Semiconductor lasers
Solid-state lasers

In a semiconductor, electron-hole pairs correspond to excited states. During recombination, the binding energy can be released in the form of radiation, if this is not in competition with other processes. In a forward-biased semiconductor, the lifetime of spontaneous radiation is very short (10 ^–10 to 10 ^–9 s), resulting in a very high gain in a population-reversal system.

To prevent the radiation from being absorbed, the energy gap is arranged to be greater than the photon's energy hν; this is achieved when the electrons in the lower limit of the conduction band recombine with the holes in the empty upper part of the valence band (four-level system)....