MOS capacitance(Cgs probolem)

[fuxinmingming]

Hi,

When i am simulating the dc operating point of a single MOS(drain connected to Vdd=3V, and Vgs=1.0542V), I print the dc operating point so that I can see the capacitance of Cgs. The print list has Cgs and Csg.
Cgs=-72.949f,
Csg=-43.405f.
My question is:
1. Why Cgs is not equal to Csg?
2. Why Cgs is -72.949f and not 72.949f?

Thanks

 

[pavel_adameyko]

Hello
if you want to find out gate-source capacitance you should better to do it through AC analysis.

 

[ayo_oc]

Hello
if you want to find out gate-source capacitance you should better to do it through AC analysis.

But why don't you suggest how to do it in AC analysis? The point of people asking questions is to get an answer, not useless hints!

 

[pavel_adameyko]

But why don't you suggest how to do it in AC analysis? The point of people asking questions is to get an answer, not useless hints!

The schematic of test circuit

doing AC analysis

in calculator type the following

output waveform is the lower curve on picture (red), the green curve is when we set AC magnitude of V1 source equal to 0 - so that's the sum of gate-source and gate-drain capacitances

Cgs=61.12 fF, Cgd= 68.98-61.12 = 7.86 fF
Regards.

 

[ayo_oc]

thanks a lot!

---------- Post added at 12:24 ---------- Previous post was at 12:17 ----------

Please can you explain what you typed into the calculator? And do you think your method is sufficient to compute for example Cgs/micron width that can be used in quick hand analysis?

 

[pavel_adameyko]

Please can you explain what you typed into the calculator? And do you think your method is sufficient to compute for example Cgs/micron width that can be used in quick hand analysis?

i=j*w*C*Vac=j*2*pi*f*C*Vac, so if Vac =1 then C=i/(2*pi*f)
that's what i typed in calculator
regarding second question - why not? i think this method is accurate enough, even more accurate than you need for hand calculations - that's my opinion.

 

[fuxinmingming]

i=j*w*C*Vac=j*2*pi*f*C*Vac, so if Vac =1 then C=i/(2*pi*f)
that's what i typed in calculator

Hi pavel,

If I want to get Cds, I plot IF("/V1/MINUS")/xval(IF("/V1/MINUS"))/6.28, however it gives me wrong answer.

Why is that ?

Regards

 

[pavel_adameyko]

Hi,
show pls me equivalent model of nmos, parameters of which you want to extract.
Do you want to know also Cgb, Cdb, Csb? or these values are unimportant to you?

regards.

 

[nus_lin]

that depends on how the bsim/EKV model is build. Cgs/Csg in model does not exactly equal to Cgs in a real mosfet.

 

[happystar]

Hi,
show pls me equivalent model of nmos, parameters of which you want to extract.
Do you want to know also Cgb, Cdb, Csb? or these values are unimportant to you?

regards.

Hi, How to extract other parameters? Thanks!