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Modeling >> Semiconductor Devices >> Overlap Capacitance
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Message started by aaron_do on Feb 6^th, 2007, 10:53pm

Title: Overlap Capacitance
Post by aaron_do on Feb 6^th, 2007, 10:53pm

Hi all,

i'm trying to calculate the gate drain capacitance in the subthreshold region for a nmos. I'm using the BSIM3.2 model, and i figured for a good approximation I only need the Gate Drain overlap capacitance. This was mentioned in the book by T.H. Lee. Is this correct?

Ok part 2. In the manual it says Cij=dQi/dVj. i.e. Cgd=dQd/dVg and Cgd is reciprocal. However, the equation given is Qd versus Vgd. Can I say Cgd=dQd/dVgd?

OK assuming I do the differentiation, I get an equation in terms of Vgd. So should I then sub in the DC value?

thanks for the help,
Aaron

Title: Re: Overlap Capacitance
Post by Geoffrey_Coram on Feb 7^th, 2007, 7:04am

aaron_do wrote on Feb 6^th, 2007, 10:53pm:

i'm trying to calculate the gate drain capacitance in the subthreshold region for a nmos. I'm using the BSIM3.2 model, and i figured for a good approximation I only need the Gate Drain overlap capacitance. This was mentioned in the book by T.H. Lee. Is this correct?

In subthreshold, Qd for the channel should be zero, so yes, that's a good approximation.

Quote:

Ok part 2. In the manual it says Cij=dQi/dVj. i.e. Cgd=dQd/dVg and Cgd is reciprocal. However, the equation given is Qd versus Vgd. Can I say Cgd=dQd/dVgd?

You mixed up your formula; Cgd = dQg/dVd. Cgd for the channel is not reciprocal. Is Cgdov only a function of Vgd? Is the equation for Qd for the channel charge or only the overlap charge?

Title: Re: Overlap Capacitance
Post by aaron_do on Feb 7^th, 2007, 5:54pm

thanks for your reply.

I rechecked the BSIM3.2 manual, and it definately says Cgd is reciprocal. In the manual, Cgs is a function of Wactive (approximately the width), CGD0, CGD1, CKAPPA and another constant (equal to .02). I think CGD0 and CGD1 are dependant on the overlap length and Cox.

By the way, why wouldn't a capacitance be reciprocal? Is it not based on a physical model? I'm totally new to modeling and completely confused, so thanks for any help,

Aaron

Title: Re: Overlap Capacitance
Post by 337see733 on Feb 7^th, 2007, 7:57pm

Quote:

I rechecked the BSIM3.2 manual, and it definately says Cgd is reciprocal

TRUE only at Vds=0 or Vd=Vs.

Also,

Cgd,ov = dQg,ov/dVd, should not be dVgd.

But, I'm not sure whether there is anyway to calculate Qg,ov(overlap Qg) in BSIM ?

Title: Re: Overlap Capacitance
Post by aaron_do on Feb 7^th, 2007, 9:12pm

I see. thanks. I think i need to just pick up a good book...My knowledge on this topic is too basic.

thanks Anyway.
Aaron

Title: Re: Overlap Capacitance
Post by Geoffrey_Coram on Feb 8^th, 2007, 11:18am

aaron_do wrote on Feb 7^th, 2007, 5:54pm:

By the way, why wouldn't a capacitance be reciprocal? Is it not based on a physical model? I'm totally new to modeling and completely confused, so thanks for any help,

For a MOS, there are 4 terminals, and you get non-reciprocality from idealizations like charge-sheet approximation and the partitioning of the channel charge.