SOLID STATE ELECTRoniC DEVICES SECOND EDITION D.K. Bhattacharya Scientist Solid State Physics Laboratory New Delhi Rajnish Sharma Dean (Academics), Chitkara University, Himachal Pradesh 1 3 Oxford University Press is a department of the University of Oxford. It furthers the University’s objective of excellence in research, scholarship, and education by publishing worldwide. Oxford is a registered trade mark of Oxford University Press in the UK and in certain other countries. Published in India by Oxford University Press YMCA Library Building, 1 Jai Singh Road, New Delhi 110001, India © Oxford University Press 2007, 2013 The moral rights of the author/s have been asserted. First Edition published in 2007 Second Edition published in 2013 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, without the prior permission in writing of Oxford University Press, or as expressly permitted by law, by licence, or under terms agreed with the appropriate reprographics rights organization. Enquiries concerning reproduction outside the scope of the above should be sent to the Rights Department, Oxford University Press, at the address above. You must not circulate this work in any other form and you must impose this same condition on any acquirer. ISBN-13: 978-0-19-808457-0 ISBN-10: 0-19-808457-9 Typeset in Times New Roman by Time Digitech Private Limited, Noida Printed in India by Yash Printographics, Noida 201301 About the Authors D.K. Bhattacharya A PhD from the University of Delhi, he has over two decades of experience as a practising semiconductor scientist including a long association with the MEMS Division, Solid State Physics Laboratory, New Delhi. He has published more than 50 research papers in national and international journals and conference proceedings. He is also the author of Engineering Physics (OUP, 2010). Rajnish Sharma A PhD from Kurukshetra University and National Physical Laboratory, New Delhi, he has served BITS, Pilani as a faculty for almost six years. He has successfully handled a research project funded by the Ministry of Science and Technology, Government of India and has presented his work at the MRS Symposium held at San Francisco and at Risoe National Laboratory, Denmark. His current research interests are in the area of device technologies for next-generation ICs and circuit design at radio frequencies. Preface to the Second Edition Solid state electronic devices are devices that are built entirely from solid materials and in which electrons, or other charge carriers, are confi ned entirely within the solid material. Prior to the use of solid state devices, most electronic devices used vacuum tubes, in which electricity passed through various elements inside a heated vacuum tube. In many applications, solid state devices have replaced vacuum tubes since they last longer and are smaller, cheaper, and more effi cient and reliable. While solid state devices can be built from crystalline, polycrystalline, and amorphous solids, the building material is most often a crystalline semiconductor. Common solid state devices include transistors, microprocessor chips, and dynamic random access memory (DRAM) chips. The fi rst solid state device was the ‘cat’s whisker’ detector which was fi rst used in 1930s as radio receivers. The transistor, invented in 1947 by Bell Labs and named an IEEE Milestone in 2009, was the fi rst solid state device to come into commercial use in the 1960s. More recently, the integrated circuit (IC), the light-emitting diode (LED), and the liquid crystal display (LCD) have evolved as further examples of solid state devices. The semiconductor industry is seeing steady change, based on continued technological innovations and the requirement of reducing cost. The next big development in semiconductor manufacturing is the transition from 300 mm wafers to 450 mm wafers, which can lead to reduction in die cost and subsequently production cost. Other exciting recent developments in semiconductor industry are the extreme ultraviolet (EUV) lithography and nano-chip technology. A nano-chip can carry billions of transistors, and its applications include high-performance servers and supercomputers, virtual reality and advanced electronic games, and ultra-fast telecommunications devices. It is these types of applications that could lead to a new revolution in how electronics goods are designed and manufactured. Recognizing the importance of this industry in our everyday lives, this second edition of Solid State Electronic Devices offers an improved coverage of the fundamental concepts of solid state electronics. This edition is an attempt to incorporate most of the feedback