First, the structure and luminous principle of LED
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 LED structure diagram is shown in the following figure.
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 chip is 3-5mm square, so it can be made into devices with various shapes and is suitable for changing environments.
4. Stability: 654.38+100000 hours, and light attenuation is 50% of the initial value.
5. Response time: The response time of incandescent lamp is millisecond, and that of LED lamp is nanosecond.
6. Environmental pollution: no harmful metal mercury.
7. Color: the color can be changed by changing the current, and the energy band structure and band gap of the material can be easily adjusted by chemical modification of light-emitting diodes, so as to realize multi-color emission of red, yellow, green, blue and orange. For example, an LED that is red at low current can turn orange, yellow and finally green with the increase of current.
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.
Three. Types and development history of monochromatic LED
The earliest LED light source based on the principle of semiconductor pn junction was produced 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
At first, LED was used as the indicating light source of instruments. 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.
Verb (abbreviation of verb) Development of white LED
For general lighting, people need a 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. (as shown in the figure below)
Table 1 lists the current types of white LEDs and their luminous principles. At present, the first product that has been commercialized is blue light monolithic plus YAG yellow phosphor, and its best luminous efficiency is about 25 lumens/watt. YAG is mostly imported from Japan, Japan and Asia, and the price is 2000 yuan/kg. The second is that Sumitomo Electric Company of Japan has also developed a white LED of ZnSe, but the luminous efficiency is poor.
It can also be seen from the table that some kinds of white LED light sources are inseparable from four kinds of phosphors: tricolor rare earth red, green and blue powder and garnet structure yellow powder. In the future, it is expected to use three-wavelength light, that is, inorganic ultraviolet chip and R.G.B trichromatic phosphor to package LED white light. It is expected that there will be commercial opportunities for three-wavelength white LEDs this year. However, the tricolor phosphor here requires smaller particle size and higher stability, and its specific application is still being explored.
Table 1 Types and Principles of White LED
Chip number
excitaton source
luminescent material
Luminous principle
1
Blue LED
InGaN/YAG
The blue light of InGaN is mixed with the yellow light of YAG to form white light.
Blue LED
InGaN/ phosphor
Red, green and blue phosphors excited by InGaN blue light 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.
Ultraviolet light emitting diode
InGaN/ phosphor
The ultraviolet excited red, green and blue phosphors of InGaN emit white light.
2
Blue LED
Yellow-green LED
InGaN、GaP
Two chips with complementary colors are packaged together to form a white LED.
three
Blue LED
Green LED
Red LED
Ingan
AlInGaP
Three kinds of small pieces emitting three primary colors are packaged together to form a white LED.
multiple
Light-emitting diode with multiple light colors
InGaN、GaP
AlInGaP
Various optical chips all over the visible area are packaged together to form a white LED.
Using LED light source for lighting will first replace incandescent lamps that consume electricity, and then gradually enter the whole lighting market, which can save a lot of electricity. Recently, the power consumption of white LED has reached more than 1 watt, and the light output is 25 lumens, which also increases its practicability. Table 2 and Table 3 list the efficiency progress of white LED.
Table 2 progress of energy efficiency of single white light L ED
age
Luminous efficiency (lumen/watt)
comment
1998
five
199
15
Similar to incandescent lamp
200 1
25
Similar to tungsten halogen lamp
2005
50
estimate
Table 3 Long-term development goals
Single white LED
input power
10W
luminous efficiency
100 lumen/watt
Output light energy
1000 lumens/watt
Overview of intransitive verbs industry
Among LED manufacturers, Nichia Chemical took the lead in developing high-brightness LEDs with different wavelengths and blue-violet semiconductor lasers (laser diodes; ; LD) is a heavyweight enterprise with blue LED patent right in the industry. After obtaining a number of basic patents such as blue LED production and electrode structure, Japan Asia Chemical insisted on not providing authorization to the outside world, only adopting its own production strategy, with the intention of monopolizing the market, which LED to the high price of blue LED. However, other manufacturers with capacity are quite disapproving. Some Japanese LED manufacturers believe that Japan's strategy of Asian Chemistry will gradually make Japan take the lead in the blue-and-white LED competition of LED manufacturers in Europe, America and other countries, causing serious damage to the overall LED industry in Japan. Therefore, many manufacturers try their best to develop and produce blue LED. At present, in addition to Japan's Asia Chemical and Sumitomo Electric, Toyota Synthetic, Romu, Toshiba and Sharp, Cree of the United States, the world's three major lighting factories, Singularity, Philips, OSRAM, Hewlett-Packard, Siemens, Research and EMCORE, have all invested in the research and development of this product, which has played a positive role in promoting the industrialization and marketization of white LED products.