1, LED light emitting structure and principle
It has been about 50 years ago, the semiconductor material can produce light of the basic knowledge, the first commercial diode produced in 1960. LED is the English light emitting diode (LED) acronym, and its basic structure is a piece of electroluminescent semiconductor materials, placed on a wire rack, and then sealed with epoxy resin around, protect the internal core wire effect, so LED's seismic performance.
The core part of the light-emitting diode is a p-type semiconductor and n-type semiconductor chip formed in the p-type semiconductor and n-type semiconductor there was a transition between, known as the pn junction. In some semiconductor materials, PN junction, the injection of minority carrier and majority carrier recombination extra time to time to the form of light energy released, which convert light energy directly into electrical energy. PN junction reverse voltage, minority carrier injection is difficult, it is not light. This use of principle of injection electroluminescence diodes called LEDs produced, known as LED. When it is in working condition when the positive (that ends with a positive voltage), current flows from the cathode LED anode, the semiconductor crystal on the issue from the ultraviolet to infrared light of different colors of light intensity and current.
2, LED light source characteristics
1. Voltage: LED using low-voltage power supply, power supply voltage between 6-24V, vary according to product, so it is more secure than using high voltage power supply, especially for public places.
2. Effectiveness: consumption of energy efficient than incandescent light with 80% reduction
3. Applicability: small, small piece of each LED unit is 3-5mm square, it can be fabricated into various shapes of the devices, and volatile environment for
4. Stability: 100,000 hours, the light fades to 50% of the initial
5. Response time: the incandescent millisecond response time, LED lights nanosecond response time
6. On the environmental pollution: no harmful metal mercury
7. Color: you can change the current color, light-emitting diodes by chemical modification methods to easily adjust the material band structure and band gap, to achieve multi-color light orange red Huanglv Lan. Such as the small current when the red LED, with the current increase, can turn into orange, yellow, and finally to green
8. Price: LED's are expensive, compared to incandescent lamps, a few LED's price on the price with an incandescent lamp rather, each signal usually require from the previous 300 to 500 diodes.
3, monochromatic LED types and history
The first application of principles of semiconductor PN junction light-emitting LED light source made of 60 came in the early 20th century. The material is then used GaAsP, hair red ( p = 650nm), the drive current of 20 mA, the luminous flux of only a few lumens per thousand, the corresponding luminous efficiency of about 0.1 lumens / watt.
Mid-70s, the introduction of elements of In and N, to generate green LED ( p = 555nm), yellow ( p = 590nm) and orange ( p = 610nm), optical efficiency is also increased to 1 lumen / watt.
To the early 80s, appeared in the LED light source GaAlAs make the red LED light efficiency of 10 lumens / watt.
The early 90s, hair, red and yellow of GaAlInP and hair green, blue GaInN developed two new materials to make LED light effect was greatly improved. In 2000, the former is made of LED in red and orange zone ( p = 615nm) light efficiency to 100 lm / W, which made the green LED in the region ( p = 530nm) light efficiency can reach 50 lm / watts.
Fourth, the application of monochromatic LED
Initially used as instruments of instruction LED light source, then all kinds of light-colored LED in traffic lights and large screens in the widely used and produced good economic and social benefits. To 12-inch red traffic lights, for example, could have been used in the United States long life, low-light 140-watt incandescent effective as the light source, it produces 2000 Lumens of white. The red filter, the optical loss of 90%, only 200 lumens of red light. In light of the new design, Lumileds has used a red LED light source 18, including circuit losses, including a total power of 14 watts of light can produce the same effect.
Automotive LED lights are also important areas of light application. In 1987, China began to install high car brake lights, LED response as fast (nanosecond), can be as early as possible so that the trailing vehicle drivers know that driving performance and reducing rear-end accident.
In addition, LED lights at the outdoor red, green, blue full-color display, mini flashlight key chain have been the areas of application.
Fifth, the development of white LED
For general lighting, the white light that people need. 1998 white light LED developed. This is the GaN LED chip and yttrium aluminum garnet (YAG) made package together. GaN chip of blue ( p = 465nm, Wd = 30nm), made of high-temperature sintering of YAG phosphor containing Ce3 + excited by this blue light after the yellow emission peak 550nm. Blue LED substrate installed in the reflective cavity bowl, cover with a mixture of YAG thin layer of resin, about 200-500nm. Issued blue LED substrate partially phosphor absorbed, and some blue mixed with yellow phosphor issued, can be too white. Now, for the InGaN / YAG white LED, YAG phosphor by changing the chemical composition and adjust the thickness of the phosphor layer, available colors and color temperature 3500-10000K white.
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