received from educators in terms of improvement of content presented in the book. About the Book This book contains 14 chapters which provide a thorough coverage of solid state electronic devices. Starting with the fundamentals of solid state physics such as electron dynamics, growth and crystal properties of semiconductors, energy bands, and charge carriers, the book goes on to the study of p-n junctions, metal–semiconductor contacts, BJTs, and FETs (including JFET, MESFET, MOSFET, and HEMT technologies). An analysis of special devices, such as opto-electronic devices, microwave devices, and power devices, follows. Finally, the book covers ICs, MEMS, rectifi ers, and power supplies. vi Solid State Electronic Devices Each chapter has been divided into small sections that are independent in themselves. A unique feature of the book is the additional information in shaded boxes at the end of relevant sections. These inputs offer a few extra facts outside the limits of the curriculum to draw the interest and attention of students. A large number of numerical problems along with answers and hints have been provided at the end of each chapter. This is followed by a recapitulation of the salient features of the chapter. Numerous review exercises and numerical problems along with answers and hints have been provided as well. A list of references is presented at the end of the book for those interested in further reading. A special attempt has been made to include topics that are a part of courses offered by a large cross-section of educational institutions. New to the Second Edition ∑ New sections such as reciprocal lattice, band structure modifi cation, electrons and holes in quantum wells, Early effect, short channel MOSFET I-V characteristics, and photoluminescence and electroluminescence ∑ Detailed explanation and coverage ∑ New illustrations Extended Chapter Material Chapter 1 Applications of cathode ray tubes (CRTs) have been added. Chapter 2 Reciprocal lattice, diffraction due to crystal planes, fl oatzone (FZ) method of crystal growth, and four-probe method for the measurement of conductivity of semiconductors are explained. Chapter 3 The concept of phonon has been introduced and band structure modifi ca- tion in semiconductors has been described. Chapter 4 Deep impurity levels, Auger recombination process, and gradient in quasi-Fermi levels have been described. Chapter 7 Formation of practical ohmic contacts has been explained and a new section on quantum confi nement of carriers has been introduced. Chapter 8 Input and output characteristics of BJTs, Early effect in BJTs, thermal runaway and thermal stability in BJTs, Kirk effect, and Webster effect have been described. Chapter 9 Signifi cantly expanded by introducing sections on short channel effect, control of threshold voltage, substrate bias effects, sub-threshold characteristics, equivalent circuits for MOSFET, MOSFET scaling and hot electron effects, drain- induced barrier lowering, short channel and narrow width effect, gate-induced drain leakage, and comparison of BJTs with MOSFETs. Chapter 10 The phenomenon of photoluminescence has been explained. Chapter 11 Two power semiconductor devices have been introduced – Gate Turn-off Thyristor (GTO) and Insulated-Gate Bipolar Transistor (IGBT), and also the formula to calculate intrinsic stand-off ratio has been included. Preface to the second edition vii Chapter 12 The process of photolithography, different etching techniques such as wet etching and dry etching, and Moore’s law have been described. Coverage Chapter-wise details of content coverage are as follows: Chapter 1 presents important aspects of electron dynamics, including the analysis of motion of charged particles under the infl uence of electric and magnetic fi elds. The last section of this chapter discusses the cathode ray tube in detail. Chapter 2 presents the salient properties of semiconductor materials with a special emphasis on the crystalline forms of these materials. Important concepts used for classifying and defi ning crystal lattices have been outlined in this chapter. Bulk and epitaxial growth techniques have also been included. Chapter 3 introduces semiconductor physics, including energy bands and charge carriers. E-k diagrams have been discussed in detail as they form the basis of understanding the operation of many solid state electronic devices. It also presents a comprehensive treatment of the two most important physical processes, namely drift and diffusion of charge carriers. To make the treatment complete, a section has been added to include graded impurity profi les. Chapter 4 offers details about the nature