Excitons

An exciton is a Coulomb correlated electro/hole pair in a semiconductor, of which it represents the elementary excitation.

A vivid picture of an exciton formation is as follows: a photon collides into a semiconductor, which effect is to excite an electron of the valence band into the conduction band. The missing electron in the valence band leaves a hole behind it, of opposite electric charge and consequently to which it is attracted by Coulomb force. The exciton results as the binding of the electron with its hole. The wavefunctions of the bound state are of the hydrogenoid type, with however a binding energy much smaller and a size much bigger than hydrogen atoms.