MOSFET

In a high-frequency C-V measurement of a MOS capacitor on a p-type substrate, what happens to the capacitance when a large positive gate voltage drives the surface into strong inversion?

  1. AIt stays nearly constant at its minimum value
  2. BIt becomes infinite
  3. CIt reaches a maximum equal to the oxide capacitance Cox
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Answer: A. It stays nearly constant at its minimum value

At high frequency, generation of minority carriers (electrons) in the inversion layer cannot keep up with the signal, so the inversion layer does not contribute to the capacitance. The capacitance therefore stays nearly constant at its minimum, set by Cox in series with the depletion capacitance at maximum depletion width. At low frequency the inversion layer responds and the capacitance returns to Cox.

  • A. Correct. At high frequency the inversion layer cannot follow, so the capacitance stays at its minimum.
  • B. The capacitance never becomes infinite.
  • C. It equals Cox in accumulation, or in inversion at low frequency.
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More MOSFET questions

  1. In a MOSFET with a very thin gate insulator, what is the effect of the electron distribution in the inversion layer peaking slightly away from the interface?
  2. Which expression appropriately gives the small-signal output resistance ro of a MOSFET in saturation, using the channel-length modulation coefficient λ and drain current ID?
  3. Which is the common condition for "strong inversion" used to define the threshold voltage of a MOSFET? (φF: Fermi potential)
  4. What is the condition for an enhancement-mode n-channel MOSFET to turn on (conduct)? (VGS: gate-source voltage, Vth: threshold voltage)
  5. If only the thickness of the gate insulator is reduced, without changing its material or area, what happens to the gate capacitance per unit area?