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Simulators >> Circuit Simulators >> Does overlap capacitance Cgdovl factor into the Cgd and Cgsovl factor into Cgs?
https://designers-guide.org/forum/YaBB.pl?num=1273462471

Message started by casual on May 9^th, 2010, 8:34pm

Title: Does overlap capacitance Cgdovl factor into the Cgd and Cgsovl factor into Cgs?
Post by casual on May 9^th, 2010, 8:34pm

Using spectre simulator to extract capacitance like Cgs, Cdg, Cdd, etc, by using OP command.

Does overlap capacitance Cgdovl factor into the Cgd and Cgsovl factor into Cgs?

bsim3v3 model.

Title: Re: Does overlap capacitance Cgdovl factor into the Cgd and Cgsovl factor into Cgs?
Post by Geoffrey_Coram on May 10^th, 2010, 3:54am

Did you try reading the manual? Or setting up a simple experiment?

I just set up a simple experiment, setting cgso=0.3, and I saw that the op-pt value cgs got large, meaning it is included. There are other op-pt values (cgsbi, cgdbi) that claim to be intrinsic parts.

Title: Re: Does overlap capacitance Cgdovl factor into the Cgd and Cgsovl factor into Cgs?
Post by casual on May 11^th, 2010, 7:06am

I believe you use HSPICE to extract it. But I do not have HSPICE!
I only can use spectre.
It does not have cgsbi, cgdbi in the OP list

Now i suspect Cgs does not include overlap cap since it is small .. u could see my another thread in http://www.designers-guide.org/Forum/YaBB.pl?num=1273302986/7#7

Could you do a comparison with my Cgs value? see the link above.
Thank you!

Title: Re: Does overlap capacitance Cgdovl factor into the Cgd and Cgsovl factor into Cgs?
Post by Geoffrey_Coram on May 17^th, 2010, 5:21am

I don't see a modelcard in the other thread, so I can't do a comparison.

If you have the modelcard,
model nmos bsim3v3 ...

then make a copy
model nmos_alt bsim3v3 ...

and change the value of CGSO. Then see if CGS changes.

What version of Spectre are you using?

Title: Re: Does overlap capacitance Cgdovl factor into the Cgd and Cgsovl factor into Cgs?
Post by casual on May 17^th, 2010, 5:33am

I am using IC5141.
Spectre from MMSIM71

I am not sure I could post the transistor bsim3v3 model...

Title: Re: Does overlap capacitance Cgdovl factor into the Cgd and Cgsovl factor into Cgs?
Post by Geoffrey_Coram on May 18^th, 2010, 8:14am

I don't need to you to post it. I want to you look at it and see if there is a CGSO value specified.

If so, then change that value by a factor of 1000, and see what happens to the operating-point information that is reported.

Title: Re: Does overlap capacitance Cgdovl factor into the Cgd and Cgsovl factor into Cgs?
Post by casual on Jun 2^nd, 2010, 10:36pm

i hacked cgso bsim3v3 in the model file.
But the cgs remains the same.
so it is not included?

Title: Re: Does overlap capacitance Cgdovl factor into the Cgd and Cgsovl factor into Cgs?
Post by casual on Jun 2^nd, 2010, 11:01pm

again, i obtained the cgsovl value from OP after hacking the cgso

I found that hacking cgso in the model file does not change the cgsovl & Cgs value at all!! (i am sure that i hack the correct model file and session)

it puzzles me!

Title: Re: Does overlap capacitance Cgdovl factor into the Cgd and Cgsovl factor into Cgs?
Post by casual on Jun 2^nd, 2010, 11:05pm

After reading chapter4 from http://www-device.eecs.berkeley.edu/~bsim3/get.html

it states that:

Gate Overlap Charge
In the above expressions, if CGS0 and CGD0 (the overlap capacitances
between the gate and the heavily doped source/drain regions, respectively)
are not given, they are calculated according to the following
CGS0 = (DLC*Cox) - CGS1 (if DLC is given and DLC > 0)

---------------------------------------
I have checked the model file, it does contain DLC & Cgsl parameters.
How do i know the simulator will use which one?? the given cgso or the calculated cgso from dlc & cgsl..

Title: Re: Does overlap capacitance Cgdovl factor into the Cgd and Cgsovl factor into Cgs?
Post by Geoffrey_Coram on Jun 4^th, 2010, 3:52am

Your quote from the BSIM3 manual implicitly states that if CGSO is given, then that specified value will be used.

I wonder, though: are you specifying cgs0 or cgso ? I noticed "CGS1" instead of "CGSL" in your calculation, and then when I copied&pasted your "CGS0" I could tell that it was a zero instead of the letter O.

Title: Re: Does overlap capacitance Cgdovl factor into the Cgd and Cgsovl factor into Cgs?
Post by casual on Jun 11^th, 2010, 6:20am

checked. It is really CGSl (L) in my foundry library. While in the Bsim3v3 document it is CGS1 (one)

Still puzzles me... Whether the library got problem or not...
since it gave me the fixed value after hacking