The concept of excitons was first proposed by Yakov Frenkel in 1931. The absorption of a photon by an inter band transition in a semiconductor creates an electron in conduction band and a hole in valence band. These oppositely charged particles attract each other through Coulomb interaction and there may be a probability of formation of a neutral electron hole pair called exciton. An exciton is a bound state of an electron and hole which ae attracted to each other by an electrostatic force. When the photon is absorbed by a semiconductor, an electron hole pair is created. The electron in the conduction band is now attracted to the localized hole by repulsive Coulomb forces from the large number of electrons surrounding the hole and excited electrons. This attraction provides a stabilizing energy balance. Type of excitons: Frenkel excitons (tight bound excitons): When there is a strong electron hole attraction, like in ionic crystals, the electrons and holes are tightly bound to each other, this type of exciton is called Frenkel exciton. Wannier–Mott excitons (free exciton): When the electron hole separation is much larger to the lattice constant, then the exciton is called Wannier–Mott exciton.
