Electron-hole recombination
The process in which an electron, which has been excited from the valence band to the conduction band of a semiconductor, falls back into an empty state in the valence band, which is known as a hole. See Band theory of solids
Light with photon energies greater than the band gap can be absorbed by the crystal, exciting electrons from the filled valence band to the empty conduction band (illus. a). The state in which an electron is removed from the filled valence band is known as a hole. It is analogous to a bubble in a liquid. The hole can be thought of as being mobile and having positive charge. The excited electrons and holes rapidly lose energy (in about 10-12 s) by the excitation of lattice phonons (vibrational quanta). The excited electrons fall to near the bottom of the conduction band, and the holes rise to near the top of the valence band, and then on a much longer time scale (of 10-9 to 10-6 s) the electron drops across the energy gap into the empty state represented by the hole. This is known as electron-hole recombination. An energy approximately equal to the band gap is released in the process. Electron-hole recombination is radiative if the released energy is light and nonradiative if it is heat. See Phonon
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Electron-hole recombination requires an excited semiconductor in which both electrons and holes occupy the same volume of the crystal. This state can be produced by purely electrical means by forward-biasing a pn junction. The current passing through a pn diode in electrons per second equals the rate of electron-hole recombination (illus. b). A major application of this phenomenon is the light-emitting diode. See Light-emitting diode, Luminescence, Semiconductor diode
Efficient radiative recombination between free electrons and holes takes place only in direct-bandgap semiconductors. During an optical transition, momentum is conserved, and since the photon carries away negligible momentum, transitions take place only between conduction-band and valence-band states having the same momentum. This is easily satisfied in direct-bandgap semiconductors, because electrons and holes collect at the conduction band at minimum and the valence band at maximum, and both extrema have the same momentum. However, for indirect-bandgap semiconductors, the conduction-band minimum and valence-band maximum have very different momenta, and consequently optical transitions between free electrons and holes are forbidden. Radiative electron-hole recombination is possible in indirect-band-gap semiconductors when the transition is assisted by lattice phonons and impurities. See Crystal
Apart from its application in light-emitting diodes and laser operation, radiative recombination, especially at low temperatures (approximately 2 K or -456°F), has been a very important tool for studying the interaction of electrons and holes in semiconductor crystals. See Exciton
Competing with radiative recombination are the nonradiative recombination processes of multiphonon emission and Auger recombination. It is suspected that nonradiative recombination by multiphonon emission drives the movement of atoms at room temperature that are responsible for device degradation phenomena such as the climb of dislocations found in GaAs light-emitting diodes and lasers. Auger recombination has been shown to limit the performance of long-wavelength (1.3–1.6 micrometer) lasers and light-emitting diodes used in optical communication systems. See Auger effect, Laser, Semiconductor
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