How to choose the optimum value of m for a fixed ratio of W/L?

[lhlbluesky]

recently i met a strange question,that is
for the same transistor with aspect ratio of W/L=40u/0.8u(or some other value),if i choose W/L=40/0.8(m=1) and W/L=40/0.8(m=20,W/L=20*2/0.8),the simulation result(in spectre) is rather different,if i use other value for m,the result is different ,too.i'm really confused.
if so,how should i choose the optimum value of m for a fixed ratio of W/L?
and if the ratio is not integer,for example,4.3u/0.8u,how to decide the value of m?

please help me.
thanks.[/u]

 

[safwatonline]

a strange question

the difference is the parasitic capacitance value, using fingures make use of charged junction

 

[mbyoussef]

Re: a strange question

The stress effect would make the threshold voltage different in the two cases you were simulating

 

[Fom]

Re: a strange question

I think because of model binning Spectre uses different model for different size. You can check model parameters for each transistor using calculator.

 

[rock_zhu]

a strange question

different m have different source and drain area which reuslt in different parasitic capacitator.
m also affact Vth.
Generally we decide a unit width to get good matching.