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LED light emission principle

Update:30 Mar 2018
Summary:

LED (Light Emitting Diode), a light emitting diode, is […]

LED (Light Emitting Diode), a light emitting diode, is a solid-state semiconductor device capable of converting electrical energy into visible light, which can directly convert electricity into light. The heart of the LED is a semiconductor wafer. One end of the wafer is attached to a support, one end is negative, and the other end is connected to the positive electrode of the power supply, so that the entire wafer is encapsulated with epoxy resin.

The semiconductor wafer consists of two parts, one is a P-type semiconductor, in which holes are dominant, and the other end is an N-type semiconductor, which is mainly electrons. When the two semiconductors are connected, a P-N junction is formed between them. When current flows through the wire to the wafer, the electrons are pushed to the P area. In the P area, the electrons recombine with the holes, and then they emit energy in the form of photons. This is the principle of the LED light. The wavelength of light, which is the color of light, is determined by the material that forms the P-N junction.

The LED can emit red, yellow, blue, green, cyan, orange, violet, and white light directly. [1] 

Initially, LED was used as the indicator light source for instrumentation. Later, various light-colored LEDs were widely used in traffic signals and large-area displays, resulting in good economic and social benefits. Take the 12-inch red traffic signal as an example. In the United States, a long-life, low-light-efficient 140 watt incandescent lamp is used as a light source, which produces 2,000 lumens of white light. After passing through the red filter, the light loss was 90%, leaving only 200 lumens of red light. In the newly designed lamp, Lumileds uses 18 red LED light sources, including the circuit loss, a total of 14 watts of power, can produce the same luminous efficiency. Automotive signal lights are also an important area for the application of LED light sources.

For general lighting, people need a white light source. In 1998, the white LED development was successful. The LED is made by encapsulating GaN chips and yttrium aluminum garnet (YAG) together. The GaN chip emits blue light (λp=465 nm, Wd=30 nm), and the Ce3+-containing YAG phosphor produced by high-temperature sintering is excited by this blue light to emit yellow light, and the peak value is 550 n LED lamp m. The blue LED substrate is mounted in a bowl-shaped reflective cavity covered with a thin layer of resin mixed with YAG, approximately 200-500 nm. The blue light emitted by the LED substrate is absorbed by the phosphor, and the other 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 a color temperature of 3500-10000K can be obtained. This method of obtaining white light through a blue LED is simple in construction, low in cost, and high in technology maturity, and therefore it is used most.

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