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This book is concerned with the physics of electronic devices, processes of device operation and techniques for modelling devices. Developed to serve as a textbook on solid state devices, microelectronics and optoelectronics, it provides an integrated approach to the subject by including modern developments in device design, VLSI and microelectronics along with device physics that form a part of traditional courses.

Angsuman Sarkar is an assistant professor in the Department of Electronics and Communication Engineering, Kalyani Government Engineering College, West Bengal. He received his MTech degree in VLSI and microelectronics from Jadavpur University, Kolkata, and is now working towards his PhD from the same university. His current research interest is in the study of short channel effects of sub 100 nm MOSFETs and nano device modelling. His publications include research papers in refereed journals and conference proceedings and several textbooks.

Chandan Kumar Sarkar is a professor in the Department of Electronics and Telecommunication Engineering in Jadavpur University, Kolkata. He obtained his MSc degree in Physics from Aligarh Muslim University, Aligarh, his PhD from Calcutta University (1979) and his DPhil from Oxford University, Oxford, UK (1984). Professor Sarkar was a Postdoctoral Fellow supported by the Royal Commission for the Exhibition of 1851 at Clarendon Laboratory of Oxford University. He was also Junior Research Fellow ofWolfson College, Oxford University. He is an active researcher in the area of semiconductor devices and nanoelectronics. He has published a number of research papers in refereed journals and conference proceedings. He has also authored several textbooks and guided many PhD students in electronics engineering. Professor Sarkar is presently IEEE EDS distinguished lecturer and also Chair of the IEEE EDS chapter, Calcutta section, India. He has been Visiting Professor in many universities abroad.

Preface xvii
1 Physics of Semiconductors 1
1.1 Introduction 1
1.2 Recapitulation from Previous Studies 1
1.2.1 Atomic bonding 1
1.2.2 Covalent bonds 2
1.2.3 Concept of holes 2
1.2.4 Intrinsic and extrinsic semiconductors 3
1.2.5 Elemental and compound semiconductors 4
1.2.6 Significance of the symbols n+, n, n-, p, p+, p- 4
1.2.7 Summary of the recapitulations 4
1.3 Crystal Structure 5
1.3.1 Various types of solids 5
1.3.2 Structure of a crystal 7
1.3.3 Basic crystal structures 8
1.3.4 Lattice point calculation 11
1.3.5 Structure of silicon and GaAs 11
1.3.6 Index system for crystal planes (crystallographic notations) 15
1.3.7 Crystal direction 17
1.4 Introduction to Atoms and Electrons 19
1.4.1 Journey from the classical model to quantum numbers 19
1.4.2 Limitations of classical physics 22
1.4.3 Quantum mechanics 24
1.5 Band Formation theory of Semiconductors 40
1.5.1 Band formation in silicon 45
1.5.2 Semiconductors, insulators and metals 46
1.5.3 Band gap energy 47
1.5.4 Band structure in compound semiconductors 48
1.6 E–k Diagram 48
1.6.1 Concept and theory of E–k diagram 48
1.6.2 Drift current due to movement of electrons 51
1.6.3 Concept of holes, negative effective mass concept for holes and
current due to holes 53
1.6.4 Direct band gap and indirect band gap semiconductors 56
1.7 Transport of Carriers 58
1.7.1 Drift 60
1.7.2 Diffusion 62
1.7.3 Diffusion and drift of carriers: Built in or induced field and the
Einstein relation 64
1.7.4 Pair generation in semiconductors 65
1.7.5 Recombination process and life time of carriers 66
1.7.6 Excess carriers and the significance of life time 67
1.8 Carrier Concentrations and Introduction to Fermi Levels 67
1.8.1 Different distribution laws 68
1.8.2 Fermi-Dirac distribution 68
1.8.3 Metals and insulators with respect to the Fermi–Dirac distribution 69
1.8.4 Fermi-Dirac distribution for semiconductors 70
1.8.5 Electrons and hole concentrations at equilibrium 71
1.8.6 Ionisation energy 77
1.8.7 Degenerate semiconductors 78
1.9 Mobility and Scattering 79
1.9.1 Drift velocity and carrier mobility 82
1.9.2 Different types of scattering 83
1.9.3 High field effects and velocity saturation 84
1.10 Excess Carriers 84
1.10.1 Injection of excess carriers 84
1.10.2 Quasi Fermi level 86
1.10.3 Continuity equation 87
1.10.4 Steady-state carrier injection and diffusion length 90
1.11 Appendix 91
1.11.1 Gauss’s law 91
1.11.2 Poisson’s equation 91
1.11.3 Hall effect 92
1.11.4 Density of states in an energy band 94