and behaviour of excess carriers created by external stimuli. A combination of drift and diffusion also governs the lives of excess carriers. It also presents the very important concept of the continuity equation that forms the core of any useful device physics model. Chapter 5 initiates the study of solid state devices with the p-n junction. Beginning with a brief discussion about the different methods used for fabricating p-n junctions, the chapter also presents forward- and reverse-biased junctions. Chapter 6 discusses the small-signal model of a p-n junction. An important aspect of p-n junctions is the way it behaves when the polarity of the applied bias is suddenly changed. A thorough treatment of transients that are critical to many applications is provided in this chapter. Chapter 7 discusses metal–semiconductor contacts or junctions, including ohmic and Schottky contacts. Chapter 8 discusses the bipolar junction transistor, without which the stupendous growth in the fi eld of solid state electronics would not have been possible. The treatment includes different models of the device along with an explanation of their relative merits and demerits. Ordinary transistors cannot be operated at very high frequencies. Methodologies involved in the design of high-frequency transistors are presented in this chapter. Chapter 9 focuses on the fi eld effect transistor which is a key ingredient for the development of IC technology. Since the MOS structure forms the basis of this device, the chapter also includes important features of this structure. Chapter 10 describes opto-electronic devices, such as photovoltaic cells, photo- detectors, LEDs, and laser diodes, which fi nd a variety of applications in the fi eld viii Solid State Electronic Devices of light wave communication. It also includes the basic principle of operation and important semiconductor structures involved. Chapter 11 develops a focused understanding of the special measures required to enable solid state devices to handle high power, through the discussion of some important aspects pertaining to power devices. Some special devices, such as semiconductor-controlled rectifi ers (SCR) and unijunction transistors (UJT) are also presented. Chapter 12 discusses how the individual devices discussed up to Chapter 11 are put together using standard processes to build widely used ICs. The three most important technologies— namely MOSFET, MESFET, and bipolar technologies—are discussed along with their salient features. This chapter also includes a discussion on micro- electromechanical systems. Chapter 13 focuses on some important microwave devices such as Gunn, IMPATT, TRAPATT, and BARITT diodes. This chapter attempts to present the basic physics of these devices so that students can understand their operation at microwave frequencies. Chapter 14 discusses some basic circuits such as rectifi ers, fi lters, and regulators that combine to form power supplies, which are one of the most important application areas of solid state devices. This is because all modern electronic systems need power supplies to operate. The last section of this chapter deals with switched mode power supply (SMPS), which is being increasingly used in various electronic systems. Authors would be grateful for further suggestions and feedback with regard to this edition. D.K. Bhattacharya Rajnish Sharma Symbols F Force tp0 Minority carrier hole lifetime a Acceleration s Surface recombination velocity m Mass V B uilt-in voltage bi W Work done C Depletion region capacitance dep V Voltage p Hole concentration in n-type n m Rest mass semiconductor 0 B Magnetic fi eld n Electron concentration in p-type p r Radius semiconductor e Electronic charge L Diffusion length of electrons n p Pitch L Diffusion length of holes 1 p D Defl ection s Conductivity S D efl ection sensitivity for electric g Conductance e defl ection c D iffusion capacitance d S Defl ection sensitivity for r S pecifi c contact resistance m C magnetic defl ection I C ollector current C p Pressure I Base current B k Segregation coeffi cient I Emitter current s E y Wave function b Common-emitter current gain m Effective mass A Voltage gain eff V n Equilibrium electron R D iffusion resistance 0 e concentration C Emitter junction capacitance je p Equilibrium hole concentration C Parasitic capacitance between 0 p n Intrinsic carrier concentration base and emitter i f(F) Fermi–dirac distribution function v Electron saturation velocity S N Effective density of states in R Collection-region series resistance c C conduction band C Collection-to-substrate S m * Density of states effective mass capacitance n N Effective