MOS Capacitor or a mosfet used as a cap (D-S-B connected to gnd)

[borabilgic]

Hi, in Cadence Spectre simulations should I use a MOS cap instance or a mosfet (D-S-B connected to gnd) to simulate a real life MOS-cap? I tried both of them and they gave different results i.e. MOS-cap instance charges to a higher voltage.

 

[dick_freebird]

MOS caps may be different construction
than a "logic" FET (e.g. depletion or intrinsic
doping).

If you want a MOS varactor you'd be well
off to use a primitive which has those
attributes modeled as a "care-about" and
not an afterthought (logic FETs' terminal
capacitances just have to be good enough
for timing closure, specific relation to the
terminal voltages may be of little interest).

 

[borabilgic]

Shoul I tie D and S to ground while building a cap from mosfet? or just bulk connection to gnd is enough?