There are many kinds of LED lights, including LED lighting, LED strips, LED light cups. Buy LED lights, LED energy-saving lamps, LED decorative lights, LED underground lights, LED contour lights, and LED floodlights. . . .
1. The structure and light-emitting principle of LED
50 years ago, people already understood the basic knowledge that semiconductor materials can produce light. The first commercial diode was produced in 1960. LED is the abbreviation of light emitting diode in English. Its basic structure is a piece of electroluminescent semiconductor material, placed on a leaded shelf, and then sealed with epoxy resin to protect the internal core wires. function, so LED has good earthquake resistance.
The LED structure diagram is shown in the figure below
The core part of the light-emitting diode is a wafer composed of p-type semiconductor and n-type semiconductor. There is a A transition layer, called a p-n junction. In the PN junction of some semiconductor materials, when the injected minority carriers recombine with the majority carriers, excess energy is released in the form of light, thereby directly converting electrical energy into light energy. When a reverse voltage is applied to the PN junction, it is difficult for minority carriers to be injected, so it does not emit light. This type of diode made using the principle of injection electroluminescence is called a light-emitting diode, commonly known as LED. When it is in the forward working state (that is, forward voltage is applied to both ends), when the current flows from the LED anode to the cathode, the semiconductor crystal emits light of different colors from ultraviolet to infrared. The intensity of the light is related to the current.
2. Characteristics of LED light source
1. Voltage: LED uses a low-voltage power supply, and the supply voltage is between 6-24V, which varies according to different products, so it is a High voltage power supply is a safer power supply, especially suitable for public places.
2. Efficiency: Energy consumption is 80% less than 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 devices of various shapes and is suitable for variable environments
4. Stability: 100,000 hours, the light decay is the initial 50%
5. Response time: The response time of incandescent lamps is milliseconds, and the response time of LED lamps is nanoseconds
6. Environmental pollution: no harmful metal mercury
7. Color: changing the current can Color-changing, light-emitting diodes can easily adjust the energy band structure and band gap of the material through chemical modification methods to achieve multi-color light emission of red, yellow, green, blue and orange. For example, an LED that is red when the current is small can turn into orange, yellow, and finally green as the current increases
8. Price: LEDs are relatively expensive. Compared with incandescent lamps, they are almost The price of LED alone is equivalent to that of an incandescent lamp, and usually each set of signal lights needs to be composed of 300 to 500 diodes.
3. Types of monochromatic light LEDs and their development history
The earliest LED light source made by applying the semiconductor P-N junction luminescence principle came out in the early 1960s. The material used at that time was GaAsP, which emits red light (λp=650nm). When the driving current is 20 mA, the luminous flux is only a few thousandths of a lumen, and the corresponding luminous efficiency is about 0.1 lumens/watt.
In the mid-1970s, the elements In and N were introduced to make LED produce green light (λp=555nm), yellow light (λp=590nm) and orange light (λp=610nm), and the light efficiency was also increased to 1 Lumens/Watt.
In the early 1980s, GaAlAs LED light sources appeared, making the light efficiency of red LEDs reach 10 lumens/watt.
In the early 1990s, two new materials, GaAlInP, which emits red and yellow light, and GaInN, which emits green and blue light, were successfully developed, greatly improving the light efficiency of LEDs.
In 2000, the luminous efficiency of LEDs made by the former reached 100 lumens/watt in the red and orange areas (λp=615nm), while the luminous efficiency of LEDs made by the latter in the green area (λp=530nm) could reach 50 lumens/watt. watt.
4. Application of monochromatic LEDs
Initially, LEDs were used as indicator light sources for instruments. Later, LEDs of various light colors were used in traffic lights and large-area display screens. It is widely used and has produced good economic and social benefits. Take a 12-inch red traffic light as an example. In the United States, a long-life, low-efficiency 140-watt incandescent lamp is used as the light source, which produces 2,000 lumens of white light. After passing through the red filter, the light is lost by 90%, leaving only 200 lumens of red light. In the newly designed lamp, Lumileds uses 18 red LED light sources, which consume a total of 14 watts of power including circuit losses to produce the same light effect.
Automotive signal lights are also an important area of ??LED light source application. In 1987, my country began to install high-mounted brake lights on cars. Due to the fast response speed of LEDs (nanosecond level), drivers of following vehicles can know the driving conditions early and reduce the occurrence of rear-end collisions.
In addition, LED lights have been used in outdoor red, green, and blue full-color displays, keychain miniature flashlights, and other fields.
