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.


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The transition of an electron from the conduction band to the valence band that results in the emission of a photon is known as radiative recombination.  Certain materials, known as direct bandgap semiconductors (DBS), have their radiative recombinations converted primarily into photon emissions, making them efficient for light emission purposes. 


In DBS materials, the momentum of the electrons at the bottom of the conduction band and that of the holes at the top of the valence band are equal.  As such, interband transitions in DBS materials that result in photon emission are highly probable, with the energy of the emitted photons equal to the bandgap energy of the DBS.


Interband transitions are unlikely in indirect bandgap semiconductors (IBS), since their radiative recombination processes involve phonons and other scattering agents. Thus, IBS materials need to be 'altered' in order to enhance their radiative processes. This is usually accomplished by introducing specific impurities (such as nitrogen) into the IBS to form efficient radiative recombination centers within them.


Light-emitting diodes, or LED's, are basically p-n junctions that emit external spontaneous optical radiations (infrared, visible, or ultraviolet) under forward-biased conditions. Visible Light Emitting Diodes, or visible LED's, involve semiconductors with energy bandgaps greater than 1.8 eV, since this is the minimum energy of light to which the human eye is sensitive (λ = 0.7 m). The color of the light emitted by an LED depends on the semiconductor material used, which is characterized by its own energy bandgap.


Relative eye sensitivity, V(λ), the measure of the effectiveness of light in stimulating the human eye, is a function of wavelength, with maximum stimulation attained for λ = 0.555 m, for which V(0.555) = 1.0. The value of V(λ) decreases as λ moves away from 0.555 m to either side, reaching almost zero at λ = 0.39 m and λ = 0.77 m.   The luminosity of radiant energy, on the other hand, describes the effectiveness of the available radiant energy on human vision.     


There are three loss mechanisms that decrease photoemission: 1) absorption of the photons within the LED material itself; 2) fresnel loss, which occurs when the photons pass from one medium type to another (e.g., from LED material to air), causing some photons to be reflected back to the media interface; and 3) critical angle loss, caused by the over-all internal reflection of photons incident to the surface at angles greater than a critical value defined by Snell's Law. This critical angle is about 16 degrees for GaAs and 17 degrees for GaP.


Generally, red LED's are fabricated on substrates of GaAs, which is a direct bandgap semiconductor.  LED's of other common colors (orange, yellow, and green) are usually fabricated on substrates of GaP, which is an indirect bandgap semiconductor. Blue LED's can be manufactured using ZnS or SiC.



See Also:  Electromagnetic Spectrum




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