MOSFET Modeling for RF Circuit Design Kenneth Yau MASc Candidate Department of Electrical and Computer Engineering University of Toronto Toronto, ON M54.

MOSFET Modeling for RF Circuit Design

Kenneth YauMASc Candidate

Department of Electrical and Computer EngineeringUniversity of TorontoToronto, ON M54 3G4

Canada

[email protected] 5, 2003

2Kenneth Yau, Copyright © 2003 - MOSFET Modeling for RF Circuit Design

Outline Problems in existing models Proposed solutions Conclusions


Problems in Existing Models

Existing models Consider only modern models. LEVEL 1

and 2 (square law) are almost artifacts BSIM3, most common modern MOSFET

model nowadays BSIM4, successor to BSIM3 someday? EKV, less common Concentrate mostly on BSIM3/EKV


Problems in Existing Models (Cont)

Problems Quasi-static (QS) models

Although BSIM3 has a non-quasi-static (NQS) model, it is less robust than the QS model

QS assumption can be violated at RF Parasitic passive elements

Include: gate, drain and source resistances and capacitances

Present in BSIM3 as “soft” resistances and “invisible” in AC simulations [Enz, 2000]

Introduce high frequency poles [Enz, 2000]


Problems in Existing Models (Cont)

Problems (cont) Signal substrate coupling

Not accounted for in available models Coupling of drain to source and to bulk Can account for up to 20% of output

admittance (Y22) [Cheng and Enz, 2000]


Proposed Solutions Completely new RF MOSFET models

Complicated Derivation very involved Will not be covered in this presentation

Subcircuit approach Use existing models (e.g. BSIM3) for intrinsic

MOS device Add extrinsic parasitic elements and/or

dependent sources Advantages: base on proven models and can

be implemented as a SPICE subcircuit


Subcircuit Approach Separate MOSFET into two parts

Intrinsic part Models the transistor itself Can use existing models (e.g. BSIM3) for this

part May also use a NQS model

Extrinsic part Models parasitic resistances and capacitances Also need to model substrate coupling (it could

account for 20% of output admittance) May account for NQS operation


Extrinsic Parasitic Network Depend on accuracy

(or frequency), can be very simple (resistors) or very complicated

Simple network ok up to ~10GHz [Enz, 2000]

Depends on layout Number of fingers Location of bulk

contactsSource: Cheng and Enz, 2000

Intrinsic transistor


Equivalent Subcircuit

Intrinsic transistor

Substrate network

Source: Enz and Cheng, 2000


Substrate network Substrate network

is layout dependent

Equations given by


1 1

1 1

1

1 1 1 1

1 1

s d

s d

f

N N

jsb js k jdb jd kk k

N N

k ksb sb k db db k

N

kdsb dsb k

C C C C

R R R R

R R


Non-Quasi Static Effects Can approximate

by adding voltage controlled current sources in parallel with intrinsic elements

The parameters Ygsnqs and Ymnqs are frequency dependent



Conclusions BSIM3 alone is sufficient for low

frequency analog circuit simulation Effects of parasitic elements become

important in RF Can model substrate coupling by a

passive network However, the modeling is dependent

on layout Approximate NQS effects by adding

VCCS


References Cheng, Yuhua and Christian Enz et.al. MOSFET Modeling for RF

Circuit Design, IEEE 2000 Enz, Christian, An MOS Transistor Model for RF IC Design Valid

in All Regions of Operation, IEEE Transactions on Microwave Theory and Techniques, vol.50, no.1, January 2002

Enz, Christian and Yuhua Cheng MOS Transistor Modeling for RF IC Design, IEEE Transactions on Solid-State Circuits, vol.35, no.2, February 2000

Hsiao, Chao-Chih, Ching-Wei Kuo and Yi-Jen Chan, A Modified BSIM 0.35µm MOSFET RF Large-Signal Model for Microwave Circuit Application, IEEE

Liu, William, MOSFET Models for SPICE Simulation including BSIM3v3 and BSIM4, John Wiley & Sons, Inc. 2001.

Tin, Suet Fong, Ashraf A. Osman and Kartikeya Mayaram BSIM3 MOSFET Model Accuracy for RF Circuit Simulation, IEEE 1998

MOSFET Modeling for RF Circuit Design Kenneth Yau MASc Candidate Department of Electrical and Computer Engineering University of Toronto Toronto, ON M54.

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