Excitons

If photons of energy comparable to the band gap are incident on a semiconductor, then they can be absorbed by the electrons forming atomic bonds between neighbouring atoms. Within the band theory of solids, this would be described as exciting an electron from the valence band across the band gap into the conduction band. If the energy of the photon is larger than the band gap, then a free electron is created and an empty state is left within the valence band. The empty state within the valence band behaves very much like an air bubble in a liquid and rises to the top the lowest-energy state. This ‘hole’ behaves as though it was positively charged and hence often forms a bond with a conduction-band electron. This bound electron-hole pair is known as an exciton. This chapter highlights importance of the non-resonant and resonant generation of excitons.