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

[uday mehta]

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?

 

[rahdirs]

• 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/