density of states in C B-c junction capacitance V BC valence band f Cut-off frequency T N Donor concentration HBT Heterojunction bipolar transistor d N Acceptor concentration f Potential barrier for electron a n m Mobility injection V Hall voltage f Potential barrier for hole injection H p R Hall coeffi cient V Collector supply voltage H CC D Diffusion coeffi cient of electrons V Base supply voltage n BB D Diffusion coeffi cient of holes t Delay time of transistor p D G Generation rate t Rise time of transistor R R Recombination rate t Storage time of transistor S E Band gap L Channel length of JFET g d Excess electron concentration W Channel width of JFET n d Excess hole concentration 2d Channel depth of JFET p E Trap energy V Gate voltage with respect to t GS tn0 Minority carrier electron lifetime source for JFET xxii Solid State Electronic Devices V Drain voltage with respect to V Potential drop across the DS oxide(0) source for JFET oxide for zero gate voltage R R esistance f Surface potential for zero s0 r R esistivity applied gate voltage A A rea f Work function difference ms N D onor concentration V Voltage across oxide d ox W Width of depletion region Q Charge per unit area in the d S I D rain current semiconductor D V Drain–source voltage at r Charge density in DS(sat) S saturation semiconductor e Dielectric constant of Q Charge per unit area in metal S M semiconductor E Electric fi eld across oxide ox V V oltage t Oxide thickness ox I P inch-off current e Dielectric constant of oxide P ox V Pinch-off voltage C Total capacitance P g D rain conductance C O xide capacitance D ox V T hreshold voltage C Depletion-layer capacitance T J HEMT High electron mobility C Minimum capacitance min transistor of MOS under strong JFET J unction fi eld-effect transistor inversion MODFET Modulation doped fi eld-effect V Flat band voltage FB transistor Q Charge per unit area within o MOS Metal–oxide–semiconductor oxide E Conduction band edge E Electric fi eld within oxide C 0 E I ntrinsic fermi energy level Q I nterface-trapped charge Fi it E Fermi energy level Q Fixed oxide charge F f E Valence band edge Q O xide-trapped charge v ot f P otential Q Mobile ionic charge m f Surface potential Z C hannel width s N A cceptor concentration g ¢ C hannel conductance a D fpF Difference between EFi MOSFET M etal–oxide–semiconductor and EF for a p-type fi eld-effect transistor semiconductor E Energy W Maximum space-charge n F requency dT width h P lanck’s constant fnF Difference between EFi C Velocity of light and EF for an n-type Eg Energy band gap. semiconductor In P hoton fl ux with frequency n f ¢ M odifi ed work function of a A bsorption coeffi cient m metal g¢ Generation rate of EHP due f Work function of metal to photons m c E lectron affi nity of I P hotocurrent L semiconductor I Short-circuit current SC c ¢ M odifi ed electron affi nity of V Open-circuit current OC semiconductor P Power Symbols xxiii V Voltage across solar cell under V Base-to-base voltage m BB maximum power condition V Peak-point voltage P I Current through solar cell V V alley-point voltage m v under maximum power G Conductance condition R Sheet resistance  P Maximum power MEMS M icro-electromechanical m P Incident optical power system in h Conversion effi ciency of DRIE Deep reactive ion etching solar cell LIGA LI (Röntgen lIthographie), FF F ill factor G (Galvanik), AM A ir mass A (Abformung). R Series resistance SMPS Switched mode power S DE Conduction band step supply c DE Valence band step V Peak voltage v m s Conductivity R Load resistor L G Photo generation rate of v Load voltage L L excess carriers I Rms current rms v Drift velocity P A c power d ac t Electron transit time h C onversion effi ciency n G P hotoconductor gain PRV Peak reverse voltage ph t Transit time g R ipple factor t f Modulating frequency T T ime period m q C ritical angle w Angular frequency C n Refractive index I Zener diode current Z PT M aximum rated power Vi I nput voltage R On-state resistance V Output voltage ON o R Channel contribution to P M aximum power CH max resistance t On time ON R D rain contact resistance RFI Radio frequency D R Thermal resistance interference th R Thermal resistance between IMPATT I mpact-avalanche th(d-p) device and package transit-time R Thermal resistance between TRAPATT T rapped-plasma avalanche- th(p-s) package and heat sink triggered-transit R Thermal resistance between BARITT Barrier-injected transit-time th(s-a) heat sink and ambient m Mobility in central valley G P Maximum power dissipation v Avalanche-zone velocity D(max) Z T Device temperature MSM M etal–semiconductor–metal dev T Maximum junction j(max) temperature