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Electronic, magnetic and optical materials

What is the main reason silicon is hardly ever used as the emitting material in light-emitting diodes?

  1. AIt is a direct-gap semiconductor, so electrons and holes recombine immediately by giving off phonons rather than emitting light
  2. BIts band gap of about 3.4 eV is too large, so it can emit only ultraviolet light with wavelengths shorter than visible light
  3. CIt is an indirect-gap semiconductor, so radiative recombination requires phonons, and the emission efficiency is low
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Answer: C. It is an indirect-gap semiconductor, so radiative recombination requires phonons, and the emission efficiency is low

Silicon is an indirect-gap semiconductor, with the conduction band minimum and the valence band maximum at different crystal momenta. For an electron and hole to recombine by emitting light, a phonon must participate at the same time to make up the momentum difference, so the probability of radiative recombination is very low and most recombination is nonradiative through defect levels, producing heat. Direct-gap semiconductors such as GaAs and InGaN emit light efficiently and are used in LEDs and lasers. Being direct-gap is in fact favorable for light emission. About 3.4 eV is the band gap of GaN, not silicon; silicon's is about 1.12 eV, so any light it emits is infrared at a wavelength of about 1.1 µm.

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