In this chapter, we discuss semiconductor lasers and light-emitting diodes (LEDs). Both semiconductor lasers and LEDs are semiconductor light sources based on electroluminescence, which results from the radiative recombination of electrons and holes in a semiconductor. A semiconductor laser emits coherent laser light with a relatively small divergence, whereas the emission of an LED is incoherent and divergent. These semiconductor devices have several unique properties. They are rugged devices that are reliable and have long operating lifetimes because of their very small, compact sizes with integrated solid-state structures. They have very high efficiencies and consume very little power in comparison with other light sources of similar brightness because they are cold light sources operating at temperatures that are much lower than the equilibrium temperatures of their emission spectra. They can be electrically pumped by current injection at relatively low current and voltage levels and can be directly current modulated with very fast response for high-speed applications, including broadband optical communications. Their compatibility with semiconductor fabrication and processing technologies allows them to take advantage of semiconductor electronics technology for easy integration into electronic systems. Furthermore, the mature nature of semiconductor electronics technology allows them to be mass produced at a low cost. These unique properties make semiconductor lasers and LEDs the light sources of choice in many practical applications.
The general characteristics of electron–hole recombination processes in a semiconductor are discussed in Section 12.3.