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Modeling >> Semiconductor Devices >> Negative value of Cgs in MOS transient op. point!
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Message started by vakilia on May 20^th, 2003, 12:52pm

Title: Negative value of Cgs in MOS transient op. point!
Post by vakilia on May 20^th, 2003, 12:52pm

Hi;
I am using BISIM models in cadence and I need to count on the values of the capacitaces like Cgs and Cbs that show up in the transient operating point utput file . But I figured that some of those capacitances are negative that I don't have any idea why they are negative. Could you explain me why they are negative and if we could use their absolute values for our hand calculation stuff?

Thank you
-Babak

Title: Re: Negative value of Cgs in MOS transient op. poi
Post by Andrew Beckett on May 27^th, 2003, 9:58pm

Here's a note I wrote on this a few years back. You
can use the mapping table at the bottom to get
the "traditional" capacitances which you can then use
in hand calculations.

Quote:

The values you are seeing are correct because the values of these
parameters are the partial derivatives of the terminal charges with
respect to the terminal voltages, so they can be negative. They are NOT
physical two terminal capacitors. See the UCB bsim3v3 documentation,
equations 4.3.28 - 4.3.31.

An earlier spectre implementation only gave the values of three
capacitors which were closer to the intuitive value of what the
capacitance should be, but many customers asked for the complete set of
partial derivatives, so that is what spectre currently provides.

Here is a summary of the differences in the capacitance operating point
figures between 4.4 and 4.4.1 BSIM3V3 models in spectre.
---------------------------------------------------------------

In the following section Cxx refers to the parameter calculated by
the bsim evaluator inside the code. The lower case cxx refers to the
value reported by the operating point.

In 4.4:

Code:

cgs = -Csg + pModel->OverlapCgs * pInst->Width * pInst->MFactor
cgd = -Cdg + pModel->OverlapCgd * pInst->Width * pInst->MFactor
cgb = -Cbg + pModel->OverlapCgb * pInst->Length * pInst->MFactor
cbd = Cjd
cbs = Cjs

In 4.4.1:

Code:

Cgsovl = pInst->pSDModel->OverlapCgs * pInst->MFactor;
Cgdovl = pInst->pSDModel->OverlapCgd * pInst->MFactor;
Cgbovl = pInst->pSDModel->OverlapCgb * pInst->MFactor;

cgs = Cgs - Cgsovl
csg = Csg - Cgsovl
cgd = Cgd - Cgdovl
cdg = Cdg - Cgdovl
cbd = Cbd
cbs = Cbs
cjd = Cjd
cjs = Cjs

Here's a mapping between the parameter name in 4.4, and what it
corresponds to in 4.4.1:

4.4 4.4.1
------------

Code:

cgs = -csg
cgd = -cdg
cgb = -cbd
cbd = cjd
cbs = cjs

The parameter names are those reported by the operating point in the two
versions.

Title: Re: Negative value of Cgs in MOS transient op. poi
Post by vakilia on May 30^th, 2003, 12:46pm

Andrew;
Thanks for your helpful comments.
I have another question also. If we look at the model parameters, we see Cgg, Cdd and Css as well.
Are these values sum of all device capacitances connected to each correcpanding node even considering the miller effects of other capacitances between nodes? I mean for Cdd , is it like Cdb pluse miller effect of Cgd and is it the value of the cap. that we should consider connected to the drain node comming from device (apart from the load capacitance or so)?
I apprecite if you could kindly point this out as well.

Thanks
-Babak

Title: Re: Negative value of Cgs in MOS transient op. poi
Post by Andrew Beckett on Jun 2^nd, 2003, 12:31am

Babak,

I'm not really sure, to be honest. The parameters are all partial derivatives,
so that means cdd is dQd/dVd (those d's should be Greek deltas). Quite
what that means in practical terms, I'm not sure...

Andrew.

Title: Re: Negative value of Cgs in MOS transient op. poi
Post by Mighty Mouse on Jun 2^nd, 2003, 7:40am

Cgg is dQ_g/dV_g, so it is the capacitance you see looking into the gate if all of the terminals are connected to voltage sources. The same is true for the others. So there is no Miller effect.

-MM-