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LED principle

LED (Light Emitting Diode), a 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 support, one end is the negative pole, and the other end is connected to the positive pole of the power supply, so that the entire chip is encapsulated by 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, which is mainly electrons. But when these two semiconductors are connected, a P-N junction is formed between them. When the current acts on the chip through the wire, the electrons will be pushed to the P area, where the electrons and holes recombine, and then emit energy in the form of photons. This is the principle of LED light emission. The wavelength of light is also the color of light, which is determined by the material forming the P-N junction.

 

Initially, LEDs were used as indicator light sources for instruments and meters. Later, LEDs of various light colors were widely used in traffic signal lights and large-area display screens, resulting in good economic and social benefits. Take a 12-inch red traffic signal light as an example. In the United States, a 140-watt incandescent lamp with long life and low optical efficiency was originally used as the light source, which produces 2000 lumens of white light. After passing through the red filter, the light loss is 90%, leaving only 200 lumens of red light. In the newly designed lamp, Lumileds company uses 18 red LED light sources, including circuit losses, a total of 14 watts of power consumption, can produce the same light effect. Automotive signal lights are also an important field of LED light source applications.

 

For general lighting, people need white light sources more. In 1998, the white LED was successfully developed. This LED is made by encapsulating a GaN chip and Yttrium Aluminum Garnet (YAG) together. The GaN chip emits blue light (λp=465nm, Wd=30nm), and the Ce3+-containing YAG phosphor made by high-temperature sintering is excited by this blue light and emits yellow light with a peak value of 550nLED lamp m. The blue LED substrate is installed in a bowl-shaped reflective cavity, 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 LEDs has simple structure, low cost, and high technological maturity, so it is most used.

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