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LED Basic Knowledge

[1] LED structure and light-emitting principle

People have understood the basic knowledge that semiconductor materials can produce light 50 years ago, and the first commercial diode was produced in 1960. LED is the abbreviation of English light emitting diode (light emitting diode), its basic structure is a piece of electroluminescent semiconductor material, placed on a leaded shelf, and then sealed with epoxy resin around to protect the internal core wire Function, so the seismic performance of the 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 p-n junction. In the PN junction of certain semiconductor materials, when the injected minority carriers and the majority carriers recombine, the excess energy is released in the form of light, thereby directly converting electrical energy into light energy. With reverse voltage applied to the PN junction, it is difficult to inject minority carriers, so it does not emit light. This kind of diode made by injection electroluminescence principle is called light-emitting diode, commonly known as LED. When it is in a positive working state (that is, a positive 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] Features of LED light source

1. Voltage: LED uses a 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 high-voltage power supply, especially suitable for public places.

2. Efficiency: Energy consumption is reduced by 80% compared with incandescent lamps with the same luminous efficiency

3. Applicability: small, each unit LED chip is 3-5mm square, so it can be prepared into various shapes of devices, and is suitable for variable environments

4. Stability: 100,000 hours, the light decay is 50% of the initial

5. Response time: the response time of the incandescent lamp is milliseconds, and the response time of the LED lamp is nanoseconds

6. Environmental pollution: no harmful metal mercury

7. Color: Change the color 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. For example, when the current is small, the LED is red, as the current increases, it can turn into orange, yellow, and finally green.

8. Price: LEDs are more expensive. Compared with incandescent lamps, the price of a few 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.

[3] Types and development history of monochromatic LEDs

The first LED light source made using the principle of semiconductor P-N junction light emission 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 lumens, and the corresponding luminous efficiency is 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 luminous efficiency was also increased to 1 lm/W.

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, which greatly improved the light efficiency of LEDs. In 2000, the LED made by the former had a luminous efficiency of 100 lumens/watt in the red and orange zone (λp=615nm), while the LED made by the latter had a luminous efficiency of 50 lumens/watt in the green zone (λp=530nm). watt.

[4] Application of monochromatic light LED

Initially, LEDs were used as indicator light sources for instruments and meters. Later, LEDs of various light colors were widely used in traffic signal lights and large-area display screens, resulting in 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 was originally used as the light source, which 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 company uses 18 red LED light sources, including circuit losses, a total of 14 watts of power consumption, can produce the same light effect.

Automotive signal lights are also an important field of LED light source applications. In 1987, my country began to install high-position brake lights on cars. Due to the fast response speed of LEDs (nanosecond level), drivers of following vehicles can be informed of driving conditions as early as possible, and the occurrence of rear-end collisions of cars can be reduced.

In addition, LED lights have been used in outdoor red, green, and blue full-color display screens, key ring miniature flashlights and other fields.