MOSFET

Which correctly describes the flat-band voltage of a MOS structure?

  1. AThe gate voltage at which the channel reaches strong inversion
  2. BThe gate voltage at which the gate insulator breaks down
  3. CThe gate voltage at which the bands at the semiconductor surface are not bent (are flat)
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Answer: C. The gate voltage at which the bands at the semiconductor surface are not bent (are flat)

The flat-band voltage is the gate voltage at which the energy bands at the semiconductor surface are flat, without bending. It is set by the work function difference between the gate and semiconductor and by charges in the insulator and at the interface. The threshold voltage is this flat-band voltage plus the voltage needed to create inversion.

  • A. The voltage at strong inversion is the threshold voltage.
  • B. The voltage at which the insulator fails is the breakdown voltage.
  • C. Correct. It is the gate voltage at which the surface bands are flat.
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More MOSFET questions

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