MOSFET

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)?

  1. AAbout 600 mV/dec
  2. BAbout 6 mV/dec
  3. CAbout 60 mV/dec
Show answer and explanation

Answer: C. About 60 mV/dec

Even below the threshold voltage, drain current flows and depends exponentially on gate voltage. At room temperature, the theoretical lower limit set by thermal energy is about 60 mV/dec. Because switching cannot be steeper than this, lowering the threshold voltage increases off-state leakage, which is a barrier to lowering supply voltages.

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  1. How does a MOSFET behave in the linear region, where the drain-source voltage VDS is sufficiently small?
  2. What is the purpose of the LDD (lightly doped drain) structure in a MOSFET?
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  5. In a MOS structure on a p-type substrate, what is the state of the semiconductor surface when a negative voltage is applied to the gate?