Solid–State Devices and Applications PDF
Preview Solid–State Devices and Applications
Solid-State Devices and Applications Rhys Lewis B.Sc.Tech., C. Eng., M.I.E.E. Lecturer in Electronics, Llandaff Technical College, Cardiff London Newnes-Butterworths THE BUTTERWORTH GROUP ENGLAND BUTTERWORTH & CO. (PUBLISHERS) LTD. LONDON: 88 Kingsway, WC2B 6AB AUSTRALIA BUTTERWORTH & CO. (AUSTRALIA) LTD. SYDNEY: 20 Loftus Street MELBOURNE: 343 Little Collins Street BRISBANE: 240 Queen Street CANADA BUTTERWORTH & CO. (CANADA) LTD. TORONTO: 14 Curity Avenue, 374 NEW ZEALAND BUTTERWORTH & CO. (NEW ZEALAND) LTD. WELLINGTON: 49/51 Ballance Street AUCKLAND: 35 High Street SOUTH AFRICA BUTTERWORTH & CO. (SOUTH AFRICA) (PTY.) LTD. DURBAN: 33/35 Beach Grove First published in 1971 by Newnes-Butterworths, an imprint of the Butterworth Group © Butterworth & Co. (Publishers) Ltd., 1971 ISBN 0 408 00050 3 Standard 0 408 00051 1 Limp Filmset and printed in England by Page Bros. (Norwich) Ltd. Preface The very broad implications of a title such as Solid-State Devices and Applications make it essential that the scope and purpose of such a text should be described rather more closely. The book is presented specifically for technicians, students who are about to enter or have already entered the final stages of courses such as the City and Guilds of London Institute Electrical Technicians Course No. 57, or the Higher National Certificate in Electronics, or those who have completed these courses in recent years and are endeavouring to maintain their level of knowledge to that demanded by a technology of ever-increasing complexity and innovation. In these days of ultra-specialisation, a work of this length, chosen, it should be said, in the best interests of economy, presentation and convenience of handling, cannot be and is not intended to be a specialist work. It is primarily an introduction to solid-state theory, devices and applications, and aims to present, in as concise a manner as possible, a summary of all the major solid-state devices available at the present time, their theory, manufacture and main applications. Where possible, the book deals with the two main types of transistor, bipolar and unipolar, side by side and includes where relevant the integrated-circuit forms. The book falls logically into three sections. The first part summarises the fundamental points of semiconductor theory and describes solid-state devices under four headings, diodes, bipolar transistors, unipolar transistors and integrated circuits. Part two is devoted to the applications of these devices commencing with a discussion of basic design considerations relevant to all applications and continuing under five sections to describe a.f. and i.f. amplifiers, wideband, d.c. and operational amplifiers, sinusoidal oscillators and nonlinear applications such as mixers and converters, logic-circuit applications and finally power and heavy-current applications. A point of interest in the logic-circuit presentation is that multivibrators are considered from a logic gate viewpoint rather than the usual discrete component approach. Part three is devoted to relevant theory and presents three chapters. The first gives a comprehensive treatment of all the equivalent circuits available and how they are derived from the general four-terminal network approach. A detailed analysis of bipolar amplifiers using hybrid parameters and of unipolar amplifiers using conductance parameters is included here. The remaining chapters consider in detail the essentials of Boolean algebra and manipulation and minimisation techniques using Veitch diagrams and de Morgan's theorems. A considerable part of the text is devoted to the theory and application of logic circuits since their use is no longer confined merely to computers. The arrangement of the text in this manner considerably improves its scope since students and others at earlier technician or craft levels will still be able to make maximum use of the first two parts, going on to the more academic theory as and when they desire. The assistance of the following firms in the provision of data for this text is gratefully acknowledged: EMI Electronics Ltd. Emihus Microcomponents Ltd. Erie Electronics Ltd. International Rectifier Co. Ltd. Marconi-Elliot Microelectronics Ltd. Mitsubishi Electric Corporation Motorola Semiconductors Ltd. Mullard Ltd. National Semiconductor Corporation Newmarket Transistors Ltd. Plessey Microelectronics Raytheon Company RCA Electric Components SDS (Portsmouth) Ltd. Texas Instruments Ltd. I am particularly grateful for the specialist assistance provided by International Rectifier, the National Semiconductor Corporation and Mullard Ltd. All the photographs are included by courtesy of Mullard Ltd. On a more personal note I would like to thank Mrs. V. White for her painstaking efforts in the typing and preparation of the manuscript and last, but by no means least, my wife for her patience and encouragement during the many long hours devoted to the preparation of the text. Rhys Lewis Llandaff October 1970 CHAPTER 1 Semiconductor Fundamentals 1.1 Introduction The most widely used semiconductor materials are germanium and silicon. Semiconductors are characterised by a resistivity lying between those of conductors and insulators and a negative tempera ture coefficient of resistance. A typical value of resistivity of a semiconductor material is about 10 times that of a conductor, such 7 as copper, and about 10~ times that of an insulator, such as glass 13 or porcelain. The negative temperature coefficient of resistance means that as the temperature is increased the conductivity of the semiconductor increases. This is the opposite effect to that of conductors. Another notable difference between conductors and semiconduc tors lies in the nature of the electric current which flows within the material when a potential difference is applied to them Whereas current in a conductor consists of electric charge being carried by one kind of carrier, the electron, conduction in a semiconductor utilises two types of carrier, the electron and the hole. The concept of the hole is unique to semiconductors and is described in more detail below. 