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A light-emitting diode ('''LED''') is a Semiconductor Diode that emits Incoherent narrow-spectrum Light when electrically Biased in the forward direction of the P-n Junction . This effect is a form of Electroluminescence . An LED is usually a small area source, often with extra optics added to the chip that shapes its radiation pattern.1 The Color of the emitted light depends on the composition and condition of the semiconducting material used, and can be Infrared , Visible , or near- Ultraviolet . HISTORY In the early 20th century, Henry Round of Marconi Labs first noted that a semiconductor junction could produce light. Russian Oleg Vladimirovich Losev independently created the first LED in the mid 1920s; his research, though distributed in Russian, German and British scientific journals, was ignored.2 Rubin Braunstein of the Radio Corporation Of America reported on infrared emission from Gallium Arsenide (GaAs) and other semiconductor alloys in 1955. Experimenters at Texas Instruments , Bob Biard3 and Gary Pittman, found in 1961 that gallium arsenide gave off infrared (invisible) light when electric current was applied. Biard and Pittman were able to establish the priority of their work and received the patent for the infrared light-emitting Diode . Nick Holonyak Jr. , then of the General Electric Company and later with the University Of Illinois At Urbana-Champaign , developed the first practical visible-spectrum LED in 19624 and is seen as the "father of the light-emitting diode".5 Holonyak's former graduate student, M. George Craford, invented in 1972 the first yellow LED and 10x brighter red and red-orange LEDs.6 for his invention.7 LED TECHNOLOGY Physical function Like a normal Diode , an LED consists of a chip of semiconducting material impregnated, or '' Doped '', with impurities to create a '' P-n Junction ''. As in other diodes, current flows easily from the p-side, or Anode , to the n-side, or Cathode , but not in the reverse direction. Charge-carriers— Electron s and Holes —flow into the junction from electrodes with different Voltage s. When an electron meets a hole, it falls into a lower Energy Level , and releases Energy in the form of a Photon . The Wavelength of the light emitted, and therefore its color, depends on the Band Gap energy of the materials forming the ''p-n junction''. In Silicon or Germanium diodes, the electrons and holes recombine by a ''non-radiative transition'' which produces no optical emission, because these are Indirect Band Gap materials. The materials used for an LED have a Direct Band Gap with energies corresponding to near-infrared, visible or near-ultraviolet light. LED development began with infrared and red devices made with Gallium Arsenide . Advances in Materials Science have made possible the production of devices with ever-shorter Wavelength s, producing light in a variety of colors. LEDs are usually built on an n-type substrate, with an electrode attached to the p-type layer deposited on its surface. P-type substrates, while less common, occur as well. Many commercial LEDs, especially GaN/InGaN, also use Sapphire substrate. Substrates that are transparent to the emitted wavelength, and backed by a reflective layer, increase the LED efficiency. The Refractive Index of the package material should match the index of the semiconductor, otherwise the produced light gets partially reflected back into the semiconductor, where it is absorbed and turned into additional heat, thus lowering the efficiency. In 2007 experiments tried to avoid multiple internal reflection by roughening the chip. Again at the surface from the package to a low refractive index medium like a glass fiber or air total internal reflection is avoided by using a sphere shaped package, with the diode in the center, so that the light rays hit the surface quite perpendicular, and Anti-reflection Coating may be added. The package may be cheap plastic, which may be colored, but this is only for cosmetic reasons or to improve the contrast ratio; the color of the packaging does not substantially affect the color of the light emitted. Conventional LEDs are made from a variety of inorganic Semiconductor Materials , producing the following colors:
With this wide variety of colors, arrays of multicolor LEDs can be designed to produce unconventional color patterns.9 Ultraviolet and blue LEDs GaN LED.]] Blue LEDs are based on the wide Band Gap semiconductors GaN ( Gallium Nitride ) and InGaN (indium gallium nitride). They can be added to existing red and green LEDs to produce the impression of White light, though white LEDs today rarely use this principle. |
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