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  1. #1
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    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?

    Thanks in advance

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  2. #2
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    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. #3
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    Dominik Przyborowski's Avatar
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    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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