Transcript yau_MOSFETmodeling - University of Toronto

MOSFET Modeling for RF
Circuit Design
Kenneth Yau
MASc Candidate
Department of Electrical and Computer Engineering
University of Toronto
Toronto, ON M54 3G4
Canada
[email protected]
December 5, 2003
Outline
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Problems in existing models
Proposed solutions
Conclusions
Kenneth Yau, Copyright © 2003 - MOSFET Modeling for RF Circuit Design
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Problems in Existing Models
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Existing models
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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
Kenneth Yau, Copyright © 2003 - MOSFET Modeling for RF Circuit Design
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Problems in Existing Models (Cont)
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Problems
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Quasi-static (QS) models
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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
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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]
Kenneth Yau, Copyright © 2003 - MOSFET Modeling for RF Circuit Design
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Problems in Existing Models (Cont)
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Problems (cont)
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Signal substrate coupling
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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]
Kenneth Yau, Copyright © 2003 - MOSFET Modeling for RF Circuit Design
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Proposed Solutions
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Completely new RF MOSFET models
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Complicated
Derivation very involved
Will not be covered in this presentation
Subcircuit approach
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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
Kenneth Yau, Copyright © 2003 - MOSFET Modeling for RF Circuit Design
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Subcircuit Approach
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Separate MOSFET into two parts
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Intrinsic part
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Models the transistor itself
Can use existing models (e.g. BSIM3) for this part
May also use a NQS model
Extrinsic part
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Models parasitic resistances and capacitances
Also need to model substrate coupling (it could
account for 20% of output admittance)
May account for NQS operation
Kenneth Yau, Copyright © 2003 - MOSFET Modeling for RF Circuit Design
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Extrinsic Parasitic Network
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Depend on accuracy
(or frequency), can be
very simple (resistors)
or very complicated
Simple network ok up
to ~10GHz [Enz,
2000]
Depends on layout
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Number of fingers
Location of bulk
contacts
Intrinsic transistor
Source: Cheng and Enz, 2000
Kenneth Yau, Copyright © 2003 - MOSFET Modeling for RF Circuit Design
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Equivalent Subcircuit
Intrinsic transistor
Substrate network
Source: Enz and Cheng, 2000
Kenneth Yau, Copyright © 2003 - MOSFET Modeling for RF Circuit Design
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Substrate network
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Substrate network is
layout dependent
Equations given by
Ns
Nd
C jsb   C js  k
C jdb   C jd  k
Ns
1
1

Rsb k 1 Rsb  k
Nd
1
1

Rdb k 1 Rdb  k
k 1
k 1
N
f
1
1

Rdsb k 1 Rdsb  k
Source: Enz and Cheng, 2000
Kenneth Yau, Copyright © 2003 - MOSFET Modeling for RF Circuit Design
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Non-Quasi Static Effects
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Can approximate by
adding voltage
controlled current
sources in parallel
with intrinsic elements
The parameters
Ygsnqs and Ymnqs
are frequency
dependent
Source: Enz and Cheng, 2000
Kenneth Yau, Copyright © 2003 - MOSFET Modeling for RF Circuit Design
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Conclusions
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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
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References
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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
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