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MOS capacitance when Vgs is negative

  1. Feb 9, 2019 #1
    I have a question on what happens to MOS capacitance Cgs+Cgd, when Vgs goes negative in the NMOS shown. I see that when Vgs goes negative, the channel is full of holes because of p- substrate, which means we see the Cox capacitance (without any other capacitance in series), but I am not sure if this is between Cgs (or Cgd) or Cgb. My intuition says it's between Cgb, but if that's the case, then there is no capacitance between Cgs (or Cgd), ignoring the overlap capacitance.
    Attached the figure here

  2. jcsd
  3. Feb 11, 2019 at 9:41 PM #2
    The capacitance (between gate and source/drain) of NMOS in depletion is reduced because no channel mean Cgb is in series with Csb+Cdb.
    with negative voltage, the capacitance is limited by Csb+Cdb, and reverse proportional to square root of voltage between gate and drain/source - because of depletion regions forming around source/drain
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