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Simulators >> Circuit Simulators >> how to know "ad as pd ps" parameter?
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Message started by xwj623 on Aug 18^th, 2008, 10:53pm

Title: how to know "ad as pd ps" parameter?
Post by xwj623 on Aug 18^th, 2008, 10:53pm

Are the following two formula correct which i used to calc the four parameter used in spice?
ad(as)=W*L
pd(ps)=2(W+L)
Or what should i use?

they mazed me very much.

Title: Re: how to know "ad as pd ps" parameter?
Post by vivkr on Aug 19^th, 2008, 5:57am

Hi,

What are the W & L here? Are these the W & L of the gate? Then, your formulae are all wrong.

You cannot know the diffusion dimensions for source & drain just by looking at the schematic. This
depends on the minimum width allowed for these by the design rules.

If I consider source or drain, then 1 dimension (let's call it the length, Ldiff) equals the width of your gate.

So, Ldiff = W

As for Wdiff, this depends on the design rules as I mentioned. So you need to instantiate the layout of a unit transistor of interest, and see how wide the diffusion there is. In a 0.5 um process, this may for instance be 0.8 um.

Then, you can compute ad(as) and pd(ps) by substituting Wdiff and Ldiff in the formulae you present here.

On more advanced features:
Naturally, you need to guarantee that you are using the same diffusion width Wdiff for all transistors. The minimum is fixed by design rules, but you may make a poor layout, or have other reasons to dimension differently, and get a higher Wdiff, or not a constant Wdiff for all transistors. Moreover, if you make multi-finger transistors, then adjacent transistors share the same diffusion implant. Thus, the effective area ad(as) and pd(ps) are reduced by a factor of 2.

Regards,
Vivek

Title: Re: how to know "ad as pd ps" parameter?
Post by xwj623 on Aug 20^th, 2008, 12:07am

thanks vivkr.
as the figure attached,i have three questions:
1. as=w*Ldif , right?
2. ps=2*(w+Ldif) or ps=w+2*Ldif ? Is it inclued the gate edge?
it seems that didn't consider the gate edge when calc the
side wall cap( Csw).
3. Does diffusion length to gate which is called LD, has some influence?

Title: Re: how to know "ad as pd ps" parameter?
Post by vivkr on Aug 20^th, 2008, 6:05am

xwj623 wrote on Aug 20^th, 2008, 12:07am:

1. Correct !!!
2. I don't know either, but I don't think the gate edge is included. So, I would use ps=2*(w+Ldif). In any case, models which include the effect of the gate edge already will usually also include the ad/as/pd/ps parameters automatically.
3. I don't think so, but again, I don't know. Usually, LD is small and you should be able to ignore it.

But the basic form of equations you have ought to be enough to model everything quite accurately.

Regards,
Vivek

Title: Re: how to know "ad as pd ps" parameter?
Post by xwj623 on Aug 21^st, 2008, 12:54am

vivkr wrote on Aug 20^th, 2008, 6:05am:

xwj623 wrote on Aug 20^th, 2008, 12:07am:

ok. I understand.
Many thanks ,Vivek.

Title: Re: how to know "ad as pd ps" parameter?
Post by Geoffrey_Coram on Oct 20^th, 2008, 1:07pm

xwj623 wrote on Aug 20^th, 2008, 12:07am:

2. ps=2*(w+Ldif) or ps=w+2*Ldif ? Is it inclued the gate edge?

BSIM4 has a parameter, PERMOD, which determines how the gate edge is computed; sometimes the gate edge junction capacitance differs significantly from the isolation edge junction capacitance.