1.2 Fundamentals of atomic theory At the time of writing, the best simplified approach to an apprecia tion of the mechanics of semiconductor action is via the familiar model of the atom developed from the original ideas of Niels Bohr (1913). In this the atom is represented by a heavy nucleus made up of two main particles, neutrons and protons, surrounded by electrons 1 2 SEMICONDUCTOR FUNDAMENTALS orbiting at discrete distances in defined orbits or 'shells'. Though we now know that the behaviour of an electron is not in fact com pletely described by comparison to a solid particle and that in fact the electron exhibits a dual nature of both particle and wave motion, the original analogy is still the best approach to a first understanding It applies to the majority of semiconductor phenomena and only falls down in certain cases, such as, for example, the 'tunnelling' effect described in Chapter 2 The shells or orbits described above are defined by energy levels and it is found that electrons may have energy only in certain definite amounts and may thus only exist in certain defined orbits around the central nucleus. The basic physical and chemical behavi our of all materials is determined by the number of orbits and the number of electrons contained within the orbits. An electron may leave one orbit and enter another, if there is room, provided that its energy level changes. An electron must be given energy for it to move to an orbit further out from the nucleus, or indeed leave the atom altogether, and must give up some of its energy if it is to move to an orbit closer to the nucleus. The freeing of electrons to take part in charge carrying through a vacuum in a thermionic valve is made possible by the provision of additional energy, in the first Conduction band Valence band (a) (b) (c) Fig. 1.1 Energy-band diagrams (a) Insulator (b) Semiconductor (c) Conductor instance, being provided by the heat energy of the heater filament In solid-state devices the electrons are freed from the atoms but do not leave the material altogether, as in valves, and the conduction process takes place within the solid; hence the general name. The energy levels which an electron may have may be grouped together in bands which are named according to the function which the electrons in the respective bands carry out within the material Thus we have the valence band containing energy levels of those electrons which, by virtue of their atomic position and energy, are capable of linking with neighbouring atoms and thus take part in SEMICONDUCTOR FUNDAMENTALS 3 chemical or physical reactions, and secondly the conduction band containing energy levels of those electrons which, again because of their atomic position and energy, are capable of leaving the atom and transferring electric charge through the material. The two bands are separated for materials classified as insulators and semicon ductors by a band which contains energy levels that electrons cannot occupy. This band is the 'forbidden' band, and its width, being a measure of the number of discrete energy levels it contains, determines the ease with which conduction may be set up within the material The conduction and valence bands of a conductor overlap and there is no forbidden band Typical energy-band dia grams are shown in Fig. 1.1 for the three categories of material. Consider the band diagram of an insulator. In order to provide electrons for conduction they must be transferred from the valence band to the conduction band, that is, they must be given sufficient energy for their energy level to be raised from that in the valence band to that in the conduction band, since an electron within an insulator cannot occupy an energy level within the forbidden band, owing to the basic atomic structure of such a material If an amount of energy is given to a valence electron sufficient to raise its energy level to one of those in the forbidden band, the electron falls back to the valence band and the extra energy is given up. This explains why in order to break down an insulator a potential gradient of several million volts per metre is required. It will be recalled that the volt is a special name given to the 'joule per coulomb'; i.e. a unit in terms of energy per unit charge. The forbidden band in a semiconductor is very much smaller and thus less energy is required per electron for conduction to take place. In a conductor there is no distinguishable difference between energy levels of valence and conduction electrons and they may be freely interchanged Thus the energy supplied is entirely available for carrier movement since none of it is required to free the electrons from their parent atoms. A pure or intrinsic semiconductor has a resistivity at room tem perature such that the current flowing through a 1 cm cube with 1 V applied might be of the order of 0Ό2 A. Addition of impurities as described below effectively reduces the forbidden-band width and much lower resistivities are possible. A semiconductor which has been doped in this manner is called extrinsic. 