MOSFET

Which expression appropriately gives the small-signal output resistance ro of a MOSFET in saturation, using the channel-length modulation coefficient λ and drain current ID?

  1. Aro ≈ 1 / (λ × ID)
  2. Bro ≈ ID / λ
  3. Cro ≈ λ × ID
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Answer: A. ro ≈ 1 / (λ × ID)

Including channel-length modulation, the drain current in saturation increases slightly with VDS as ID(1 + λVDS). The reciprocal of this slope is the output resistance, ro ≈ 1/(λID). A longer channel has a smaller λ and a larger output resistance.

  • A. Correct. ro ≈ 1/(λID).
  • B. ID/λ has units of current times voltage, not resistance.
  • C. λ × ID has units of current per voltage, not resistance.
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  3. 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?
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