2 Diodes 121
2.1 Introduction 121
2.2 p-n Junction Under Zero Bias (Unbiased) Conditions 122
2.2.1 Formation of depletion region 126
2.2.2 Contact potential 127
2.2.3 EquilibriumFermi levels 131
2.2.4 Expression for electric field and space–charge width 132
2.3 Forward and Reverse Bias 135
2.3.1 Forward bias 135
2.3.2 Reverse bias 138
2.3.3 Drift and diffusion currents in the biased diode 140
2.4 Current Calculation in p-n Junction 140
2.4.1 Assumptions for deriving the current expression in a p-n junction 145
2.4.2 Minority and majority currents in the p-n diode 145
2.4.3 Static and dynamic resistance 149
2.5 Applications of p-n Diodes 149
2.6 Reverse Bias Breakdown 150
2.6.1 Avalanche breakdown 151
2.6.2 Zener breakdown 153
2.6.3 Differences between avalanche and Zener diodes 157
2.7 Tunnel Diode 157
2.7.1 I–V characteristics of tunnel diode 157
2.8 Capacitance of p-n Junctions 159
2.8.1 Junction capacitance 159
2.8.2 Varactor diode 160
2.8.3 Diffusion capacitance 161
2.8.4 One-sided junction 162
2.8.5 Graded junction 163
2.9 Switching Characteristics of a Semiconductor Diode 164
2.9.1 The turn off transient 165
2.9.2 Switching diode 167
2.9.3 Rectifier diode 167
2.10 Metal–semiconductor Contacts 169
2.10.1 Comparison of Schottky and p-n diodes 172
2.10.2 Ohmic contacts 174
2.11 Photovoltaic Effect 176
2.12 Solar Cell 179

3 Bipolar Junction Transistors 204
3.1 Introduction 204
3.1.1 Three terminal device and the general concept of a control input terminal205
3.2 Simplified Structure and Modes of Operation 206
3.2.1 Regimes of operation 207
3.3 Band Diagram of a Transistor 208
3.4 Various Current Components in an n-p-n BJT 211
3.5 Bipolar Transistor: A Conceptual Picture 213
3.6 Transistor Action 217
3.7 Operation of the n-p-n Transistor in the Active Mode 218
3.8 How a BJT Provides Amplification 223
3.8.1 Minority carrier profile and band diagram in different modes 225
3.9 Equivalent Circuit Model of the Forward Active Mode 226
3.10 Models of Reverse Active Mode BJT 227
3.11 Combining Models of Forward Active and Reverse Active: Ebers–Moll Model 229
3.11.1 First use of Ebers–Moll model: Current in forward active mode 230
3.11.2 Second use of Ebers–Moll model: Current in the saturation mode 231
3.12 Load Line and Modes of Operation 232
3.13 Early Effect or Base Width Modulation 233
3.14 Common Emitter Characteristics and Common Emitter Current Gain 235
3.15 Saturation Voltage and Saturation Resistance 237
3.16 Common Base Characteristics 239
3.17 The Collector Saturation Current and Transistor Breakdown 241
3.17.1 Avalanche multiplication breakdown 241
3.17.2 Breakdown due to punch-through 245
3.18 BJT Functioning as an Amplifier and a Switch 246
3.18.1 Large signal operation 246
3.18.2 Amplifier gain 248
3.18.3 Operation as a switch 249
3.19 Large Signal Model 250
3.19.1 Small signal operation and models 251
3.19.2 Concept of transconductance 251
3.19.3 Small signal collector current and transconductance 253
3.19.4 Small signal base current and input resistance at the base 253
3.19.5 Small signal emitter current and the input resistance at the emitter 254
3.19.6 Small signal voltage gain 255
3.20 Hybrid _ Model 255
3.20.1 Inclusion of early effect in the Hybrid _ model 256
3.21 h(hybrid) Parameter Model 257
3.22 Kirk Effect 260
3.23 Collector Current Fall off at Low and High Currents 261
3.24 Future Trends in BJT Design 261

4 Junction Field Effect Transistors (JFETs) 287
4.1 Introduction 287
4.2 Gate Isolation 287
4.3 Structure of JFET 288
4.3.1 Basic JFET operation 289
4.4 The Working Principle of JFET Explained with
Equations 292
4.5 Ideal dc Current–voltage Relationship 296
4.6 Comparison Between JFET and BJT 296
4.7 Parameters of JFET 297

