Semiconductor physics

Why does the carrier density of n-type silicon drop sharply when it is cooled to cryogenic temperatures?

  1. AThe number of impurities decreases at low temperature
  2. BMobility becomes zero at low temperature
  3. CThere is not enough thermal energy to excite electrons from donors into the conduction band (carrier freeze-out)
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Answer: C. There is not enough thermal energy to excite electrons from donors into the conduction band (carrier freeze-out)

Although donor levels are shallow, a little thermal energy is still needed to release electrons into the conduction band. At cryogenic temperatures this energy is insufficient, so electrons stay bound to the donors and the carrier density drops sharply. This is called carrier freeze-out.

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More Semiconductor physics questions

  1. Which recombination process becomes significant at high carrier concentrations, where the released energy is given to another carrier instead of being emitted as light?
  2. What is the approximate resistivity of n-type silicon with an electron density of 1×10¹⁶ cm⁻³ and an electron mobility of 1000 cm²/V·s? (Ignore holes; elementary charge 1.6×10⁻¹⁹ C)
  3. What is the energy difference between the vacuum level and the bottom of the conduction band of a semiconductor called?
  4. In lightly doped silicon, what is the main reason mobility decreases as temperature rises?
  5. Which value is closest to the intrinsic carrier density of silicon at room temperature?