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What is the difference between a diode and a led? I know a led is a diode and I know how they are used and so on but I'm not really sure why a regular is not also light emitting. Therefore, what is the key aspect a diode is missing so it can emit light?

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As others already pointed out, LEDs need to have a direct band gap, which means that the bottom of the conduction band and the top of the valence band must have the same momentum. This is necessary, because a photon at band gap energy carries almost no momentum, therefore the source and target state of the electron must have the same momentum for the process to take place. If the bandgap is indirect, i.e. the hole and the electron have different momenta, a recombination process can only take place with the assistance of a phonon, which carries away the difference in momentum.

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In semiconductor physics, there is 2 types of band gap, a direct band gap and an indirect band gap.

Light-emitting and laser diodes are almost always made of direct band gap materials, and not indirect band gap ones like silicon.

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