MOSFET MODELING & BSIM3 USER’S GUIDE Yuhua Cheng Chenming Hu MOSFET MODELING & BSIM3 USER’S GUIDE This page intentionally left blank. MOSFET MODELING & BSIM3 USER’S GUIDE by Yuhua Cheng Conexant Systems, Inc. and Chenming Hu University of California, Berkeley KLUWER ACADEMIC PUBLISHERS NEW YORK, BOSTON , DORDRECHT, LONDON , MOSCOW eBook ISBN 0-306-47050-0 Print ISBN 0-792-38575-6 ©2002 Kluwer Academic Publishers New York, Boston, Dordrecht, London, Moscow All rights reserved No part of this eBook may be reproduced or transmitted in any form or by any means, electronic, mechanical, recording, or otherwise, without written consent from the Publisher Created in the United States of America Visit Kluwer Online at: http://www.kluweronline.com and Kluwer's eBookstore at: http://www.ebooks.kluweronline.com Contents Contents .................................................................................................. v Preface .................................................................................................... xiii Chapter 1 Introduction ......................................................................1 1.1 Compact MOSFET Modeling for Circuit Simulation....................................1 1.2 The Trends of Compact MOSFET Modeling.......................................................5 1.2.1 Modeling new physical effects ................................................................................5 1.2.2 High frequency (HF) analog compact models ........................................................6 1.2.3 Simulation robustness and efficiency ...............................................................7 1.2.4 Model standardization........................................................................................8 References ....................................................................................................8 Chapter 2 Significant Physical Effects In Modern MOSFETs ...13 2.1 MOSFET Classification and Operation .......................................................13 2.1.1 Strong inversion region (Vgs>Vth) ..................................................................17 2.1.2 Weak and moderate inversion or the subthreshold region..............................18 2.2 Effects Impacting the Threshold Voltage .....................................................18 2.2.1 Non-uniform doping effects ..............................................................................19 2.2.2 Normal short channel effects ...........................................................................23 vi MOSFET Modeling & BSIM3 User’s Guide 2.2.3 Reverse short channel effects ........................................................................... 23 2.2.4 Normal narrow-width effects ........................................................................... 25 2.2.5 Reverse narrow-width effects ........................................................................... 27 2.2.6 Body bias effect and bulk charge effect............................................................28 2.3 Channel Charge Theory ................................................................................ 30 2.3.1 Accumulation.................................................................................................... 33 2.3.2 Depletion .......................................................................................................... 33 2.3.3 Inversion........................................................................................................... 34 2.4 Carrier Mobility............................................................................................ 37 2.5 Velocity Saturation ....................................................................................... 39 2.6 Channel Length Modulation......................................................................... 41 2.7 Substrate Current Due to Impact Ionization .................................................44 2.8 Polysilicon Gate Depletion........................................................................... 48 2.9 Velocity Overshoot Effects........................................................................... 51 2.10 Self-heating Effect ........................................................................................ 53 2.11 Inversion Layer Quantization Effects ...........................................................55 References .................................................................................................... 57 Chapter 3 Threshold Voltage Model .............................................65 3.1 Threshold Voltage Model for Long Channel Devices .................................. 65 3.2 Threshold Voltage Model with Short Channel Effects.................................67 3.2.1 Charge sharing model...................................................................................... 68 3.2.2 Quasi 2-D models for drain induced barrier lowering effect .......................... 71 3.3 Narrow Width Effect Model.........................................................................77 3.4 Threshold Voltage Model in BSIM3v3 ........................................................ 80 3.4.1 Modeling of the vertical non-uniform doping effects ....................................... 80 3.4.2 Modeling of the RSCE due to lateral non-uniform channel doping................ 83 3.4.3 Modeling of the short channel effect due to drain induced barrier lowering ..85 3.4.4 Modeling of the narrow width effects ............................................................... 88 3.4.5 Complete Vth model in BSIM3v3 ..................................................................... 90 3.5 Helpful Hints ................................................................................................ 92 References ...................................................................................................... 101 Chapter 4 I-V Model.....................................................................105 4.1 Essential Equations Describing the I-V Characteristics .............................105 CONTENTS vii 4.2 Channel Charge Density Model .................................................................106 4.2.1 Channel charge model in the strong inversion region ...................................106 4.2.2 Channel charge model in the subthreshold region.........................................107 4.2.3 Continuous channel charge model of BSIM3v3 .............................................109 4.2.4 Continuous channel charge model with the effect of Vds .............................. 112 4.3 Mobility Model ..........................................................................................114 4.3.1 Piece-wise mobility models ............................................................................114 4.3.2 Mobility models in BSIM3v3 ..........................................................................116 4.4 I-V Model in the Strong Inversion Region ................................................. 117 4.4.1 I-V model in the linear (triode) region ...........................................................117 4.4.2 Drain voltage at current saturation, Vdsat....................................................118 4.4.3 Current and output resistance in the saturation region .................................120 4.5 Subthreshold I-V Model .............................................................................124 4.6 Single Equation I-V model of BSIM3v3 ....................................................125 4.7 Polysilicon Gate Depletion Effect ..............................................................129 4.8 Helpful Hints ..............................................................................................130 References ..................................................................................................140 Chapter 5 Capacitance Model .................................................... 143 5.1 Capacitance Components in a MOSFET ................................................... 144 5.2 Intrinsic Capacitance Model.......................................................................145 5.2.1 Meyer model...................................................................................................145 5.2.2 Shortcomings of the Meyer model.................................................................. 151 5.2.3 Charge-based capacitance model ..................................................................154 5.3 Extrinsic Capacitance Model .....................................................................161 5.4 Capacitance Model of BSIM3v3 ................................................................163 5.4.1 Long channel capacitance model (capMod=0) ............................................. 164 5.4.2 Short channel capacitance (capMod=1) ........................................................ 170 5.4.3 Single-equation short channel capacitance model (capMod=2) ...................178 5.4.4 Short channel capacitance model with quantization effect (capMod=3)...... 186 5.5 Channel Length/Width in Capacitance Model ...........................................197 5.6 Helpful Hints ..............................................................................................198 References ..................................................................................................207 Chapter 6 Substrate Current Model ................................................211 6.1 Substrate Current Generation .....................................................................211