MOSFET

What is the condition for an enhancement-mode n-channel MOSFET to turn on (conduct)? (VGS: gate-source voltage, Vth: threshold voltage)

  1. AVGS is 0 V
  2. BVGS is greater than Vth
  3. CVGS is negative
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Answer: B. VGS is greater than Vth

In an enhancement-mode nMOSFET, applying a positive gate voltage above the threshold voltage Vth forms an electron inversion layer (channel) at the surface of the p-type substrate under the gate, and current flows between source and drain. At VGS = 0 V there is no channel and the transistor is off.

Cross-section of an n-channel MOSFET (on state) p-type substrate Source n⁺ Drain n⁺ Gate VGS > Vth Electron inversion layer (channel) Gate oxide
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More MOSFET questions

  1. 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?
  2. Why is the static (steady-state) power consumption of a CMOS inverter small?
  3. 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)
  4. For nMOS and pMOS transistors of the same size, the nMOS generally has a larger drive current. What is the main reason?
  5. In an n-channel MOSFET, what happens to the threshold voltage when a negative voltage is applied to the substrate (body) relative to the source (body bias)?