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UltraSim and gmin

simbamford

Hi,

I'm using spectre and simulating designs in which mosfets are backwards biased (e.g. nmos with gate voltage below source voltage), reducing the current through the transistor (in the processing I'm using) to sub-picoamp levels. In order to see this in the simulation I need to reduce the "gmin" variable from its normal value of 10*e^-12 to e.g. 10*e^-15. When I try to switch to UltraSim there is a larger current through these transistors than there should be, as if the simulator is assuming a certain minimum conductance across these transistors, however there is no "gmin" option. Is there another way to get this level of accuracy from UltraSim, or is it simply not designed for such accuracy?

Thanks very much,

Sim Bamford

  • Quek

    Hi Sim Bamford

    You can add the following option to set gmin:

    .options gmin=10e-15

    There is also a gmin_allnodes option to force a conductance for all nodes. It can be set using:

    .usim_opt gmin_allnodes=10e-15

    You can search for "gmin" in $MMSIMHOME/doc/Ultrasim_User/Ultrasim_User.pdf. There is also another option named gmin_float. : )


    Best regards
    Quek

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