Photoelectric effect

Internal photoelectric effect is a process in which the absorption of a photon in a semiconductor results in the excitation of an electron from the valence band to the conduction band. A semiconductor, such as Si and Ge, has a filled valence band and an empty conduction band, but in contrast to an insulator, a relatively small gap exists between these bands.

Band theory: The molecular orbitals of a solid become a series of continues energy levels, or a band of electronic energy levels, can be used to explain the behavior or conductors, semiconductors and insulators.