Why Does Id Increase with Constant Vgs Despite Body Effect?

[salil87]

I found on my notes that for an NMOS with increase in Vsb voltage Body Effect increases, Vt also increases. But it is mentioned that with Vgs constant id increases which I feel is wrong since id is proportional to (Vgs - Vt)2. Hence I think id should decrease. Please show me the light :-)

Thanks
Salil

 

[es1]

I think your notes are wrong. I would also expect Id to decrease with increasing Vsb for a fixed Vgs and assuming the transistor is limiting factor for the drain current.

 

[es1]

Actually, I just thought of a quick way of mentally verifying this. Since Vgs is fixed, you can solve for the Vsb to make Vth above Vgs. This would make Id zero as the FET would be off. Since you can always increase Vsb to make Id=0 by increasing Vth for any fixed Vgs (in the mathematical model anyway) you should conclude increasing Vsb reduces Id.

 

[salil87]

Great... thnks a lot for the confirmation :-)