5 Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) 308
5.1 Introduction 308
5.2 Basic Operation 311
5.2.1 Operation without gate bias 311
5.2.2 Operation with a positive gate bias 312
5.2.3 Operation with a small VDS 314
5.2.4 Operation as VDS is increased 314
5.3 MOS Capacitor 316
5.3.1 Accumulation 317
5.3.2 Depletion 319
5.3.3 Inversion 320
5.3.4 Detailed analysis of depletion 320
5.3.5 Detailed analysis of inversion 324
5.4 Flat Band Voltage: Effect of Real Surfaces 328
5.4.1 Equalisation of the Fermi levels 328
5.4.2 Charges in the oxide 329
5.4.3 Interface traps 329
5.4.4 Flat-band voltage 330
5.5 Expression of Threshold Voltage 331
5.6 Capacitance–Voltage Characteristics of the
MOS Structure 333
5.7 I-V Characteristics of a MOSFET 334
5.8 Transconductance (gm) 342
5.9 pMOS and its I-V Characteristics 344
5.10 Aspect Ratio and its Implication 345
5.11 Channel Length Modulation 347
5.12 Substrate Bias Effects 350
5.12.1 Effect of substrate bias on threshold voltage 351
5.12.2 Influence of substrate bias on device characteristics 353
5.13 Large Signal Model of a MOSFET 353
5.14 Small Signal Equivalent Circuit Model 355
5.15 MOS and VLSI 357
5.15.1 SSI, MSI, LSI and VLSI 359
5.16 MOS Inverter 360
5.16.1 Resistive load inverter 361
5.16.2 CMOS inverter 362
5.16.3 Appendix 366

6 Charge Coupled Devices 390
6.1 Introduction 390
6.2 Photoelectric Effect: Vasis of CCD Operation 392
6.3 Dynamic Effects in MOS Capacitors 392
6.4 Surface-channel CCD (SCCD) and Buried-channel CCD (BCCD) 393
6.5 Structure of a CCD 395
6.6 Electric Field in CCD 395
6.7 Charge Generation in a CCD 398
6.8 Working Principle of CCD 398
6.9 Fabrication of CCD 400
6.10 An Analogy (Bucket Brigade) to Explain CCD Operation 400
6.11 Movement of Charge Packet in a CCD 402
6.12 Thick Front-side Illuminated CCD 407
6.13 Thinned Back-side Illuminated CCD 408
6.14 Other CCD Structures 408
6.15 How do CCDs Record Colour? 409
6.16 CCDs as Imaging Devices 409

7 Elements of Fabrication Technology 414
7.1 Introduction 414
7.2 Making a Wafer Base: From Crystal Growth to Wafer Preparation 416
7.3 Changing Layer Composition or Doping Methods 418
7.3.1 Ion implantation 418
7.3.2 Diffusion 419
7.4 Adding a Layer or Deposition 420
7.4.1 Epitaxial deposition 420
7.4.2 Chemical vapour deposition (CVD) 420
7.4.3 Oxide growth or oxidation 422
7.4.4 Metallisation by “sputtering” 423
7.4.5 “Metallisation” by evaporation 423
7.5 Removing a Layer or Etching 424
7.6 Photolithography or Pattern Transfer 424
7.7 Electrical Probing and Die Separation 428
7.8 Example Fabrication to Create Hole/block 428
7.9 nMOS Fabrication Process Steps 430
7.10 CMOS Fabrication Process 430
7.10.1 CMOS fabrication process by n-well on p-substrate 432
7.10.2 CMOS fabrication process by p-well on n-substrate 435
7.11 Resistor Within an IC 437
7.12 Capacitor Within an IC 438
7.13 Diode Within an IC 439
7.14 BJT Fabrication 440

8 Sub-micron MOSFETs 450
8.1 Introduction 450
8.2 Scaling 450
8.2.1 Benefits of scaling 451
8.2.2 Short channel effects: Result of aggressive scaling 451
8.2.3 Types of scaling 452
8.3 Short channel effects 458
8.3.1 Reduction of effective threshold voltage 459
8.3.2 Hot electron effects 462
8.3.3 Avalanche breakdown and parasitic bipolar action 464
8.3.4 DIBL (drain induced barrier lowering) 465
8.3.5 Channel length modulation 466
8.3.6 Punch-through 467
8.3.7 Ballistic transport 467
8.3.8 Velocity saturation 468
8.3.9 Mobility degradation 471
8.3.10 Sub-threshold conduction 473
8.3.11 Output impedance variation with drain–source voltage 477
8.3.12 Summary of scaling and short channel effects 478
8.4 VLSI Device Structures 479
8.4.1 Gate stack and gate material 479
8.4.2 Source–drain structures 479
8.4.3 Channel doping structure 481
8.4.4 Reverse short-channel effect (RSCE) 481
8.5 Silicon on Insulator (SOI) MOSFET 483
8.6 Introduction to high-k MOSFETs 484
8.7 Introduction to SiGe or strained Si MOSFET for higher mobility 486
8.8 Double-gate (DG) MOSFET 487
8.9 FinFET 488
8.10 Trends and limiting factors for the scaling of MOSFETs (beyond 100 nm) 490

