MOSFET

What is the main reason the channel mobility of a MOSFET is lower than the mobility inside bulk silicon?

  1. ABecause the temperature in the channel is extremely low
  2. BBecause carriers in the channel flow near the Si/SiO2 interface and are easily scattered by interface roughness and charges
  3. CBecause the channel contains no impurities at all
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Answer: B. Because carriers in the channel flow near the Si/SiO2 interface and are easily scattered by interface roughness and charges

A MOSFET channel forms very close to the interface with the gate insulator. Carriers are easily scattered by interface roughness and charges near the interface, and the more strongly the gate voltage pulls them toward the interface, the more scattering increases, so mobility is lower than in the bulk crystal.

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More MOSFET questions

  1. In a short-channel MOSFET, what is the main reason the drain current falls below the (VGS − Vth) square law and approaches a linear dependence?
  2. Which expression roughly represents the delay of a CMOS logic gate? (C: load capacitance, V: supply voltage, I: drive current)
  3. What is the sign of the threshold voltage (relative to the source) of an enhancement-mode p-channel MOSFET?
  4. What is the effect in which the gate-drain capacitance appears (1 + gain) times larger when seen from the input of an amplifier?
  5. What problem results from depletion of the polysilicon on the gate electrode side of a polysilicon gate?