Fifty years ago, people already knew the basic knowledge of semiconductor materials that can produce light. The first commercial diode was produced in 1960. LED is the abbreviation of light emitTIng diode (light emitting diode) in English. Its basic structure is a piece of electroluminescent semiconductor material, placed on a leaded rack, and then sealed with epoxy resin around it to protect the internal core wire. Function, so the seismic performance of LED is good.
The core part of the light-emitting diode is a wafer composed of a p-type semiconductor and an n-type semiconductor. There is a transition layer between the p-type semiconductor and the n-type semiconductor, which is called a pn junction. In the PN junction of some semiconductor materials, the injected minority carriers and majority carriers will release excess energy in the form of light, thereby directly converting electrical energy into light energy. The reverse voltage is added to the PN junction, and minority carriers are difficult to inject, so they do not emit light. This kind of diode made by the principle of injection electroluminescence is called light-emitting diode, commonly known as LED. When it is in the forward working state (that is, the forward voltage is applied to both ends), when the current flows from the anode to the cathode of the LED, the semiconductor crystal emits light of different colors from ultraviolet to infrared. The intensity of the light is related to the current.
Characteristics of LED light source
1. Voltage: LED uses low-voltage power supply, the power supply voltage is between 6-24V, depending on the product, so it is a safer power supply than the use of high-voltage power supply, especially suitable for public places.
2. Efficiency: Energy consumption is reduced by 80% compared to incandescent lamps with the same light efficiency
3. Applicability: very small, each unit LED chip is 3-5mm square, so it can be prepared into various shapes of devices, and is suitable for a variable environment
4. Stability: 100,000 hours, the light attenuation is 50% of the initial
5. Response time: the response time of incandescent lamps is in the order of milliseconds, and the response time of LED lamps is in the order of nanoseconds
6. Environmental pollution: no harmful metal mercury
7. Color: The color can be changed by changing the current. The light-emitting diode can easily adjust the energy band structure and band gap of the material through chemical modification methods to achieve red, yellow, green, blue and orange multi-color light emission. For example, when the current is small, the LED is red, and as the current increases, it can turn into orange, yellow, and finally green
8. Price: The price of LEDs is more expensive. Compared with incandescent lamps, the price of a few LEDs can be comparable to the price of an incandescent lamp. Usually, each group of signal lamps needs to be composed of 300 to 500 diodes.
Types and development history of monochromatic LED
The earliest LED light source made using the principle of semiconductor PN junction light emission came out in the early 1960s. The material used at that time was GaAsP, which emitted red light (λp = 650nm). When the driving current was 20 mA, the luminous flux was only a few thousandths of lumens, and the corresponding luminous efficiency was about 0.1 lumens / watt.
In the mid-1970s, the elements In and N were introduced to make the LED produce green light (λp = 555nm), yellow light (λp = 590nm) and orange light (λp = 610nm), and the light efficiency was also increased to 1 lumen / watt.
In the early 1980s, GaAlAs LED light sources appeared, which made the red LED's light efficiency reach 10 lumens / watt.
In the early 1990s, the development of two new materials, GaAlInP that emits red and yellow light, and GaInN that emits green and blue light, led to a significant increase in the light efficiency of LEDs. In 2000, the light efficiency of LEDs made by the former in the red and orange regions (λp = 615nm) reached 100 lumens / watt, while the light efficiency of LEDs made by the latter in the green region (λp = 530nm) could reach 50 lumens / watt.
Monochromatic LED application
At first, LEDs were used as indicator light sources for instrumentation. Later, LEDs of various light colors were widely used in traffic lights and large-area display screens, 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-efficiency 140-watt incandescent lamp was used as the light source. It 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 uses 18 red LED light sources, including circuit losses, and consumes a total of 14 watts to produce the same light effect.
Automotive signal lights are also an important field of LED light source applications. In 1987, China began to install high-level brake lights on cars. Due to the fast response speed of LEDs (nanosecond level), the driver of the trailing vehicle can be informed of the driving situation as early as possible to reduce the occurrence of rear-end collisions.
In addition, LED lights have been used in outdoor red, green, and blue full-color displays, key ring miniature flashlights and other fields.
White LED development
For general lighting, people even need white light sources. 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 produced by high-temperature sintering emits yellow light after being excited by this blue light, with a peak value of 550nm. 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. The blue light emitted by the LED substrate is partially 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 LEDs, by changing the chemical composition of the YAG phosphor and adjusting the thickness of the phosphor layer, various colors of white light with a color temperature of 3500-10000K can be obtained. 
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