. These electrons are called valence electrons and give rise to the four valences of silicon. Silicon atoms are able to combine their valence electrons with other silicon atoms using what is called a covalent bond (Figure 2.1). In covalent bonding, valence electrons are shared between different atoms, resulting in the formation of a crystal.

As the temperature of the crystal increases, thermal vibrations of the lattice lead to the breaking of some valence bonds. As a result of this, some of the electrons that previously participated in the formation of valence bonds are split off and become conduction electrons. In the presence of an electric field, they move against the field and form electricity.

However, when an electron is released in the crystal lattice, an unfilled interatomic bond is formed. Such “empty” spaces with missing bonding electrons are called “holes.” The appearance of holes in a semiconductor crystal creates additional opportunity for charge transfer. Indeed, the hole can be filled by an electron transferred under the influence of thermal vibrations from a neighboring atom. As a result, normal communication will be restored in this place, but a hole will appear in another place. Any of the other bond electrons, etc., can in turn go into this new hole. The sequential filling of a free bond with electrons is equivalent to the movement of a hole in the direction opposite to the movement of electrons. Thus, if in the presence of an electric field electrons move against the field, then holes will move in the direction of the field, i.e. the way positive charges would move. Consequently, in a semiconductor there are two types of current carriers - electrons and holes, and the total conductivity of the semiconductor is the sum of electronic conductivity (n-type, from the word negative) and hole conductivity (p-type, from the word positive).

Along with transitions of electrons from a bound state to a free state, there are reverse transitions in which a conduction electron is captured in one of the vacant positions of bond electrons. This process is called electron-hole recombination. In a state of equilibrium, such a concentration of electrons (and an equal concentration of holes) is established at which the number of straight lines and reverse transitions per unit time is the same.

The considered conduction process in pure semiconductors is called intrinsic conductivity. Intrinsic conductivity increases rapidly with increasing temperature, and this is a significant difference between semiconductors and metals, whose conductivity decreases with increasing temperature. All semiconductor materials have a negative temperature coefficient of resistance.

Pure semiconductors are an object of mainly theoretical interest. Major semiconductor research concerns the effects of adding impurities to pure materials. Without these impurities, most semiconductor devices would not exist.

Pure semiconductor materials such as germanium and silicon are kept at room temperature a small amount of electron-hole pairs and can therefore conduct very little current. Alloying is used to increase the conductivity of pure materials.

Doping is the addition of impurities to semiconductor materials. Two types of impurities are used. Impurities of the first type - pentavalent - consist of atoms with five valence electrons, for example, arsenic and antimony. The second type of impurity - trivalent - consists of atoms with three valence electrons, for example, indium and gallium.



When a pure semiconductor material is doped with a pentavalent material such as arsenic (As), some of the semiconductor atoms are replaced by arsenic atoms (Figure 2.2). The arsenic atom introduces four of its valence electrons into covalent bonds with neighboring atoms. Its fifth electron is weakly bound to the nucleus and can easily become free. The arsenic atom is called a donor atom because it donates its extra electron. The doped semiconductor material contains a sufficient number of donor atoms, and therefore free electrons, to maintain the current.

At room temperature, the number of additional free electrons exceeds the number of electron-hole pairs. This means that the material has more electrons than holes. Therefore, electrons are called majority carriers. Holes are called minority carriers. Since majority carriers have a negative charge, such a material is called an n-type semiconductor.

When a semiconductor material is doped with trivalent atoms, such as indium (In) atoms, these atoms will place their three valence electrons among three neighboring atoms (Figure 2.3). This will create a hole in the covalent bond.

The presence of additional holes will allow electrons to easily drift from one covalent bond to another. Since holes easily accept electrons, atoms that introduce additional holes into a semiconductor are called acceptor atoms.



Under normal conditions, the number of holes in such a material significantly exceeds the number of electrons. Therefore, holes are the majority carriers and electrons are minority carriers. Because the majority carriers have a positive charge, the material is called a p-type semiconductor.

