Ohh Yes I see !
Is the bulk of your MOS tied to the Source ?
If not, your VTH will be strongly impacted by the Body Effect.
Remember that the VTH equation is roughly given by :
Vth = Vth0 + Gamma*{sqrt(abs(Vsb-2*PhiFs))-sqrt(abs(2*PhiFs))} with :
Vth0 is the Th volltage at Vsb=0 (i,e Vsource tied to Vbulk)
Gamma = The body effect factor
Vsb = Source to Bulk voltage
PhiFS = The Fermi Level stuff ... (never remember the name)
So if Vsb = 0, then Vth = Vth0 otherwise Vth will be greater.
You may to take this into account.
This is a weak point of the NMOS compared to the P-one. Since the Pmos
is drawn in an N-well, you can always tie the S-B nodes. But the NMOS
is in the global substrate ...
This could be solved if you process offers Deep-Nwells that can help
in creating isolated Pwells.
Other parameters like Well Proximity Effects (WPE), Shallow Trench
Isolation (STI) Stress ... impact the value of the VTh but it is a
third order impact ...
Look at your MOs model do***entation (MM9, BSIM3, BSIM4 ...) for more
details.
i don't know what you're designing but in a case of diff pair for
example, the current is fixed by the current source/mirror, the Diff-
pair transistors will then adjust their VGS to match the current fixed
by the source. The value of the VGS id of course dependent of your W/L
ratio.
Good luck anyway !
Riad.
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