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Modeling >> Semiconductor Devices >> capacitive currents in MOSFET
https://designers-guide.org/forum/YaBB.pl?num=1100786435

Message started by miro masar on Nov 18^th, 2004, 6:00am

Title: capacitive currents in MOSFET
Post by miro masar on Nov 18^th, 2004, 6:00am

Hi,

I tried to examine (using spectre) to which freqency pinched MOSFET (BSIM3v3 model) acts as constant current soure. I gave constant dc bias on gate, dc bias on drain and superimposed ac voltage to drain and examined drain current.
I've expected capacitive current through drain to be w*Cdd*Vac, but it was 4-times bigger. Is the way I'm computing expected capacitive current wrong or why is there that difference?

thanks
miro

Title: Re: capacitive currents in MOSFET
Post by Mighty Mouse on Nov 18^th, 2004, 8:37am

Isn't the current equal to 2*pi*omega*C_dd*V_ac? The missing factor of 2pi would explain a factor of 6, meaning the capacitive current was actually a little less than you expected.

- MM -

Title: Re: capacitive currents in MOSFET
Post by gmork on Nov 18^th, 2004, 9:19am

Hi,

As far as i remember from circuit theory lessons (it's been soooo far away), the admitance of capacitor is w*C so current should be w*Cdd*Vac=2*pi*f*Cdd*Vac.
I think the problem could be that i shouldn't use Cdd but some different capacitance(but which?)

miro

Title: Re: capacitive currents in MOSFET
Post by Mighty Mouse on Nov 18^th, 2004, 3:54pm

Whoops! Sorry about that. Momentary brain freeze.

- MM -

Title: Re: capacitive currents in MOSFET
Post by Mighty Mouse on Nov 18^th, 2004, 4:00pm

Try looking at the posts by Andrew Beckett on MOSFET capacitances as reported by Spectre. You can find them at
http://www.designers-guide.com/Forum/?board=analog_design;action=display;num=1087411058;start=5#5

- MM -

Title: Re: capacitive currents in MOSFET
Post by gmork on Nov 18^th, 2004, 5:24pm

Thanks. As I see, I wasn't only one wich was confused about it. The correct expression for current is w*(cdd+cjd)*Vac and everything is consistent with AC simulation.

miro