why in si based diode most of power is dissipate in form of heat instead of light?

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uday mehta

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i read that si and germanium is not use for led, because in there when electron falls from CB to VB it dissipate 90% heat and 10% light. what is the reason that in GaAs, GaP, GaAlN etc. energy emitted in form of light?
 

  • LEDs are usually fabricated from compound semiconductor materials, such as gallium arsenide or gallium arsenide phosphide. These materials are direct-bandgap semiconductors.
  • They are semiconductors for which the minimum of the conduction band occurs at the same k, as the maximum of the valence band, have a stronger absorption of light as characterized by a larger absorption coefficient.They are usually favored when fabricating light emitting devices
  • Si is known to be an indirect semiconductor i.e. semiconductors for which the minimum of the conduction band does not occur at the same k as the maximum of the valence band, are known to have a smaller absorption coefficient and are rarely used in light emitting.
  • From Si's E vs k,we can see that when electron undergoes transition from conduction band to valence band,there is also a change of momentum unlike direct case where there was no change of momentum.
  • Phonons are emitted after recombination to conserve momentum in indirect semiconductors & it happens that phonons also have energy in addition to momentum & hence most of the energy from recombination in Si,ends up as heat

For more info,refer to thread already on Edaboard:https://www.edaboard.com/threads/61537/
 
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