N- and p-type semiconductor materials have significantly higher conductivity than pure semiconductors. This conductivity can be increased or decreased by changing the amount of impurities. The more heavily doped a semiconductor material is, the less electrical resistance.

The contact of two semiconductors with different types of conductivity is called a p-n junction and has a very important property– its resistance depends on the direction of the current. Note that such a contact cannot be achieved by pressing two semiconductors against each other. A p-n junction is created in one semiconductor wafer by forming regions with different types of conductivity in it. Methods for obtaining p-n junctions are described below.

So, in a piece of a single-crystal semiconductor, a p-n junction is formed at the boundary between two layers with different conductivities. There is a significant difference in the concentrations of charge carriers. The concentration of electrons in the n-region is many times greater than their concentration in the p-region. As a result, electrons diffuse into the region of their low concentration (in the p-region). Here they recombine with holes and in this way create a spatial negative charge of the ionized acceptor atoms, which is not compensated by the positive charge of the holes.

At the same time, diffusion of holes into the n-region occurs. Here, a spatial positive charge of the donor ions, which is not compensated by the electron charge, is created. Thus, a double layer of space charge is created at the boundary (Fig. 2.4), depleted of the main current carriers. A contact electric field Ek arises in this layer, preventing the further transition of electrons and holes from one region to another.

The contact field maintains a state of equilibrium at a certain level. But even in this case, under the influence of heat, a small part of electrons and holes will continue to pass through the potential barrier caused by space charges, creating a diffusion current. However, at the same time, under the influence of the contact field, minority charge carriers of the p- and n-regions (electrons and holes) create a small conduction current. In a state of equilibrium, these currents cancel each other out.

If an external current source is connected to the p-n junction, then the voltage indicated in Fig. 2.5 Reverse polarity will cause external field E, coinciding in direction with the contact field Eк. As a result, the width of the double layer will increase, and there will be practically no current due to the majority carriers. Only a small current is possible in the circuit due to minority carriers ( reverse current Iobr).







When the voltage of direct polarity is turned on, the direction of the external field is opposite to the direction of the contact field (Fig. 2.6). The width of the double layer will decrease, and a large forward current Ipr will arise in the circuit. Thus, the p-n junction has pronounced one-way conductivity. This is expressed by its current-voltage characteristic (Fig. 2.7).



When a forward voltage is applied to a pn junction, the current increases rapidly with increasing voltage. When a reverse voltage is applied to the p-n junction, the current is very small, quickly reaches saturation and does not change up to a certain limiting value of the reverse voltage Urev, after which it increases sharply. This is the so-called breakdown voltage at which breakdown occurs p-n junction and it collapses. It should be noted that in Figure 2.7 the scale of the reverse current is a thousand times smaller scale direct current.

From your physics course you know that there are conductors, dielectrics and semiconductors. Conductors are characterized by a conductivity of 10 2 -10 8 S/cm 3 (Sm - Siemens = 1/Ohm), for dielectrics - 10 -10 S/cm 3 and less. The interval from 10 -10 to 10 2 S/cm 3 is occupied by semiconductors. Characteristic feature What distinguishes semiconductors from metals is the increase in electrical conductivity with increasing temperature.

Semiconductor devices are electrical converting devices whose operating principle is based on phenomena occurring in the semiconductor itself or at the interface of two semiconductors with different types of conductivity.

Semiconductor devices include:

Rectifier diodes

RF and microwave diodes

Zener diodes or reference diodes

Tunnel diodes

Varicaps

Thyristors

Bipolar and field-effect transistors, etc.

For the manufacture of real semiconductor devices, germanium, silicon and gallium arsenide are usually used.

As you know, a semiconductor has a three-dimensional lattice. For simplicity, we will consider a flat lattice. Silicon atoms are connected to each other by a covalent bond. At a temperature of 0 o K, all semiconductors are ideal insulators because there are no free electrons in their structure.

Under the influence external factors(change in temperature, radiation, light radiation, etc.) the crystalline structure receives external energy, which can lead to the breaking of a covalent bond and a free electron appears in the structure, i.e. The resistance of the semiconductor will decrease.

