Indirect bandgap
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In semiconductor physics, an indirect bandgap is a bandgap in which the minimum energy in the conduction band is shifted by a k-vector, which is determined by the material's crystal structure.
Semiconductors with an indirect bandgap are inefficient at emitting light. This is because any electrons present in the conduction band quickly settle into the energy minimum of that band. Electrons in this minimum require some source of momentum allowing them to overcome the offset and fall into the valence band. Photons have very little momentum compared to this energy offset – hence, the momentum "kick" of a photon being emitted would normally not be enough to dislodge the electron from the conduction band.
Since the electron cannot rejoin the valence band by radiative recombination, conduction band electrons typically last quite some time before recombining through less efficient means. Silicon is an indirect bandgap semiconductor, and hence is not generally useful for light-emitting diodes or laser diodes.
Indirect bandgap semiconductors can absorb light, however this only occurs for photons with significantly more energy than the bandgap. This is why pure silicon appears dark grey and opaque, rather than clear.