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I'm studying some materials on semiconductors, and a section on photoconductivity has left me somewhat confused.

In an extrinsic material, is photoconductivity the result of electrons moving from the valence band to the conduction band, or is it the result of dopant atom ionization? If it is the latter, is the complete ionization assumption invalid for photoconducting devices?

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Anywhere near room temperature, most of the dopants will be ionized. The photoconductivity is indeed caused by electron-hole pair generation. In an extrinsic material (almost all silicon you will find is extrinsic), the question is what is the carrier concentration. In silicon, the intrinsic carrier concentration is a little over 1E10/cc, so "extrinsic" is anything above that. If your background doping is say 1E14, than your photo-generated carriers can easily be multiple orders of magnitude larger, and be the conduction mechanism for all practical purposes.

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