1.3 Crystal structures and doping Both germanium and silicon are crystalline in nature and are tetravalent. This means that each atom has four valence electrons 4 SEMICONDUCTOR FUNDAMENTALS capable of linking with neighbouring atoms. A two-dimensional equivalent diagram of a section through a crystal plane is shown in Fig 1.2 (a). It must be remembered that this is a two-dimensional diagram and that in practice the atoms shown as lying in one plane Semiconductor atoms Ö—Q—Ö Parent atom nucleus Electron ^—s~^—Parent atom T TT nucleus Electron (b) (a) Fig. 1.2 (a) Crystal structure (b) Valence-electron sharing are, in fact, lying in many planes, thus making it possible for the tetravalence to hold good The valence electrons link with neigh bouring atoms to form strong covalent bands. To help visualise a covalent bond Fig 1.2 (b) is included; this approach is crude and possibly not very accurate from a purist point of view, but it is an Semiconductor atoms o—o—o 0—0—0 -Pentavalent impurity o—Φ-^-ο ©DDoonnoo r electron Hole ] o—0—6 O—O <!> (a) (b) Fig. 1.3. Doping effective aid to understanding. The figure shows that neighbouring atoms share valence electrons and any particular atom is strongly held by four bonds within the crystal lattice. The intrinsic crystal is electrically neutral since each atom nucleus carries an equal and opposite positive charge to the negative charge carried by the valence electrons. SEMICONDUCTOR FUNDAMENTALS 5 The doping of a semiconductor crystal with a pentavalent impurity such as arsenic or aluminium is shown in Fig. 1.3 (a) The impurity atom shown has five valence electrons available for bonding. Four of these form covalent bonds with neighbouring semiconductor atoms and the fifth one is bonded only to the parent atom. The additional electron is more readily available for conduction since its bond is not as strong as those linked to the semiconductor atoms. Atoms such as the one shown are called donor atoms since an additional carrier is donated to the crystal. The material itself is called n-type material. Note that the n-type crystal is still electrically neutral since the impurity atom carries an equal and opposite charge on its nucleus to that carried by its valence electrons. Methods and levels of impurity doping are considered below. If a trivalent impurity such as indium or boron is introduced a rather different situation is set up, as shown in Fig 1.3 (b). The three valence electrons of the impurity atom bond with the adjacent semiconductor atoms but the fourth covalent bond is incomplete leaving an electron vacancy or hole. This hole would capture an electron should one be available and so for all intents and purposes behaves as a positive charge. Further, the hole is mobile since if an electron from a neighbouring covalent bond should leave its bond and transfer to the hole, the bond it leaves now has a vacancy and so the hole has effectively moved to the adjacent bond It should be noted that the electrons which cause hole transference are valence electrons moving within the atoms, from one atom to the one adjacent They are not conduction electrons, which actually leave the atom This explains why hole conduction which is detectable by a method described below takes place at a different rate to electron conduction Electron conduction is due to electrons which have energy levels within the conduction band Hole conduction is due to electrons having lower energy levels within the valency band and moving only from one atom to the next Holes move at about half the speed of conduction electrons through a semiconductor when a p.d. is applied. Trivalent impurity atoms are called acceptor atoms since they readily accept electrons. Semiconductor material doped with acceptor impurity is called p-type material. The phenomenon of hole conduction is unique to semiconductors. In a conductor, valence and conduction electrons are indistinguish able and the mechanism for setting up holes within the material is not present owing to the atomic structure of a conductor. Holes be have like positively charged particles and it is perhaps easier, when endeavouring to understand semiconductor action, to regard them as such Hole conduction as distinct from electron conduction may be detected using the Hall-effect apparatus shown in Fig 1.4. A transverse magnetic field in a direction shown across a p-type B 6 SEMICONDUCTOR FUNDAMENTALS semiconductor crystal results in a positive charge being experienced at the top surface, a phenomenon consistent with a stream of posi tive particles producing a magnetic field so as to react with the transverse field, resulting in an upward force being experienced by the charge carriers. Upper surface charge + + + .p material X XX ■ Hole current i—·—r^ " x xx XX X— -Direction of magnetic field l |i|t Fig. 1.4 Hall-effect apparatus For the semiconductor action required in solid-state diodes and other devices, the impurity concentration is very, very small, being only about one impurity atom to one hundred million semiconduc tor atoms. Greater concentrations may produce too high a con ductivity and diode and transistor action, to be described, will not occur. 1.4 Behaviour of doped materials p-type or n-type materials on their own behave as normal low- conductivity conductors, charge being effectively carried mainly by holes in p-type material and mainly by electrons in n-type material. A p.d applied across a crystal of one or the other material types results in current flow which reverses if the direction of the applied p.d is reversed The conductivity of either material is in the range indicated above and is determined by the level of impurity content and by temperature. With increase in temperature valence electrons can absorb additional energy which may raise them into the con duction band This increases the available charge carriers twofold since the original valence electrons leave behind holes in the valence band available for conduction in the manner already described.
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