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Modeling >> Semiconductor Devices >> Extracting MOSFET input and output Capacitances from PDK
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Message started by naumz on Jun 25^th, 2009, 5:29am

Title: Extracting MOSFET input and output Capacitances from PDK
Post by naumz on Jun 25^th, 2009, 5:29am

Hi,

Is there a way to extract MOSFET Input (Gate) and Output (drain - bulk) capacitances from the PDK model file? Just the first-order values. I need these values for scripting purposes.

I appreciate any help. Thanks.

Title: Re: Extracting MOSFET input and output Capacitances from PDK
Post by pancho_hideboo on Jun 25^th, 2009, 6:38am

I don't know what vendor's simulator you use.

However you can use DC operation imformation for MOS in any simulator.
Here you can get various capacitance informations.

Title: Re: Extracting MOSFET input and output Capacitances from PDK
Post by naumz on Jun 25^th, 2009, 10:54pm

I use Cadence Spectre for my simulations. Thanks for the information. Additionally, is there a way to obtain the required parameters from the spice definition of the MOSFET, without performing an actual simulation? My purpose is to extract these parameters in bulk in an automated manner.

Title: Re: Extracting MOSFET input and output Capacitances from PDK
Post by pancho_hideboo on Jun 26^th, 2009, 5:38am

naumz wrote on Jun 25^th, 2009, 10:54pm:

Additionally, is there a way to obtain the required parameters from the spice definition of the MOSFET, without performing an actual simulation?

What model of MOSFET do you use ?
BSIM3? BSIM4? PSP? HiSIM?

Title: Re: Extracting MOSFET input and output Capacitances from PDK
Post by Geoffrey_Coram on Jun 26^th, 2009, 5:42am

It might be a little tricky. I think you can get a 0th order approximation for the gate capacitance from COX W L, where COX = EPSOX / TOX. For the drain-bulk capacitance, you need to look at CJ, CJSW, CJSWG, multiplied by some function of W and the diffusion distance HDIF. However, when I look at the model card from a foundry for TOX, I see
.model nch.1 NMOS ... TOX = 'toxn' ... HDIF = 'hdifn' ... CJ = cjn
where toxn, hdif, and cjn are set differently for the different corners.

Title: Re: Extracting MOSFET input and output Capacitances from PDK
Post by naumz on Jun 26^th, 2009, 6:57am

@Geoffrey
Yes, that's the kind of approximation I'm liking at. I remember reading in a textbook that for modelling losses, the gate capacitance can be approximated as a linear capacitance (only to the first order of course) as 2/3 WLC_ox. Thanks for confirming it. However I did not get a reference for the drain cap. I guess it should be different for corners and center and I was looking for a similar equation to it as to the gate cap.

@pancho
I believe for a first order approximation, the model will not be important. Am I right? I was looking for a generic, if rough, expression.

Thanks.

Title: Re: Extracting MOSFET input and output Capacitances from PDK
Post by pancho_hideboo on Jun 26^th, 2009, 11:02pm

naumz wrote on Jun 26^th, 2009, 6:57am:

@pancho
I believe for a first order approximation, the model will not be important. Am I right? I was looking for a generic, if rough, expression.

A little correct.
If your purpose is very rough estimation, see MOS Level-1 Model, especially modified Meyer Capacitance model.

Title: Re: Extracting MOSFET input and output Capacitances from PDK
Post by naumz on Jun 27^th, 2009, 1:12am

@pancho
Thank you for that picture. It's exactly what I was looking for. Can you provide the path of the document that you posted the screenshot from? I assume it comes with Cadence documentation but I could not find it in my search.

Title: Re: Extracting MOSFET input and output Capacitances from PDK
Post by pancho_hideboo on Jun 27^th, 2009, 2:25am

naumz wrote on Jun 27^th, 2009, 1:12am:

Can you provide the path of the document that you posted the screenshot from?
I assume it comes with Cadence documentation but I could not find it in my search.

<Cadence_Installation_path>/doc/spectremod/spectremod.pdf

Title: Re: Extracting MOSFET input and output Capacitances from PDK
Post by naumz on Jun 27^th, 2009, 7:03am

I'm going through the manual now. Thanks for all your help! I really appreciate it.