When illuminated with an above band gap light, electrons from the valence band are transferred to the conduction band in a semiconductor. The process thus generates electrons and holes in the conduction and valence bands, respectively. Note that within the effective mass approximation, electrons and holes at the respective band extrema can be treated as free particles of mass \(m_{e}\) and \(m_{h}\) with negative and positive electronic charges, respectively. It is now conceivable that Coulomb interaction between electrons and holes can result in the formation of hydrogen atom like bound pairs. Such bound entities are called excitons.