Capacitance Trend of a Varactor

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tasctasc

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When I simulate the capacitance of a varactor (nmos in an nwell) using spectre in cadence, the capacitance increases slightly with frequency. Other devices (standard NMOS and PMOS, MIM), the capacitance decreases with frequency which is expected. Is anyone able to please explain why the slight increase in varactor capacitance is seen with frequency. I have also noticed this trend in other papers but haven't been able to find an explanation.
Thank you.
 

Could you post you're simulation results and/or testbench?

Also are you sure you've connected the Nwell to ground (a common mistake!)?
 

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