BSIM3v3 model... Lint with negative values?

Discussion in 'Cadence' started by spectrallypure, Jun 17, 2008.

  1. spectrallypure

    spectrallypure Guest

    Hi all! I am trying to do some hand calculations for design, and need
    to estimate the effective channel width and length of MOS transistors.
    I was suggested to use:

    Leff = Ldrawn - 2Lint; Weff = Wdrawn - 2Wint

    where Lint and Wint are taken from the design kit's BSIM3v3 model
    files. However, for the technology in which I am working (0.35um
    CMOS), these models for NMOS and PMOS have both NEGATIVE values for
    Lint!!!:

    lint=-5.005e-08 (NMOS), lint=-7.130e-08 (PMOS)

    Being Lint somewhat related to the length source and drain overlaps
    with the gate, how come can it be negative? Is this just the result of
    model optimization? Is this meaningful?

    Thanks in advance for any suggestions.

    Jorge.
     
    spectrallypure, Jun 17, 2008
    #1
  2. spectrallypure

    Ozgur.Ates Guest

    Jorge, BSIM3v3 model extraction programs try to match measured device
    characteristics to model behaviour within lots of MOS devices with
    different lenghts and widths. Physically LINT is of course positive,
    however for your case BSIM3v3 equations better fit to measured
    characteristics with negative LINT value.

    Ozgur
     
    Ozgur.Ates, Jun 18, 2008
    #2
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