Fifty years ago, people knew the basic knowledge that semiconductor materials can emit light. The first commercial diode was produced in 1960. LED is the abbreviation of English light emitting diode. Its basic structure is a piece of electroluminescent semiconductor material, which is put on the shelf with leads and then sealed with epoxy resin to protect the internal core wire, so the led has good seismic performance. The core of LED is a wafer composed of P-type semiconductor and N-type semiconductor. There is a transition layer between P-type semiconductor and N-type semiconductor, which is called pn junction. In the PN junction of some semiconductor materials, when the injected minority carriers recombine with the majority carriers, the excess energy will be released in the form of light, thus directly converting electric energy into light energy. When reverse voltage is applied to PN junction, minority carriers are difficult to inject, so they do not emit light. This kind of diode made by injection electroluminescence principle is called light emitting diode, commonly known as led. When working in the forward direction (that is, DC voltage is applied at both ends), when the current flows from the anode of the LED to the cathode, the semiconductor crystal emits different colors of light from ultraviolet to infrared, and the intensity of light is related to the current. Second, the characteristics of LED light source 1. Voltage: LED uses low-voltage power supply, the power supply voltage is between 6-24V, which varies according to different products, so it is safer than using high-voltage power supply, especially suitable for public places. 2. Efficiency: Compared with incandescent lamps with the same light efficiency, the energy consumption is reduced by 80%. 3. Applicability: very small, each unit LED die is 3-5mm square, so it can be made into devices with various shapes, which is suitable for changing environments. 4. Stability: 65,438+million hours, and the light attenuation is 50% of the initial value. 5. Response time: The response time of incandescent lamp is millisecond. The response time of LED lamp is nanosecond. 6. Environmental pollution: no harmful metal mercury. 7. Color: You can change the color by changing the current. LED can easily adjust the band structure and band gap of materials by chemical modification, and realize multicolor emission of red, yellow, green, blue and orange. For example, when the current is low, the red LED will turn orange, yellow and finally green. 8. Price: led is more expensive. Compared with incandescent lamps, the price of several LEDs can be equivalent to the price of an incandescent lamp. Usually, each group of signal lamps needs to be composed of 300 ~ 500 diodes. Third, the types and development history of monochromatic LEDs The earliest LED light source made of semiconductor P-N junction light emitting principle came out in the early 1960s. The material used at that time was GaAsP, which emitted red light (λp=650nm). When the driving current is 20 mA, the luminous flux is only a few thousandths of lumens, and the corresponding luminous efficiency is about 0. 1 lumen/watt. In the mid-1970s, the introduction of In and N elements led to green light (λp=555nm), yellow light (λp=590nm) and orange light (λp=6 10nm), and the light efficiency was also improved to 1 lumen/watt. In the early 1980s, GaAlAs LED light source appeared, which made the light efficiency of red LED reach 10 lumen/watt. In the early 1990s, two new materials, GaAlInP emitting red and yellow light and GaInN emitting green and blue light, were successfully developed, which greatly improved the light efficiency of LED. In 2000, the light efficiency of the LED manufactured by the former reached 100 lumens/watt in red and orange regions (λp=6 15nm), while the light efficiency of the LED manufactured by the latter reached 50 lumens/watt in green regions (λp=530nm). Fourth, the application of monochromatic light LED Initially, LED was used as the indicating light source of instruments and meters. Later, LEDs with various colors were widely used in traffic lights and large-area display screens, which produced good economic and social benefits. Take 12 inch red traffic light as an example. In the United States, 140 watt incandescent lamp with long life and low light efficiency was originally used as the light source to generate white light with 2000 lumens. After passing through the red filter, 90% of the light is lost, leaving only 200 lumens of red light. In the newly designed lamps, Lumileds Company adopted 18 red LED light sources, including circuit loss, * * * consumption 14 watt, which can produce the same light effect. Car signal lamp is also an important field of LED light source application. 1987, China began to install high-position brake lights on cars. Because of the fast response speed of LED (nanosecond level), the driver behind the car can know the driving situation as soon as possible and reduce the occurrence of rear-end collision. In addition, LED lights have been used in outdoor red, green and blue full-color display screens, keychain miniature flashlights and other fields. V. Development of White LED For general lighting, people need white light source more. 1998 white LED has been successfully developed. This kind of LED is encapsulated by GaN chip and yttrium aluminum garnet (YAG). GaN chip emits blue light (λp=465nm, Wd=30nm), and the high-temperature sintered YAG phosphor containing Ce3+ emits yellow light after being excited by this blue light, with a peak value of 550nm. The blue LED substrate is installed in the bowl-shaped reflection cavity and covered with a thin layer of YAG-mixed resin, 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. Now, for InGaN/YAG white LED, by changing the chemical composition of YAG phosphor and adjusting the thickness of phosphor layer, various white lights of 3500- 10000K can be obtained.