MOSFET

For nMOS and pMOS transistors of the same size, the nMOS generally has a larger drive current. What is the main reason?

  1. AThe nMOS gate oxide is always thicker
  2. BA pMOS has no gate electrode
  3. CThe mobility of electrons in the channel is higher than that of holes
Show answer and explanation

Answer: C. The mobility of electrons in the channel is higher than that of holes

Electrons flow in the channel of an nMOS and holes in a pMOS. In silicon, electron mobility is about 2–3 times hole mobility, so an nMOS of the same size can carry more current. In CMOS circuits, the pMOS gate width is often made wider to balance drive strength.

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More MOSFET questions

  1. 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)?
  2. Which is a characteristic of a depletion-mode MOSFET?
  3. Which value is closest to the theoretical lower limit of a MOSFET's subthreshold swing at room temperature (the gate voltage needed to change the drain current by one decade below threshold)?
  4. How does a MOSFET behave in the linear region, where the drain-source voltage VDS is sufficiently small?
  5. What is the purpose of the LDD (lightly doped drain) structure in a MOSFET?