valence & conduction band

[kidia]

Can anybody explain to me why holes are found at the top of the valence band, whereas electrons are found at the bottom of the conduction band.

[Klaynos]

Because the electrons have been excited in some way (given extra energy by some intereation i.e. absorption of a photon) and have moved up energy levels to be in the conduction band, leaving a hole in the valance band where they used to be...