5. Development of white light LEDs
For general lighting, people need white light sources. In 1998, the white-light-emitting LED was successfully developed. This kind of LED is made by packaging GaN chips and yttrium aluminum garnet (YAG). The GaN chip emits blue light (λp=465nm, Wd=30nm), and the YAG phosphor containing Ce3 made by high-temperature sintering emits yellow light after being excited by the blue light, with a peak value of 550nm. The blue LED substrate is installed in a bowl-shaped reflective cavity and covered with a thin layer of resin mixed with YAG, 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 LEDs, by changing the chemical composition of the YAG phosphor and adjusting the thickness of the phosphor layer, white light of various colors with a color temperature of 3500-10000K can be obtained. (As shown in the figure below)
Table 1 lists the current types of white LEDs and their light-emitting principles. The first product that has been commercialized is a blue light single chip plus YAG yellow phosphor. Its best luminous efficiency is about 25 lumens/watt. YAG is mostly imported from Japan's Nichia Company, and the price is 2,000 yuan/kg; The second is that Japan's Sumitomo Electric has also developed a white LED using ZnSe as the material, but the luminous efficiency is poor.
It can also be seen from the table that certain types of white LED light sources are inseparable from four kinds of phosphors: namely, the three primary colors of rare earth red, green, blue powder and garnet structure yellow powder, which will be more popular in the future. What is favored is three-wavelength light, which uses inorganic ultraviolet wafers and R.G.B three-color phosphors to encapsulate LED white light. It is expected that three-wavelength white light LEDs will have commercialization opportunities this year. However, the particle size requirements of the three primary color phosphors here are relatively small and the stability requirements are also high. Specific applications are still being explored.
Table 1 Types and principles of white LEDs
Number of chips
Excitation sources
Light-emitting materials
Light-emitting Principle
1
Blue LED
InGaN/YAG
The blue light of InGaN and the yellow light of YAG are mixed into white light
Blue LED
InGaN/Phosphor
The red, green and blue phosphors excited by the blue light of InGaN emit white light
Blue LED
ZnSe
The blue light emitted by the thin film layer and the yellow light excited on the substrate are mixed into white light
UV LED
InGaN/ Phosphors
InGaN's ultraviolet-excited red, green, and blue primary color phosphors emit white light
2
Blue LED
Yellow-green LED
InGaN, GaP
Package two chips with complementary colors together to form a white LED
3
Blue LED
Green LED
Red LED
InGaN
AlInGaP
Three kinds of small chips that emit three primary colors will be packaged Together, they form white LEDs
Multiple
LEDs of multiple light colors
InGaN, GaP
AlInGaP
Packaging a variety of light chips spread across the visible light area together to form white LEDs
Using LED light sources for lighting will first replace power-consuming incandescent lamps, and then gradually enter the entire lighting market. Save a lot of electricity. Recently, white LEDs have reached the point where a single unit consumes more than 1 watt of electricity and has a light output of 25 lumens, which also increases its practicality. Tables 2 and 3 list the performance progress of white LEDs.
Table 2 Performance Progress of Single White LED
Year
Luminous Efficiency (Lumen/Watt)
Remarks
1998
5
199
15
Similar to an incandescent lamp
2001
p>25
Similar to tungsten halogen lamp
2005
50
Estimated
Table Three long-term development goals
Single white LED
Input power
10 watts
Lighting efficiency
100 Lumens/watt
Output light energy
1000 lumens/watt
6. Industry overview
Among the LED industry players, Nichia It was the first to use the above-mentioned technology to develop high-brightness LEDs of different wavelengths and blue-violet semiconductor lasers (Laser Diode; LD). It is a heavyweight player in the industry that holds patent rights for blue LEDs. After Nichia obtained many basic patents such as blue LED production and electrode structure, it insisted on not providing external authorization and only adopted its own production strategy in order to monopolize the market, making blue LED prices high. However, other companies that already have production capacity are quite unconvinced. Some Japanese LED companies believe that Nichia's strategy will gradually allow Japan to lose the upper hand to LED companies in Europe, the United States and other countries in the competition for blue and white LEDs. It has caused serious harm to the overall Japanese LED industry. Therefore, many industries are doing everything possible to develop and produce blue LEDs.
At present, in addition to Nichia Chemical and Sumitomo Electric, there are also Toyoda Gosei, Romu, Toshiba and Sharp, the American company Cree, the world's three major lighting manufacturers GE, Philips, Osram, HP, Siemens, Research, EMCORE, etc. have invested in this product. The R&D and production have played a positive role in promoting the industrialization and marketization of white LED products.