有关LED的毕业论文外文翻译.doc

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Renewable and Sustainable Energy Reviews
High-brightness LEDs—Energy efficient lighting sources and their pos.
The use of light-emitting diodes marks great advancementsover existing indoor agricultural lighting. LEDs allow the control ofspectral composition and the adjustment of light intensity tosimulate the changes of sunlight intensity during the day. Theyhave the ability to produce high light levels with low radiant heatoutput and maintain useful light output for years. LEDs do notcontain electrodes and thus do not burn out like incandescent orfluorescent bulbs that must be periodically replaced. Not tomention that incandescent and fluorescent lamps consume a lot ofelectrical power while generating heat, which must be dispelledfrom closed environments such as spaceships and space stations.
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2. LED development
LED is a unique type of semiconductor diode. It consists of a chipof semiconductor material doped with impurities to create a p–njunction. Current flows easily from the p-side <anode>, to the n-side<cathode>, but not in the reverse direction.
Electrons and holes flow into the junction from electrodes withdifferent voltages. When an electron meets a hole, it falls into alower energy level, and releases energy in the form of a photon. Thecolor <wavelength> of the light emitted depends on the band gapenergy of the materials forming the p–n junction. The materialsused for an LED have a direct band gap with energies correspondingto near-infrared, visible or near-ultraviolet light.
The key structure of an LED consists of the die <or light-emittingsemiconductor material>, a lead frame where the die is placed, andthe encapsulation which protects the die <Fig. 1>.
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LEDdevelopment beganwith infrared and red devicesmade withgalliumarsenide. Advances inmaterials science havemade possiblethe production of devices with ever-shorter wavelengths, producinglight ina varietyof colors. reportedthat thefirst knownlight-em