Semiconductor physics

In an indirect-gap semiconductor such as silicon, why is the probability of electron-hole recombination with light emission low?

  1. ABecause the conduction band minimum and valence band maximum have different momenta, so the transition requires a phonon
  2. BBecause the hole mobility is lower than the electron mobility
  3. CBecause the band gap is too large
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Answer: A. Because the conduction band minimum and valence band maximum have different momenta, so the transition requires a phonon

In an indirect-gap semiconductor, the conduction band minimum and the valence band maximum are at different positions in momentum (wave-vector) space. A photon carries almost no momentum, so the transition requires the simultaneous involvement of a lattice vibration (phonon), which makes it unlikely. This is why silicon is not suited to light-emitting devices.

  • A. Correct. A phonon is needed to conserve momentum.
  • B. The difference in mobility does not determine the probability of light emission.
  • C. Silicon's band gap (about 1.12 eV) is not especially large and is not the reason it emits light poorly.
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More Semiconductor physics questions

  1. In a semiconductor driven out of thermal equilibrium by illumination or current injection, what are the levels introduced to describe the electron and hole densities separately?
  2. Which value is closest to the lattice constant of crystalline silicon?
  3. Why is the intrinsic Fermi level of silicon slightly offset from the exact center of the band gap?
  4. Which value is closest to the band gap of silicon (Si) at room temperature?
  5. Which statement is correct when phosphorus (P) is added to silicon as an impurity?