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I recently saw that the input parasitic capacitance of a MOSFET (Cgs, etc.) changes with the applied gate source voltage? Why is that?

I always thought that it was fixed since the parasitic capacitances are due to overlap capacitances or due to the gate channel (with oxide as dielectric) capacitor itself?

How can a physical overlap area or channel area (hence capacitance) change with the applied voltage???

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    \$\begingroup\$ There's a lot of junk going on the "plate" on the channel-side of the capacitor. It's not just a normal conductor. electronics.stackexchange.com/questions/66660/… \$\endgroup\$
    – DKNguyen
    May 20, 2020 at 4:58
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    \$\begingroup\$ In outline : remember that gate voltage affects the conductivity of the channel ... when it's conducting, there is much more conductive material close to the gate. So you have moved the two plates of the capacitor closer together... \$\endgroup\$
    – user16324
    May 20, 2020 at 7:31
  • \$\begingroup\$ Voting close for insufficient information -- different types of MOSFETs behave differently: some depend on Vgs, some depend on Vds. \$\endgroup\$ Aug 5, 2023 at 4:15
  • \$\begingroup\$ @TimWilliams can you give an example of a MOSFET that does NOT change its Cgs (a great part of the input capacitance) by changing Vgs? I can't think of any... \$\endgroup\$
    – Designalog
    Aug 5, 2023 at 9:10
  • \$\begingroup\$ @ErnestoG Any VDMOS I've tested -- basically any power transistor. It depends heavily on Vds, and very little or not at all on Vgs. At least over the range I tested. There should still be a MOS capacitance effect, but it wasn't perceptible at the accuracy and voltage range I tested (a few percent, so I might've missed it). Or it pulls in outside the tested voltage range (but I wouldn't expect a sudden change at like < -10Vgs, that would be weird). \$\endgroup\$ Aug 5, 2023 at 10:05

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the spacing of the gate "plates" will vary from

  • gate-oxide thickness

to

  • gate-oxide thickness plus depth-of-channel-bulk thickness
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    \$\begingroup\$ not sure why this was voted down....to me you were saying that the separation of the "plates" increases as the channel increases, thereby changing the input capacitance. Perhaps someone thought this was vague as a complete answer? Even so, I don't think it deserved a down vote.... \$\endgroup\$
    – jrive
    Jul 31, 2023 at 21:10

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