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indirect band gap semiconductor doubt ....

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munna007

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its said that indirect band gap semiconductors like silicon and germanium are not used as a light sources because they have more non-radiative recombinations rather than radiative ,which inturn decreases efficiency of optical source.


but i want to understand how indirect band gap materials produce non-radiative photons.

i have studied many books like keiser , senior on optical fibre .those all are not explanatory on this ....also googled for this ,but not helped from there.....


hoping for help here....thanks.....love to all....
 

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eladla

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Hi,

I had a few courses on these topics, hope I can help.
Materials like Silicon, which have indirect band gaps do not emmit photons when loosing energy (moving from conduction to valance band).
On the other hand materials like GaAs, which have direct band gap, do emmit photons when loosing energy.

The reason for this can be seen very graphically from the image you posted.
You can see that the minima in the conduction band is not directly above the maxima in the valance band,
so when the electrons loose energy the energy goes in to changing their momentum.
On the other hand, when the maxima in right under the minima, no energy needs to go to momentum and
so the excess energy goes in to creating a photon.

It`s all a matter of energy conservation.

Hope this helps.
Elad.
 

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