# Varactor C-V Curves In Cadence

1. ## Varactor C-V Curves In Cadence

Hi all,

I want to obtain C-V curves of a varactor in cadence. I have some questions.

1- In which simulation I can obtain C-V curves?

2- For a single NMOS I tried DC sweep. I plotted Cgg-VDS for the NMOS. The Cgg axis in "F (fN)" units. What does this unit stand for?

3- When I try to obtain Cgs-VDS corves, I get negative values (again in "F(fN)" units). Can anyone suggest why the capacitance values are negative?  Reply With Quote

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2. ## Re: Varactor C-V Curves In Cadence

I would do a transient analysis with a pulsed current source
and a trivial shunt (like 1E12 explicit resistor). I=CdV/dt, use
fixed (after initial condition=0) flat-top I, your C(V) is the
I/(dV/dt) at the voltage-point of interest; keep / plot V(t)
and I(t)/deriv(V(t)), change the X axis to be V(t) instead
of time, there's your C-V curve.

For accuracy you may want to crank down some of the transient
accuracy params, maybe pick a Euler solution method, and might
make sense to use a much larger than minimum device - like a
1000-element array - to stay up out of numerical noise / slop.

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3. ## Re: Varactor C-V Curves In Cadence

The simplest way is to use ac analysis and measure imaginary part of current.
For a very low values of Vds and/or Vgs negative capacitance or asymmetrical (Cgs not equal to Csg) is nothing suspicious, if you are using BSIM model in version 3 or 4.

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