A semiconductor can be represented in terms of energy levels (valence band, band gap, conduction band). Here DW is the band gap, the potential barrier that an electron must overcome to enter the conduction band. For the most common semiconductors, DW = 0.1-3 eV, and for dielectrics - 6 eV. For germanium DW = 0.72 eV, for silicon DW = 1.12 eV.



At the site of the bond break, a hole appears, which has the same charge as the electron, but with the opposite sign. In an ideal semiconductor, the concentration of electrons and holes is the same. If n n is the electron concentration, and n p is the hole concentration, then for an ideal semiconductor n n = n p is the intrinsic conductivity of the semiconductor.



Real semiconductor devices use impurity semiconductors. If a 5-valence element is introduced into a semiconductor as an impurity, then this semiconductor will be a semiconductor with electronic conductivity or n-type, and the impurity is called a donor impurity. In this case, the electron concentration Nn will be much greater than the hole concentration Np, i.e. N n >> N p . Thus, electrons will be the majority charge carriers, and holes will be the minority ones.




If a 3-valence element is introduced into a semiconductor as an impurity, then free holes will appear in the valence band. In this case, the hole concentration will be much greater than the electron concentration N p >> N n - this is a semiconductor with hole conductivity or p-type, and the impurity is called acceptor. Here the main charge carriers are holes.

ELECTRON-HOLE TRANSITION



When two semiconductors with different types of conductivity come into contact, as a result of diffusion, electrons move into the p-layer, and holes, on the contrary, into the n-layer. At the interface of the contact of two semiconductors, as a result of recombination, a region of stationary space charges (ions) is formed, which create an electric field that prevents further transition of the main charge carriers. A pn junction is a region depleted of charge carriers and, therefore, it has an increased resistance, which determines the electrical resistance of the entire system.

There are also two pn junction capacitances:

It is therefore obvious that the properties of a pn junction depend on the frequency of the voltage applied to the pn junction.



The current-voltage characteristic of the p-n junction is as follows:

Where I o is the reverse current of the p-n junction (thermal current). The p-n junction current in the forward direction is determined by the formula:

j T - temperature potential

From the current-voltage characteristic it is obvious that the p-n junction conducts well in the forward direction and poorly in the reverse direction, i.e. has valve properties. The current-voltage characteristic is nonlinear, which means that when alternating signals pass through a p-n junction, the signal spectrum is transformed.

On the reverse branch of the current-voltage line, the dotted line shows a sharp increase in current, i.e. breakdown of the p-n junction occurs.



Electrical breakdown is a reversible breakdown that is used to produce special devices- zener diodes. Electrical breakdowns include tunnel, avalanche and surface.

Tunnel breakdown is when, with an increase in the reverse voltage Urev, a sharp curvature of the energy zones occurs. In this case, the level of the valence band of the n-type semiconductor turns out to be at the level of the conduction band of the p-type semiconductor, i.e. a tunnel for charges appears, which leads to a sharp increase in current.

Avalanche breakdown occurs at higher p-n junction voltages than tunnel breakdown, as a result of which an avalanche-like multiplication of charge carriers begins in the p-n junction, which also leads to a sharp increase in current.

Thermal breakdown is irreversible.

EQUIVALENT ELECTRICAL DIAGRAM

p-n-junction



r - differential resistance

Usually the equivalent circuit is used for variable signals and therefore differential parameters are used.

Rk - resistance of contacts and leads

r - resistance of the p-n junction in direct or reverse connection

C is the diffusion capacitance for direct connection or the barrier capacitance for reverse connection of the p-n junction.

It follows from the diagram that at a high signal frequency, the valve properties of the p-n junction deteriorate.

DEPENDENCE OF RN-JUNCTION PARAMETERS ON TEMPERATURE



The parameters are highly dependent on the ambient temperature. As the ambient temperature increases, both forward and reverse currents increase. Changes especially strongly inverse parameters, for example, r arr decreases sharply, which can reduce the breakdown voltage U breakdown. An increase in temperature enhances the generation of minority charge carriers and, consequently, sharply increases their concentration in the semiconductor. This is a significant drawback of the pn junction and all semiconductor devices.

