The principle of LED lights

- May 30, 2019-

LED (Light Emitting Diode), a solid-state semiconductor device that converts electrical energy into visible light, converts electricity directly into light. The heart of the LED is a semiconductor wafer with one end attached to a holder, one end being the negative pole and the other end connected to the positive pole of the power supply, so that the entire wafer is encapsulated by epoxy.


The semiconductor wafer consists of two parts, one part is a P-type semiconductor, in which the hole dominates, and the other end is an N-type semiconductor, which is mainly electrons here. But when the two semiconductors are connected, they form a P-N junction. When a current is applied to the wafer through the wire, the electrons are pushed toward the P region. In the P region, the electrons recombine with the holes, and then the energy is emitted in the form of photons. This is the principle of LED light emission. The wavelength of light, which is the color of light, is determined by the material that forms the P-N junction.


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

The LED was originally used as an indicator light source for instrumentation. Later, various light-colored LEDs were widely used in traffic lights and large-area displays, resulting in good economic and social benefits. Taking a 12-inch red traffic light as an example, in the United States, a 140-watt incandescent lamp with long-life, low-light performance was used as a light source, which produced 2000 lumens of white light. After passing the red filter, the light loss is 90%, leaving only 200 lumens of red light. In the newly designed lamp, Lumileds used 18 red LED light sources, including circuit losses, to consume 14 watts of electricity, which would produce the same light effect. Automotive signal lights are also an important area for LED light source applications.


For general lighting, people need a white light source. In 1998, white LEDs were successfully developed. This LED is made by encapsulating a GaN chip and yttrium aluminum garnet (YAG). The GaN chip emits blue light (λp=465nm, Wd=30nm), and the Ce3+-containing YAG phosphor prepared by high-temperature sintering is excited by the blue light to emit yellow light, and the peak value is 550n 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, about 200-500 nm. The blue portion of the LED substrate is absorbed by the phosphor, and the other portion of the blue light is mixed with the yellow light emitted by the phosphor to obtain white light.


For the InGaN/YAG white LED, by changing the chemical composition of the YAG phosphor and adjusting the thickness of the phosphor layer, white light of various colors having a color temperature of 3500 to 10000 K can be obtained. This method of obtaining white light by a blue LED has the advantages of simple structure, low cost, and high technical maturity, and thus is most used.