Normal NMOS as Capacitor

[yannik33]

Is it possible to use a normal NMOS Transistor as a Capacitor (because
*NMOS Varactor (e.g. n-diffusion in n-well)
*Inversion type MOS varactor
aren't normal NMOS Transistor), i.e. between Gate and Source/Drain?

Quite anumber of pages show the characteristic of the GateBulk-Capacity vs. Ugs (and illustrate the differences betwee Accumulation, Inversion etc.) but how would a similar curve look for the Gate ->Drain/Source capacity? Would the oxid capacity be the dominating capacity?

 

[csolis_GT]

If you tie source and drain together, and you ensure that vgs is always higher than vth, the oxide capacitance will dominate since the transistor is ON and in triode. It is helpful for supply rails when you will have a high voltage already and the oxide of the NMOS will have a good capacitance density.

 

[dick_freebird]

Good for decoupling applications where you can count on
the DC voltage you need to drive full inversion and low
"access resistance" (channel). Poor for applications where
you require low variation in C across a wide range of DC
bias (or large signal operation). CV swing can give you
harmonic distortion issues in RF and analog amplifiers.

This is why, generally, a MOS capacitor would use N+
contacts in as heavy a NWell as can be made, ensuring
that you are well into inversion even at zero volts (a
depletion-mode MOSFET structure, if you will).