electron-hole recombination

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

Recombination of electrons and holes generated by ( a ) optical absorption and ( b ) a forward- biased pn junctionenlarge picture
Recombination of electrons and holes generated by (a) optical absorption and (b) a forward- biased pn junction

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

McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.

electron-hole recombination

[i′lek‚trän ′hōl rē‚käm·bə′nā·shən]
(solid-state physics)
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.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
[18] used [Cu.sup.2+]-Ti[O.sub.2] nanorod thin film photocatalysts reducing C[O.sub.2] into methanol and ethanol obtaining, respectively, 36.18 [micro]mol/g-cat x h and 79.13 [micro]mol/g x cat-h in which [Cu.sup.2+] acted as active sites of electron traps and could suppress the electron-hole recombination allied to high surface area of Ti[O.sub.2] nanorods.
Another major drawback of Ti[O.sub.2] is photogenerated electron-hole recombination, which deteriorates the photocatalytic activity [27].
The aim of this work is to investigate the degradation of the Malachite Green dye (MG), carbinol base, under visible light irradiation, using a simulated sunlight source, in an attempt to correlate the photocatalytic activity of titanium oxide modified with different amounts of [Bi.sub.2][O.sub.3] with their physicochemical properties in particular; the band gap energy, the microstructure, and the electron-hole recombination rate.
The increase in the barrier height and in the depletion width of the Au/BN/C devices with increasing temperature could be ascribed to many physical reasons like the increase in the electron-hole recombination rate, the thermal expansion, and the lattice constants variations with temperature.
Lower resistance of charge pathways and electron-hole recombination rate of photogenerated carriers are necessary for longer electron diffusion length and extending lifetime in Ti[O.sub.2] photoanode and to obtain good collection efficiency [7, 8].
A negative temperature impact is conditioned by increase in thermal electron-hole recombination. During thermal electron-hole recombination light is not emitted and, as the LED temperature grows, illuminance flux decays.
Introducing iron ions into the lattice of Ti[O.sub.2] provides photocatalysts not only with lower electron-hole recombination rate but also with excitability by visible light.
Shockley, "Electron-hole recombination statistics in semiconductors through flaws with many charge conditions," Physical Review, vol.
The process of emitting light is through radiative electron-hole recombination. Recombination is a process by which a conduction band electron loses energy and re-occupies the energy state of an electron hole in the valence band.
The nanocomposite film was exploited in a photoelectrochcmical solar cell, and the results showed that the photocurrent was significantly enhanced compared to the pure Ti[O.sub.2] film, that might be because the presence of Au nanorods suppressed electron-hole recombination at the interface.
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