SEMICONDUCTOR DIODES

A semiconductor diode is an electrical converting device whose properties depend on the properties and characteristics of the p-n junction. Diodes are distinguished by frequency range and power dissipation.

Based on the conversion frequency, there are low-frequency (LF) diodes (rectifier and power), high-frequency (HF) diodes and pulse diodes.

Special diodes include zener diodes, stabilizers, varicaps and tunnel diodes.

Based on dissipation power, low-power diodes are distinguished (up to 0.25 W), medium power (from 0.25 to 1 W) and high power(over 1 W).

RECTIFIER DIODES

Consider a rectifier diode. Here, the emitter is understood as a region with a high concentration of charge carriers, and the base is a region with a low charge concentration, i.e. the base is high resistance.



In the figure of the current-voltage characteristic, the dotted line indicates an ideal p-n junction.

DU b is the voltage drop across the high-resistance base.



In real semiconductor devices, the current-voltage characteristic is shifted to the right. Rectifier diodes are also characterized by differential parameters: r pr, r arr, C diff, C bar.

U rectifier diodes The capacitance usually lies in the range C = (10 - 100) pF. The capacitance depends on the area of ​​the p-n junction.

To characterize rectifier diodes, enter the following parameters:

I pr.max - direct maximum current,



U obr.add. - permissible reverse voltage, at which there is still no thermal breakdown.

Just like a pn junction, the parameters and characteristics of a rectifier diode are highly dependent on temperature.

An example of the use of a rectifier diode is a half-wave rectifier. Where the average value of the rectifier current is:



Then the input current will have a sinusoidal character then

A capacitor is usually connected in parallel to the load, which smoothes out current pulses.



ZENER DIRECTION (REFERENCE DIODE)

Rectifier diodes are capable of rectifying current from units of mA to 1000A at voltages from units of volts to 1000 V. For high currents and voltages, diode assemblies are used.

Zener diodes are used to stabilize DC voltage. The working section of the current-voltage characteristic of the zener diode is the reverse branch. It has three characteristic sections. Section I is the usual reverse current of a real diode - thermal current or generation current. Section II is the section of electrical breakdown - tunnel 1 or avalanche 2 in nature; it is this section of the current-voltage characteristic that is the working section of the zener diode. In section III, thermal breakdown occurs.

As the reverse voltage increases, the current through the diode increases, as well as the power released in the p-n junction, which leads to an increase in the temperature of the p-n junction. Increasing the diode temperature increases the generation of minority charge carriers, which in turn increases the reverse current. Thus, the temperature rises even more, which leads to the destruction of the p-n junction.

I min - selected at the initial moment of the breakdown.

I max - determined from the permissible power dissipation.

The operating point of the zener diode is usually selected in the middle of the working branch of the zener diode. As the current decreases, the operating point shifts to the region where the differential resistance of the zener diode increases, which leads to deterioration of stabilization. With a significant change in the stabilization current, the stabilization voltage Ust changes little.

The main parameters of the zener diode (nominal values) are - U st - stabilization voltage, I st - stabilization current and r diff - differential resistance.

The lower the differential resistance, the higher the quality of the zener diode. For real zener diodes, the stabilization resistance is in the range of 1 - 100 Ohms.



This is a relative change in the stabilization voltage DU st / U st to the absolute change in temperature DT. For zener diodes, TKN can be greater or less than zero. Typically, low-voltage tunnel diodes have a negative TKN, while higher-voltage avalanche diodes have a positive TKN. The dependence of TKN on stabilization voltage is shown in the figure.

The presence of negative and positive TKN in zener diodes makes it possible to carry out thermal compensation and the total TKN in this case is significantly less. In particular, you can connect an additional diode in series with the zener diode, whose TKN is negative, or you can choose two zener diodes with the same TKN, but with different signs. In this case, the circuit of two zener diodes will be more stable and the stabilization voltage will change little when the ambient temperature changes.

