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Modeling >> Semiconductor Devices >> RF modeling
https://designers-guide.org/forum/YaBB.pl?num=1189734954

Message started by modelinger on Sep 13^th, 2007, 6:55pm

Title: RF modeling
Post by modelinger on Sep 13^th, 2007, 6:55pm

follows are the my understanding of rf modeling and some questions.
can sombody tell me whether these are right.

a RF mosfet model can be divided into an intrinsic part, representing mainly the active part of the device, and
an extrinsic part responsible for most parasitic element.

The intrinsic part is often a core model such as BSIM3v3 which characterizes the I-V and C-V of device.

The parasitic elements( such as gate resistance, substrate resistances, Cgs,Cgd ) are extracted under some operationg points.
The values of these parasitic elements are sensitive to bais and frequency.

but in most articles, the authors often only represent the parasitic elements under one operation point, why?
when i model the parasitic elements, should i divide the capacitance into intrinsic capacitance and parasitic capacitance?

or i just use parasitic capacitances under RF model?

am i clear?

thank you!

Title: Re: RF modeling
Post by Geoffrey_Coram on Sep 14^th, 2007, 6:44am

I would have thought Cgd and Cgs were modeled mainly by the core device.

I also don't think that substrate resistance is bias-dependent, at least not strongly. Also, the substrate contacts are usually carefully placed close to the device to reduce any frequency dependence.

Title: Re: RF modeling
Post by 337see733 on Sep 17^th, 2007, 11:28pm

Main reason why most ppl represent the parasitics with one operating point:

Limitation in the existing compact model. The existing compact model is not developed with RF applications in mind, but rather for DC and low frequency AC applications. To really incorporate the RF effects, both "intrinsic" and "extrinsic" are to be modeled to change with respect to frequency. However, this is likely to lead to very complicated compact model. Therefore, as engineer who usually want to solve the problem, short-cut route is taken. It is assumed that the parasitics are independent of bias voltages (of course it is unlikely, you can think some of the lumped elements are actually voltage dependence resistance, capacitance and inductance, but you may not know how it is to relates to all the 4-terminals..!).

I assumed you don't have the liberty to change the intrinsic model. However, do check the capacitances for the intrinsic device. It is usually difficult to model the short channel capacitance, as usually the shortest lengths are used for a technology. If you still want to fit the S-parameters, what you can do is to use the external elements to either enhance/compensate the accuracy. In anyway, it is a challenging job..haha

GOOD LUCK!