Electroluminescence is a luminescence which arises from electrical influences. The technologically most important type of electroluminescence, as exploited in light-emitting diodes (LEDs), occurs in semiconductor p–n junctions, where an electric current can generate electron–hole pairs. Subsequent recombination of these pairs can lead to the emission of light. The quantum efficiency of such electroluminescence can be very high in direct band gap semiconductors with a low density of crystal defects: on average, nearly one photon is obtained for each electron–hole pair or for each electron sent into the device. The photon energy and thus the color of the emitted light are essentially determined by the bandgap energy of the material.
There are also electroluminescent displays, where a relatively high voltage is applied two electrodes between which there is some kind of organic or inorganic electroluminescent material (a phosphor). One electron, obtaining a much higher energy than in an LED, can lead to the emission of multiple photons. One electrode needs to be transparent or structured for extracting the generated light.
A different kind of electroluminescence, called cathodoluminescence, occurs when an electron beam hits some luminescent material (e.g. a phosphor). The impinging electrons allow the transfer of electrons from the valence band to the conduction band, i.e., the creation of electron–hole pairs. The subsequent photon emission works as described above.
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