PARAMETRIC VOLTAGE STABILIZER

where E is an unstabilized power source;

R b - ballast resistance;

R n - load resistance;

I n - load current;

I st - stabilization current;

VD - zener diode included in reverse direction.

According to Kirchhoff's second law:

Suppose that as a result of external factors the voltage of the power supply has changed to DE, then

Obviously, the expression in the denominator is always greater than one, i.e. the voltage at the output of a parametric stabilizer is significantly less than the change in voltage at the input. In order to reduce DU st, you need to reduce r st and increase R b. As R b increases, most of the power source voltage will drop across the ballast resistance R b and to maintain the stabilization voltage in a given range, it will be necessary to increase the power source voltage. In addition, the useful power of the source will also drop at the ballast resistance.

It is desirable that no more than 2 V drop across the ballast resistance Rb.

RF DIODES

Typically, radio engineering devices (detector, frequency converter, frequency mixer) use RF diodes. RF diodes differ from rectifier diodes by the small capacitance of the pn junction.

Typically, RF diodes use a point p-n junction, which has a small p-n junction area and, therefore, a small p-n junction capacitance, but also small currents through the p-n junction and low reverse voltage.



Point diodes are obtained as follows. They take an n-type semiconductor crystal, a metal needle, at the tip of which there is an acceptor impurity. A powerful current pulse of short duration is passed through the needle and crystal. A p-n junction is formed at the point of contact. The capacitance of RF diodes lies in the range C = 1 - 10 pF. The smaller the capacitance of the p-n junction, the higher frequency range RF diode operation.

PULSE DIODES





In modern digital pulse devices Pulse diodes are widely used. They belong to the class of RF diodes, but time restrictions are introduced for them. The input signal for them is a rectangular pulse, which has a very wide signal spectrum.

At moment t 1 the voltage changes sign. In this case, a sharp jump in the reverse current "I d" is observed. In the interval from t 1 to t 2, the current drops to I o - the reverse current of the diode.

treset is called the recovery time of the reverse resistance of the p-n junction, i.e. - this is the time of resorption of minority charge carriers in the base of the diode.



When switched back on, the time t rise is affected by the barrier capacitance, which is charged to the value of the reverse voltage. The current in the capacitance leads the voltage by 90 o. As the barrier capacitance is charged, the current in the capacitance decreases according to an exponential law and at time t 2 the current takes on a steady value I arr = I o.

t east "(0.1 - 1) µs - for pulse diodes.

The capacitance of the p-n junction for pulsed diodes is units of pF.

If the input pulse has a long duration t U , then the recovery time t rest is short. If t U is small, then t rise increases.

In the case of direct connection of the diode at the time of arrival of a single current pulse t 1, the current voltage on the diode reaches the maximum value U max, and then drops to a steady value equal to the single level U 1.

Then t mouth = t 1 - t 2 - time of establishment of forward voltage.

This happens because the base is high-resistance and drops across the diode maximum voltage. As charge carriers are injected from the emitter into the base, the base resistance drops, the potential barrier decreases, and this leads to a voltage drop to a steady-state value equal to U 1 .

t mouth - determined by voltage from U max to a value of 1.2 from the unit level U 1. Typically t mouth is on the order of units of microseconds.

Thus, the main parameters of a pulse diode are: I max imp pr, U arr. add. (1 - 100V), C, t east, t set.

MESADIODES



In integrated technology, it is convenient to obtain a mesadiode, which belongs to the pulse diodes and is capable of operating with very short pulses.

They are obtained as follows. An n-type substrate is taken and an acceptor impurity is introduced by diffusion or sputtering, thereby creating a p-type region. Next, mesadiodes with a small pn junction area are created using mechanical processing or etching. The plate is then cut.

The parameters of mesadiodes are the same as those of pulsed diodes, i.e. I max imp pr, U arr. add. , C, t east, t mouth.

TUNNEL DIODE



If there is a high concentration of impurities in a semiconductor, this leads to bending of the energy bands. In this case, a tunnel appears through which charge carriers move from the valence band to the conduction band.

If no external voltage is applied to the tunnel diode, then the total current through the p-n junction is zero.



