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Model Report

analogy

This is bit weird.When I was refering to gpdk045 model report, Vth for standard nmos1v transistor for W:L of 120n:45n was 0.36 volts.But as you sweep Vgs from 0 to 1v for a fixed Vds voltage of 1v, it is easy to see Vth to be ~0.5v

Is there a problem ?

  • If you're comparing the vth0 (or vtho) model parameter with the "vth" as reported by the operating point, then I would expect them to be different. If you look in the Virtuoso Simulator Components and Device Models Reference manual (<MMSIMinstDIr>/doc/spectremod/spectremod.pdf - also accessible via cdnshelp or on Cadence Online Support), in the chapter on bsim4, the Vth as reported in the operating point output is computed as described by equation 21-14 (in the MMSIM14.1 version at least, this is on page 1260, in the section labelled "Threshold Voltage Model"). You can see there that this is a complex equation, where VTH0 is only one of the parameters and there are lots of other dependencies.

    Regards,

    Andrew.

  • Hi analogy,

    If you're looking for a threshold voltage metric that provides design insight over various operating regions, you might consider the "constant-current" method of defining Vth. It's often used by foundries, trivial to set up in simulation, and makes it easy to see the shift due to DIBL. Here's the JEDEC definition, and there are more detailed references online.

    Regards,

    Stephen

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