1. MOSFET Modeling for RF Circuit Design Kenneth Yau MASc Candidate Department of Electrical and Computer Engineering University of Toronto Toronto, ON M54 3G4 Canada kyau@eecg.utoronto.ca December 5, 2003

2. 2 Kenneth Yau, Copyright © 2003 - MOSFET Modeling for RF Circuit Design Outline Problems in existing models Proposed solutions Conclusions

3. 3 Kenneth Yau, Copyright © 2003 - MOSFET Modeling for RF Circuit Design 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

4. 4 Kenneth Yau, Copyright © 2003 - MOSFET Modeling for RF Circuit Design 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]

5. 5 Kenneth Yau, Copyright © 2003 - MOSFET Modeling for RF Circuit Design 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 (Y 22 ) [Cheng and Enz, 2000]

6. 6 Kenneth Yau, Copyright © 2003 - MOSFET Modeling for RF Circuit Design 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

7. 7 Kenneth Yau, Copyright © 2003 - MOSFET Modeling for RF Circuit Design 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

8. 8 Kenneth Yau, Copyright © 2003 - MOSFET Modeling for RF Circuit Design 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 contacts Source: Cheng and Enz, 2000 Intrinsic transistor

9. 9 Kenneth Yau, Copyright © 2003 - MOSFET Modeling for RF Circuit Design Equivalent Subcircuit Intrinsic transistor Substrate network Source: Enz and Cheng, 2000

10. 10 Kenneth Yau, Copyright © 2003 - MOSFET Modeling for RF Circuit Design Substrate network Substrate network is layout dependent Equations given by Source: Enz and Cheng, 2000

11. 11 Kenneth Yau, Copyright © 2003 - MOSFET Modeling for RF Circuit Design 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 Source: Enz and Cheng, 2000

12. 12 Kenneth Yau, Copyright © 2003 - MOSFET Modeling for RF Circuit Design 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

13. 13 Kenneth Yau, Copyright © 2003 - MOSFET Modeling for RF Circuit Design 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