MOSFET

How does the threshold voltage change when the impurity concentration of the channel region (p-type substrate) of an nMOSFET is increased?

  1. AIt becomes higher
  2. BIt becomes lower
  3. CIt does not change
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Answer: A. It becomes higher

With a higher substrate concentration, more acceptor charge in the depletion layer must be balanced before the surface inverts, so a larger gate voltage is needed. The threshold voltage therefore rises. Adjusting the threshold voltage with channel implants uses this property.

  • A. Correct. More depletion charge must be overcome before inversion.
  • B. Lowering the concentration lowers the threshold; raising it does the opposite.
  • C. Substrate doping is one of the main factors that set the threshold voltage.
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More MOSFET questions

  1. If the gate insulator of an nMOSFET is made thicker with all else unchanged, how does the magnitude of the threshold voltage generally change?
  2. Which generally describes the relationship between a MOSFET's gate length (the length of the gate electrode on the mask or after patterning) and its effective channel length?
  3. Which correctly describes punch-through in a MOSFET?
  4. In a pass transistor (switch) made of a single nMOSFET with its gate at the supply voltage VDD, how high does the output rise when passing an input of VDD?
  5. In a CMOS inverter, what is the current that flows from the supply to ground while the input is switching called?