imd1
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I think that the PMOS in an n-well is exactly the structure used to implement high-quality varactors in RF VCOs... The capacitance goes from Cox (at accumulation) to Cox in series with Cchannel (in inversion), about 2X to 3X change, I think. The only difference between this MOSCAP and the PMOS is the fact that the p+ implants (S/D) are not done, there are n+ implants instead, whcih are used to contact the bulk as close as possible to minimize the loss (resistive drop).
The Spice model used for these are the same as for the PMOS (BSIM3vx), but having different parameters.
If you do a careful layout (minimize gate resistance and bulk resistance) a PMOS should work OK as decoupling cap, I think.
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