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LED light principle


LED (Light Emitting Diode) is a solid-state semiconductor device that can convert electrical energy into visible light. It can directly convert electricity into light. The heart of the LED is a semiconductor chip. One end of the chip is attached to a bracket, one end is the negative electrode, and the other end is connected to the positive electrode of the power supply, so that the entire chip is encapsulated in epoxy resin.

The semiconductor wafer is composed of two parts. One part is a P-type semiconductor, in which holes dominate, and the other end is an N-type semiconductor, where electrons are mainly present. But when these two semiconductors are connected, a P-N junction is formed between them. When current acts on the wafer through the wire, the electrons will be pushed to the P region, where the electrons will recombine with holes, and then energy will be emitted in the form of photons. This is the principle of LED light emitting. The wavelength of light, that is, the color of light, is determined by the material forming the P-N junction.

LED can directly emit red, yellow, blue, green, cyan, orange, purple and white light.

Initially, LEDs were used as indicator light sources for instruments. Later, LEDs of various light colors were widely used in traffic lights and large-area display screens, producing good economic and social benefits. Take a 12-inch red traffic light as an example. In the United States, a long-life, low-light visual efficiency 140-watt incandescent lamp is used as the light source, which produces 2,000 lumens of white light. After passing through the red filter, 90% of the light is lost, leaving only 200 lumens of red light. In the newly designed lamp, Lumileds uses 18 red LED light sources, which consume a total of 14 watts of power including circuit losses to produce the same light effect. Automotive signal lights are also an important area for LED light source applications.

For general lighting, people need white light sources. In 1998, the white-light-emitting LED was successfully developed. This kind of LED is made by packaging GaN chips and yttrium aluminum garnet (YAG). The GaN chip emits blue light (λp=465nm, Wd=30nm), and the YAG phosphor containing Ce3+ made by high-temperature sintering emits yellow light after being excited by the blue light, with a peak value of 550nLED lamp m. The blue LED substrate is installed in a bowl-shaped reflective cavity and covered with a thin layer of resin mixed with YAG, about 200-500nm. Part of the blue light emitted by the LED substrate is absorbed by the phosphor, and the other part of the blue light is mixed with the yellow light emitted by the phosphor to obtain white light.

For InGaN/YAG white LEDs, by changing the chemical composition of the YAG phosphor and adjusting the thickness of the phosphor layer, white light of various colors with a color temperature of 3500-10000K can be obtained. This method of obtaining white light through blue LED has a simple structure, low cost, and high technological maturity, so it is the most used.

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