MOSFET

Why is the static (steady-state) power consumption of a CMOS inverter small?

  1. ABecause the supply voltage is high
  2. BBecause current always flows into the gate
  3. CBecause whether the input is 0 or 1, either the pMOS or the nMOS is off, so almost no current flows from supply to ground
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Answer: C. Because whether the input is 0 or 1, either the pMOS or the nMOS is off, so almost no current flows from supply to ground

In a CMOS inverter, when the input is 0 the pMOS is on and the nMOS off, and when the input is 1 the pMOS is off and the nMOS on. In steady state there is always an off transistor between supply and ground, so almost no through current flows. Power is mainly consumed at the moment of switching (charging and discharging).

CMOS inverter VDD pMOS Out nMOS GND In When the input is 0 pMOS on, nMOS off → output 1 When the input is 1 pMOS off, nMOS on → output 0 One of them is always off, so little current flows from VDD to GND
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More MOSFET questions

  1. When a long-channel MOSFET operates in the saturation region, the drain current ID is roughly proportional to what? (VGS: gate-source voltage, Vth: threshold voltage)
  2. For nMOS and pMOS transistors of the same size, the nMOS generally has a larger drive current. What is the main reason?
  3. 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)?
  4. Which is a characteristic of a depletion-mode MOSFET?
  5. 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)?