Semiconductor and p-n junction questions

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Hi guys,

I just have a few questions regarding semiconductors and p-n junctions.
How can I explain using an E vs k diagram why a silicon based device can be an efficient light detector but not an efficient light emitter? I imagine it has something to do with the fact that an electron in the conduction band would have to make an indirect transition since the conduction band minimum has a different k value to the valence band maximum.
Are electrons mostly found at the bottom of the conduction band because there are lots of free states there? I'm guessing that Silicon can be an efficient light detector because it can absorb any photon with an energy greater than the band gap energy. Does the electron then preferentially move to the conduction band minimum once it has been promoted?

Also why are semiconductor junctions used as light emitters in preference to bulk semiconductors?

Thanks for your help!
 

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