First, the structure of LED and the principle of illumination
Fifty years ago, people learned about the basics of semiconductor materials that produced light. The first commercial diode was born in 1960. LED is the abbreviation of English light emitting diode. Its basic structure is an electroluminescent semiconductor material placed on a leaded shelf and then sealed with epoxy resin to protect the inner core. The role, so the LED's seismic performance is good.
The core of the LED is a wafer consisting 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, called a pn junction. In some PN junctions of semiconductor materials, the injected minority carriers recombine with the majority carriers to release excess energy in the form of light, thereby directly converting electrical energy into light energy. The PN junction adds a reverse voltage, and minority carriers are difficult to inject, so they do not emit light. Such a diode fabricated by the principle of injection electroluminescence is called a light-emitting diode, and is generally called an LED. When it is in the forward working state (ie, the forward voltage is applied to both ends), when the current flows from the anode of the LED to the cathode, the semiconductor crystal emits light of different colors from ultraviolet to infrared, and the intensity of the 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, depending on the product, so it is a safer power supply than using high voltage power supply, especially suitable for public places.
2. Performance: 80% less energy consumption than incandescent lamps with the same efficacy
3. Applicability: Very small, each unit LED small piece is 3-5mm square, so it can be prepared into various shapes and suitable for variable environment.
4. Stability: 100,000 hours, the light decay is the initial 50%
5. Response time: the response time of its incandescent lamp is millisecond, and the response time of LED light is nanosecond.
6. Environmental pollution: no harmful metal mercury
7. Color: Change the current to change color. The light-emitting diode can be easily adjusted by chemical modification method to adjust the energy band structure and band gap of the material to realize multi-color luminescence of red, yellow, green and blue. For example, a red LED with a small current can change to orange, yellow, and finally green as the current increases.
8. Price: The price of LED is relatively expensive. Compared with incandescent lamps, the price of several LEDs can be comparable to the price of an incandescent lamp. Usually, each group of signal lamps needs to consist of 300 to 500 diodes.
Third, the type of monochromatic LED and its development history
The earliest application of the LED light source made by the principle of semiconductor PN junction light was introduced in the early 1960s. The material used at that time was GaAsP, which emitted red light (λp=650 nm). 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 lm/W.
In the mid-1970s, the elements In and N were introduced to produce green light (λp = 555 nm), yellow light (λp = 590 nm) and orange light (λp = 610 nm), and the luminous efficiency was also increased to 1 lm/W.
By the early 1980s, the LED light source of GaAlAs appeared, making the red LED's luminous efficiency reach 10 lumens per watt.
In the early 1990s, the development of two new materials, GaAlInP, which emits red light and yellow light, and GaInN, which emits green and blue light, have greatly improved the light efficiency of LEDs. In 2000, the former made LEDs in the red and orange regions (λp = 615 nm) with a luminous efficacy of 100 lm/W, while the latter made LEDs with a luminous efficacy of 50 lumens in the green region (λp = 530 nm). watt.
Fourth, the application of monochromatic LED
Originally used as an indicator light source for instrumentation, LEDs of various light colors have been widely used in traffic lights and large-area displays, resulting in good economic and social benefits. Take the 12-inch red traffic light as an example. In the United States, a long-life, low-efficiency 140-watt incandescent lamp was used as the 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 can produce the same light effect.
Automotive signal lights are also an important area for LED light source applications. In 1987, China began to install high-position brake lights on cars. Due to the fast response speed (nanoseconds) of LEDs, drivers of trailing vehicles can be informed of driving conditions and reduce the occurrence of rear-end collisions.
In addition, LED lights have been applied in outdoor red, green, and blue full-color displays, and key-type miniature flashlights.
Fifth, the development of white LED
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=465 nm, Wd=30 nm), and the Ce3+-containing YAG phosphor prepared by high-temperature sintering is excited by the blue light to emit yellow light with a peak of 550 nm. 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. Now, 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 having a color temperature of 3500 to 10000K can be obtained.
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