MOSFET

Which is the common condition for "strong inversion" used to define the threshold voltage of a MOSFET? (φF: Fermi potential)

  1. AThe surface potential becomes −φF
  2. BThe surface potential becomes 2φF (the surface minority carrier density equals the bulk majority carrier density)
  3. CThe surface potential becomes zero
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Answer: B. The surface potential becomes 2φF (the surface minority carrier density equals the bulk majority carrier density)

The onset of strong inversion is commonly defined as the point where the surface potential reaches 2φF, that is, where the electron density at the surface equals the hole density in the bulk. The gate voltage at this point is the threshold voltage, expressed as the sum of the flat-band voltage, 2φF and the voltage due to depletion charge.

  • A. A negative surface potential puts the surface of a p-type substrate into accumulation.
  • B. Correct. A surface potential of 2φF marks strong inversion.
  • C. Zero surface potential is the flat-band condition.
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  2. 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?
  3. Why is the static (steady-state) power consumption of a CMOS inverter small?
  4. When a long-channel MOSFET operates in the saturation region, the drain current ID is roughly proportional to what? (VGS: gate-source voltage, Vth: threshold voltage)
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