MOSFET

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?

  1. AThe threshold voltage becomes zero
  2. BThe gate insulator appears effectively thicker, reducing gate capacitance (and drive current)
  3. CThe gate insulator appears effectively thinner and the gate capacitance increases
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Answer: B. The gate insulator appears effectively thicker, reducing gate capacitance (and drive current)

Because of quantum mechanical effects, inversion-layer electrons peak not right at the interface but slightly inside (about 1 nm). The charge centroid therefore sits away from the interface, which acts like a thicker insulator and reduces gate capacitance and drive current. Together with polysilicon gate depletion, it increases the effective insulator thickness.

  • A. This effect does not make the threshold voltage zero.
  • B. Correct. The effective insulator thickness increases and the capacitance decreases.
  • C. The charge centroid moves away from the interface, so the insulator appears thicker.
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More MOSFET questions

  1. 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?
  2. Which is the common condition for "strong inversion" used to define the threshold voltage of a MOSFET? (φF: Fermi potential)
  3. What is the condition for an enhancement-mode n-channel MOSFET to turn on (conduct)? (VGS: gate-source voltage, Vth: threshold voltage)
  4. 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?
  5. Why is the static (steady-state) power consumption of a CMOS inverter small?