why I have to put AD, AS, PD, PS in H.Spice?

[samelgo]

Hi, I'm starting to study MOSFET and H.Spice.

but, I have a problem with AD,AS,PD,PS.

so, I'd like to know about AD,AS,PD,PS.

I know that AD and AS are area of Drain and Source. but, I don't know why I have to put AD,AS in H.spice.

also, I know that PD and PS are length of Drain and Source. but, I don't know why I have to put PD,PS in H.spice.

AD,AS are the area and PD,PS are legth.

I'm so curious with this. Why AD,AS are the area and PD,PS are legth?

plz notice to me.

 

[keith1200rs]

The area and perimeter of the drain/source are used to model the drain/source to bulk diodes. You can leave them out but the leakage and other effects of those diodes won't be modelled so your simulations won't be as accurate.

Keith

 

[samelgo]

sorry but one more thing..

Sorry, but I don't understand exactly.

so, AD, AS, PD, PS are just concerned about modeling?

could you give me a example which is related to AD, AS, PD, PS problem or something with it, please?

I really want to know :-(

 

[keith1200rs]

The parameters are not about modelling - they are real dimensions with real effects. In the same way that you choose a transistor with a width of say 5um and length of 0.18um, it also has a drain and source area. The drain and source create three dimensional diodes to the bulk which are reverse biased or zero biased in normal use. They do affect performance due to capacitance and leakage so their effect must be taken into account.

Keith

 

[dick_freebird]

Some SPICEs let you enter LD / LS and calculate the areas
and peripheries for you by default. But that falls apart when
you get to multifinger devices or depart from the groundrule
driven minimum length.

 

[ic98]

PD and PS are peripheral of drain and source respectively, not length of those!
Spice modern device libraries calculate default AD, AS, PD, and PS according to W, L and process parameters (included in the library). These are defaults, when you have a special layout for the transistors, then AD, AS, .. differ from the defaults. You can find more info in HSpice manuals.