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Fig. 1. Schematics of a semiconductor band structure. The vertical scale is an energy scale for electrons. For an intrinsic semiconductor, the valence band has all of its level occupied by electrons, while the conduction band is empty of electrons. In this case, electrical conductivity is not possible (an electron in the valence band is frozen as it has no other state to move to). The energy difference between the maximum of the valence band and the minimum of the conduction band is called the band gap of the semiconductor material. When there are electrons in the conduction band, electrical conductivity is made possible. It is also the case when some electrons are missing in the valence band (so called holes). Finally, when a conduction band electron and a valence band hole are at the same place in the semiconductor, they can recombine (ie the electron leaves the conduction band to fill the valence band hole) and emit a photon, with an energy (and hence a wavelength) corresponding to the band gap of the material.