The section from O to A is a section of a pronounced tunnel effect (up to approximately 0.2 V). Section AB, with an increase in voltage greater than U 1, the energy zones are even more bent, which leads to a decrease in the tunnel effect current (U 2 is approximately equal to 0.4 - 0.6 V).

With a further increase in voltage (section BC), the diffusion process begins, as in a conventional diode.

Section AB is a negative differential resistance, which makes it possible to use the tunnel effect in amplifier circuits, electronic generators and pulse switching devices (multivibrator, trigger, etc.), but the power of such diodes is usually low.

Parameters: I max /I min »5, I max i.e. , I min i.e. , - r, U 1(max i.e.) , U 2(min i.e.) , DU - change in voltage during direct connection, when the maximum current of the tunnel effect becomes equal to the diffusion current.

VARICAPE





A varicap is a semiconductor diode with controlled capacitance. To describe the operation of a varicap, the capacitance-voltage characteristic is used, i.e. dependence of capacitance on applied voltage.

The characteristic is nonlinear and only part of it is used when the diode is turned back on. As the reverse voltage U reverse decreases, the capacitance increases, i.e. in a varicap a barrier capacitance is used.



Varicap parameters: C max, C max /C min ³10.

Varicaps are used in selective devices, for example in a parallel oscillating circuit.

With section - does not allow the DC component to pass into the circuit.

By changing the voltage, we thereby change the capacitance of the varicap and, therefore, resonant frequency contour. In receivers with AFC, it is the varicap that is used.

NOTATION

D9A - high-frequency, low-power diode.

Here D - means diode, 9 - series, A - features of electrical parameters. IN in this case D9A - germanium diode.

KD220 K - silicon diode, series 220.

An analogue of this designation is 2D220. The first digit here means 1 - germanium, 2 - silicon, 3 - gallium arsenide.

BIPOLAR TRANSISTORS

A transistor is an electrical converting device with two or more p-n junctions. There are two types of transistors: n-p-n-type and p-n-p-type.



The emitter is an area with a very high concentration of charge carriers. The middle region - the base - has a different type of conductivity, the concentration of carriers in it is much less than the concentration in the emitter, i.e. as in diodes, the base is high-resistance.

The collector extracts carriers from the base under the influence of external voltage. The carrier concentration in the collector is high, but slightly lower than in the emitter.

If a voltage is applied to the transistor to the emitter junction in the forward direction, and to the collector junction in the reverse direction, with E to >>E e, then the emitter junction becomes narrower, its resistance decreases and the injection of charge carriers from the emitter to the base begins.

The collector junction is closed to the majority charge carriers, but since the electrons in the base are minority carriers, under the influence of the collector voltage E k they pass into the collector and create external circuit current Ik - collector current.

Thus, the emitter current flows in the external circuit of the emitter, which is equal to:

I e = I k + I b

Moreover, as a first approximation, we can assume that I e = I k, because The base current I b is very small. In real transistors, there are minority charge carriers in the emitter, base and collector. Therefore, a current of minority charge carriers of the collector I o, or a thermal current, flows through a closed collector junction, i.e.

I e =I k + I o



In the diagrams, transistors are designated as follows

For a transistor, it is important to know the relationship between the input current I in and the output current I out, so the current transfer coefficient is introduced. In a circuit with a common base (our example), this is a - the current transfer coefficient or the emitter current transfer coefficient.

It is equal to a = I k /I e "(0.96 - 0.999) - in real transistors, i.e. a circuit with a common base does not amplify the current because a<1.

THREE TRANSISTOR CONNECTION DIAGRAMS



Connection diagram with a common base. Here the base is the common electrode for the input and output.

I in = I e, I out = I k

U in = U eb, U out = U kb

Common emitter circuit.

I in = I b, and I out = I k

U in = U e U out = U e

Circuit with a common collector.

I in = I b, I out = I e

U in = U bk U out = U eq

The most common circuits are those with a common base and a common emitter.

