Electroluminescence

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Electroluminescence

Luminescence is defined as the emission of optical radiation (ultraviolet, visible, or infrared light rays) as a result of a material being subjected to electronic excitation. Light emitting diodes (LED's) and semiconductor laser devices are examples of semiconductor applications that employ luminescence to perform their intended functions, and are therefore grouped together in the luminescent device family. Note that luminescence does not encompass the phenomenon of optical radiation as a result of the temperature of the emitting material, which is termed as 'incandescence.'

Since luminescent devices involve photons in their operation, they are also known as photonic devices. There are, however, other classes of photonic devices that do not involve luminescence. These photonic devices, which can not be considered as luminescent devices, include: 1) photodetectors, which are devices that detect optical signals; and 2) photovoltaic devices or solar cells, which convert optical radiation into electrical energy.

The variety of colors and broad energy range characterizing the radiation emitted by electroluminescent devices today stem from the discovery of many semiconductor materials that are useful for luminescent applications as well as the development of different techniques for exciting them into luminescence. These resulted in different devices that emit a vast range of wavelengths and energies.

Luminescence may be classified into four (4) categories, depending on its source of input energy: 1) photoluminescence, wherein the excitation is provided by optical radiation; 2) cathodoluminescence, wherein the excitation comes from an electron beam or a cathode ray; 3) radioluminescence, wherein the excitation involves fast particles or high-energy radiation; and 4) electroluminescence, wherein an electric field or current provides the excitation.

Electroluminescence may be triggered in various ways, e.g., intrinsic, avalanche, tunneling and injection processes. Of special interest to this discussion is injection electroluminescence, which results from injection of minority carriers into the region of a p-n junction where radiative transitions occur. A radiative transition is broadly defined as a transition between two states of a molecular entity, with the energy difference being emitted or absorbed as photons.

Photon absorption and emission can occur in three ways: 1) when an electron transitions from a filled state in the valence band to an empty state in the conduction band, in which case a photon is absorbed; 2) when a photon triggers the emission of another photon from an electron that transitions from a filled state in the conduction band to an empty state in the valence band; and 3) when an electron in the conduction band spontaneously returns to an empty state in the valence band, wherein a photon is also emitted in the process.

See Also: Electromagnetic Spectrum

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