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spectre captab: 1) negative capacitance 2) not reciprocal values

ZoltanT

HI everyone,

I wanted to get some node-to-node capacitance values of a transistor with the help of spectre's captab option. Unfortunately I faced two strange issues:

1) In some cases I got negative capacitance values for the drain-source capacitance of a transistor.

2) C_AB differ from C_BA where B and A are two arbitrary nodes. In some cases the error is more than 10x.

The results are unsettling to me. Either I set up something wrong, or the transistor models behave very strange.The formula for calculating the capacitances is written in the Spectre's User Guide. What I do not know, how is it determined? Do Spectre run AC analysis on the circuit to determine the capacitances? How error-proof the process is? Is it easy to miss something?

Thanks in advance!
Zoltan

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  • VLSIiitm

    This is because of the way partial capacitances are defined. e.g., Cgs is the change in the charge on the gate (g: first subscript) for a unit change in the voltage on the source (s: second subscript). When defined this way, capacitance will come out negative (even for a regular capacitor) because, if you increase the source voltage, there will be more positive charge on the source terminal and negative charge on the gate terminal. For the "conventional" capacitance definition, use Cgg, which is the change in the charge on the gate (g: first subscript) for a unit change in the voltage on the gate (g: second subscript).

    Asymmetry comes from nonlinearity and the way channel charge is apportioned between drain and source. Chapter 7 in the book below has the details.


    Operation and Modeling of the MOS Transistor (The Oxford Series in Electrical and Computer Engineering) 3rd Edition
    by Yannis Tsividis (Author), Colin McAndrew (Author)

    Nagendra

  • RFStuff
    in reply to VLSIiitm

    If one wants to find the Cgd for small signal modeling, how the reported simulation results can be used ?

Reply
  • RFStuff
    in reply to VLSIiitm

    If one wants to find the Cgd for small signal modeling, how the reported simulation results can be used ?

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