Exciton

An exciton is a combination of an electron and a hole in a semiconductor or insulator in an excited state. The hole behaves as a positive charge, and the electron is attracted to it to form an "exotic atom" state akin to that of a hydrogen atom. The probability of the electron falling into the hole is limited by the difficulty of losing the excess energy, so that the exciton may have a relatively long life. The existence of these states may be inferred from the absorption of light associated with their excitation. Alternatively, an exciton may be thought of as an excited state of an atom or ion, the excitation wandering from one cell of the lattice to another.

see also: positronium