LED It is a kind of semiconductor diode, can convert electric energy into light energy. Light-emitting diodes and ordinary diodes are composed of a PN junction, but also has a one-way conductivity. When the forward voltage is applied to the light emitting diodes, the holes injected from the P region into the N region and the electrons injected from the N region into the P region are separated from the electrons of the N region and the P region Hole complex, resulting in spontaneous emission of fluorescence. The energy states of electrons and holes in different semiconductor materials are different. When the electrons and holes combined when the release of the energy of the number of different, the more energy released, the shorter the wavelength of light emitted. Commonly used is red, green or yellow diodes. The reverse breakdown voltage of the LED is greater than 5 volts. Its positive volt-ampere characteristic curve is very steep and must be used in series to limit the current through the diode.
White LED: Most of the white LEDs currently produced are made by covering a layer of light yellow phosphor on a blue LED (near-UV, wavelengths from 450 nm to 470 nm). This yellow phosphor is usually made by mixing Cerium yttrium aluminum garnet (Ce3 +: YAG) crystals are ground into powder and mixed in a dense adhesive. When the LED chip is blue, part of the blue light will be very efficient conversion of this crystal into a broad spectrum (spectral center of about 580nm) mainly yellow light. (In fact, the monocrystalline Ce-doped YAG is treated as a scintillator than the phosphor.) Since the yellow light stimulates the red and green receptors in the naked eye and then blends the blue light of the LED itself, it looks like white light , And its color is often referred to as "moonlight white". To adjust the color of light yellow light, other rare earth metal terbium or gadolinium can be used to replace cerium (Ce) doped with Ce3 +: YAG, or even to replace part or all of aluminum in YAG. Based on the characteristics of its spectrum, red and green objects in this LED irradiation will not look like the broad spectrum of light when the light so clear. In addition, due to the variation of production conditions, the color temperature of the finished product of this LED is not uniform, from warm yellow to cold blue, so in the production process will be out of its characteristics to distinguish.
Another method of making white LED is a bit like a fluorescent lamp, emitting near-ultraviolet LED will be coated with two kinds of phosphor mixture, one is red and blue light europium, the other is green, Copper and aluminum doped with zinc sulfide (ZnS). But because the UV will make the adhesive in the epoxy resin cracking deterioration, so the production is more difficult, and life is shorter. Compared with the first method, it is less efficient and produces more heat (because StokesShift the former is larger), but the advantage is better spectral characteristics, the light produced relatively well. And because the UV light LED power is high, so its efficiency is relatively low compared to the first method, the brightness is similar.
The latest method of making white LEDs is no longer used on phosphors. The new approach is to grow the epitaxial layer of zinc selenide on zinc selenide (ZnSe) substrates. When the active area will be issued when the blue light and the substrate will be yellow, mixed together is white light.
LED unidirectional continuity: LED can only turn on one direction (power), called forward bias (forward bias), when the current flows, the electrons and holes in which complex and issued monochromatic light, This is called electroluminescence effect, and the wavelength of light, color and the type of semiconductor materials used with the incorporation of elemental impurities. With high efficiency, long life, easy to break, high switching speed, high reliability and other traditional light source advantages. White LED luminous efficiency, in recent years has been significantly improved, at the same time, in the purchase price per thousand lumens, but also because of the market competition among manufacturers, and significantly decreased. Although more and more people use LED lighting for office, furniture, decoration, signs and even street light use, but in technology, LED in the photoelectric conversion efficiency (effective illumination on the ratio of electricity consumption) is still lower than the new fluorescent lamp, After the country to develop civilian whereabouts.