Why not indirect band gap for led

onqun
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Hey I was studying for my test, in my notes it says si cannot be used for led, because it has indirect band gap.Why indrect band gap is not useful for led?
 
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It is a less efficient. It requires not only the available states, but the available phonons to provide the necessary change in momentum.

Zz.
 
Thank you very much
 

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