The LED (Light Emitting Diode) , a Light-Emitting Diode, is a solid semiconductor device that converts electrical energy into visible Light. It converts electricity directly into Light. The heart of an LED is a semiconductor chip with one end attached to a bracket, one end connected to a negative electrode, and the other end connected to a positive electrode of the power supply, making the chip epoxy.

A semiconductor chip is made up of two parts, one part is a P type semiconductor, in which the holes dominate, the other part is an N type semiconductor, on this side are mainly electrons. But when the two semiconductors are connected, a P-N junction is formed between them. When an electric current flows through the wire on the chip, the electrons are pushed toward the p region, where they are combined with the holes, and they emit energy in the form of photons, which is how leds glow. The wavelength of light, the color of light, is determined by the material that forms the P-N junction.

LED can directly issued red, yellow, blue, green, green, orange, purple, white light.

At first, LED was used as the indicating light source of instruments and meters. Later, LED with various light colors was widely used in traffic lights and large area display screen, which produced good economic and social benefits. Take, for example, the 12 inch red traffic light, which in the United States is powered by a long lived, low light efficiency 140 watt incandescent that produces 2,000 lumens of white light. After the red filter, light loss of 90% , leaving only 200 lumens of red light. In the new design of the lamp, Lumileds company used 18 red LED light source, including circuit losses, a total of 14 watts, can produce the same light effect. Automobiles signal lamp is also an important field of LED light source application.

For general lighting, people need more white light source. In 1998, White LED was successfully developed. The LED is made by encapsulating the GAN chip with yttrium aluminum garnet (YAG) . GAN chips emit blue light (λp = 465nm, WD = 30nm) , and the high temperature sintered YAG phosphor containing CE 3 + emits yellow light with a peak value of 550nLED m. Blue LED substrate installed in a bowl-shaped cavity, covered with YAG resin thin layer, about 200-500nm. The blue light emitted by the LED substrate is partly absorbed by the phosphor, and the other part of the blue light is mixed with the yellow light emitted by the phosphor to obtain the white light.

For InGaN/YAG white LED, by changing the chemical composition of YAG phosphor and adjusting the thickness of phosphor layer, white light with color temperature of 3500-10000K can be obtained. The method of obtaining white light through blue LED is simple in construction, low in cost and high in technical maturity, so it is used most.