VOLTAMPER CHARACTERISTICS OF THE TRANSISTOR



Let's consider a family of input and output current-voltage characteristics, although there are also transient and feedback characteristics of the current-voltage characteristic.

The input current-voltage characteristic of a transistor in a connection circuit with a common base is the dependence of the input current on the input voltage Iin = f(Uin) with Uout = const or otherwise

I e = f(U eb) at U kb = const.



This is a characteristic of an open emitter junction. The current-voltage characteristic is affected by the voltage at the collector p-n junction. The higher the voltage on it, the more to the right the input current-voltage characteristic of the transistor shifts. This occurs as a result of modulation of the base thickness. If the base decreases in thickness, then its resistance increases.

The output current-voltage characteristic is the dependence of the output current on the output voltage Iout = f(Uout) at Iin = const. The family of output characteristics are the characteristics of a closed collector p-n junction.

Here Iko is the thermal collector saturation current.

As the input current increases, the output current increases proportionally (I e4 > I e3 > I e2 > I e1 > 0). The output collector current is practically independent of the output voltage U kb.

The range of voltage values ​​at U kb kb = 0 collector current in the output circuit is due to the presence of the electric field of the high-resistance base, the potential difference of which is similar to the potential difference of the previously considered p-n junction.

SCHEME PARAMETERS WITH A COMMON BASE



at U KB = const. r e - differential resistance of the emitter junction.

Base diffusion resistance

Volume resistance of the base (depends on the concentration of carriers in the base)



at I e = const. r k is the differential resistance of the collector junction.



This is the voltage feedback factor.

Note that the feedback ratio is the ratio of the input voltage to the output voltage. The ratio of the output voltage to the input voltage is the forward transfer factor (or gain?)

at U kb = const - this is the coefficient of direct current transfer.



EQUIVALENT ELECTRICAL DIAGRAM OF A TRANSISTOR

Usually the equivalent circuit is used on alternating current. Here C e is the diffusion capacitance of the emitter p-n junction; it is usually neglected.

mU kb - equivalent current (voltage?) generator.

mU kb = U eb

B’ is the internal point of the base.

r b = r’ b + r” b

m = (10 -3 - 10 -5) - therefore, in real transistors it is neglected.

The output circuit includes rk, barrier capacitance Ck and an equivalent current generator aI e = Ik. The barrier capacitance Ck cannot be neglected, because The resistance of the collector junction r to is high. As a result, the equivalent electrical circuit of the transistor is simplified.

The parameters r e, r b, r k, C k are given in reference books.

VOLTAMPER CHARACTERISTICS OF A COMMON EMITTER CIRCUIT

Input characteristics are the dependences of the base current on the voltage between the base and emitter I b = f (U b e) at U k e = const. These are the characteristics of an open pn junction.

At a voltage of less than 0.3 V, a reverse current I o flows in the base circuit. With increasing voltage between the collector and emitter Uke, the characteristic shifts to the left, i.e. value of the specified input current appears at a lower base-emitter voltage U be, because part of the voltage Uke is also applied to the emitter junction.

Output current-voltage characteristics are the dependence of the output collector current on the output voltage, i.e. in this case I c = f(U b e) at a constant input base current I b = const. These are the characteristics of a closed collector p-n junction.



I brass is the through saturation current in a circuit with a common emitter. This is zero through collector current, it flows through the entire transistor.

As the input current increases, the output current also increases (I b4 > I b3 > I b2 > I b1 >0). Moreover, the greater the input current, the greater the dependence of the collector current Ik on the output voltage Uke.



As for the parameters of the equivalent circuit, it is important to know the relationship between the input and output currents. By analogy with a common base circuit, we can imagine the following equivalent circuit. Here the parameters r b, r e, r k, C k are the same as in the scheme with a common base, but this scheme not convenient, because there is no connection between the input current I b and the output current I c. You can write

I e = I k + I b From the scheme with a common base I k = aI e + I k, we substitute the previous one into this expression, then

I k = aI e + aI b + I k and from here we get

And a is the current transfer coefficient in a circuit with a common base, then

b is the flux transfer coefficient in a circuit with a common emitter.