MOSFET

Power MOSFETs are easy to use in parallel. Which statement about the temperature dependence of on-resistance explains why?

  1. AOn-resistance is independent of temperature
  2. BOn-resistance falls as temperature rises, so current concentrates further in the hottest device
  3. COn-resistance rises with temperature, so a device that heats up naturally carries less current and the current spreads out
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Answer: C. On-resistance rises with temperature, so a device that heats up naturally carries less current and the current spreads out

A MOSFET's on-resistance increases with temperature because mobility decreases (a positive temperature coefficient). If one of several parallel devices heats up, its resistance rises and its current falls, shifting current to the others. Thermal runaway is therefore unlikely, and parallel connection is easy.

  • A. On-resistance changes considerably with temperature.
  • B. Resistance falling with temperature and concentrating current is the property behind thermal runaway in bipolar transistors.
  • C. Correct. The positive temperature coefficient naturally spreads the current.
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More MOSFET questions

  1. Under Dennard (constant-field) scaling, which statement is correct when dimensions and voltages are each scaled by 1/k?
  2. Which correctly describes the flat-band voltage of a MOS structure?
  3. In a high-frequency C-V measurement of a MOS capacitor on a p-type substrate, what happens to the capacitance when a large positive gate voltage drives the surface into strong inversion?
  4. In a MOSFET with a very thin gate insulator, what is the effect of the electron distribution in the inversion layer peaking slightly away from the interface?
  5. 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?