9 Heterostructure Semiconductor Devices 499
9.1 Introduction 499
9.1.1 General properties of heterostructures 500
9.1.2 Different design options for heterostructures 501
9.2 Band Diagram 502
9.2.1 Steps for drawing band diagram 504
9.3 Quantum Well 505
9.3.1 Quantum confinement in MOSFET 507
9.4 Modulation Doping 507
9.4.1 Benefits of modulation doping 510
9.5 HEMT (high electron mobility transistor) 510
9.5.1 Structure 512
9.6 GaAs/AlGaAs HBTs 513
9.7 Si–Ge Heterostructures 516

10 Power Electronic Devices 520
10.1 Introduction 520
10.2 Difference Between Power and Linear Electronics 521
10.3 Power Bipolar Junction Transistor 522
10.3.1 Construction of a power BJT 522
10.3.2 Power BJT characteristics 523
10.4 Darlington Pair Configuration 527
10.5 Power MOSFET 528
10.5.1 Constructional features of a power MOSFET 529
10.5.2 Principle of operation 531
10.5.3 Parasitic BJT in a MOSFET cell 533
10.5.4 Safe operating area (SOA) 534
10.6 Silicon Controlled Rectifier (SCR) or Thyristor 535
10.6.1 Basic operating principle of a thyristor 536
10.6.2 Triggering the SCR 538
10.6.3 Different methods to turn on a SCR 539
10.6.4 SCR turn off 539
10.6.5 Current voltage characteristics of SCR 540
10.6.6 Energy band diagrams for the p-n-p-n diode 541
10.6.7 Hole flow and electron flow in a p-n-p-n diode 541
10.7 DIAC 543
10.8 TRIAC 543
10.9 Insulated Gate Bipolar Transistor (IGBT) 545
10.9.1 Constructional features of an IGBT 545
10.9.2 Principle of working 547
10.9.3 Steady state characteristics of an IGBT 548

11 Negative Resistance Devices 559
11.1 Importance and use of negative resistance devices 559
11.2 Gunn Diode 560
11.2.1 Gunn effect 561
11.2.2 Two valley mode theory 562
11.2.3 Structure and fabrication of GaAs Gunn diodes 568
11.3 Read Diodes 569
11.4 IMPATT Diode 569
11.4.1 IMPATT diode structure 570
11.4.2 IMPATT diode operation 571
11.4.3 Advantages and disadvantages of IMPATT diode 573
11.5 Uni-junction Transistors (UJTs) 574
11.6 Example Problems 575

12 MEMS 580
12.1 Introduction 580
12.2 What is Micro-engineering? 581
12.3 Definition of MEMS 581
12.4 How MEMS Work 582
12.4.1 Micro-sensors technology 582
12.4.2 Micro-actuator technology 584
12.5 Importance of MEMS 585
12.6 MEMS Markets 586
12.7 MEMS Applications 586
12.8 MEMS and ICs: Two of a Kind 587
12.9 Materials used in MEMS 588
12.10Micro-machining 589
12.10.1 Basic fabrication process 589
12.10.2 Bulk micro-machining 590
12.10.3 Surface micro-machining 594

13 Optoelectronic Devices 599
13.1 Introduction 599
13.2 Optical Processes 600
13.2.1 Transitions 600
13.2.2 Direct and indirect band gap semiconductor 601
13.2.3 Semiconductors suitable for optoelectronics 602
13.2.4 Intrinsic band-to-band generation-recombination processes 604
13.2.5 Electron–hole recombination 605
13.2.6 Photon generation 609
13.2.7 Heat generation 609
13.2.8 Photoemission in a p-n diode 610
13.2.9 Spontaneous emission: Electroluminescence 611
13.3 Requirements of An Optical Source 612
13.4 Light Emitting Diode 612
13.4.1 Processes in LED 614
13.4.2 Power efficiency of LED 615
13.4.3 Basic theory of a hetero-junction LED 615
13.4.4 Different LED structures 617
13.4.5 Advantages, disadvantages and applications of LED 617
13.5 LASER 617
13.5.1 Two requirements for lasing action and optical gain 618
13.5.2 Basic components and the role of feedback 618
13.5.3 Basic steps required to produce laser beam 620
13.5.4 Einstein relationships for stimulated emission 623
13.5.5 Relationship between stimulated and spontaneous emission 624
13.5.6 Advantages of lasers 625
13.5.7 Population inversion 625
13.5.8 Stimulated emission in a p-n junction 625
13.5.9 The salient points about laser action 626
13.5.10 Operational parameter of lasers 627
13.6 Photodetector 627
13.6.1 p-n junction photodiode 629
13.6.2 V–I characteristics of a p-n junction photodiode 629
13.6.3 Disadvantage of p-n junction photodiode 630
13.6.4 p-i-n photodiode 630
13.6.5 Avalanche photodiode (APD) 632
13.7 OEIC (optoelectronic integrated circuits) 633
Index 640