In a semiconductor, the energy gap between the valence band and the conduction band is small enough that when given enough energy, valence electrons can move into the conduction band which permits the conduction of mobile electrons.
Fischer-Cripps. A.C., The Electronics Companion. Institute of Physics, 2005.
Roulston. David. J., An Introduction to the Physics of Semiconductor Devices. Oxford University Press, 1999.
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