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, that is, solid packaging, which can protect the internal core wire, so the led has good seismic performance.
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.
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.
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 with a peak value of 550nm after being excited by this blue light. 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. This method of obtaining white light through blue LED is widely used because of its simple structure, low cost and high technical maturity.
In 1960s, scientists and technicians developed LED according to the principle of semiconductor PN junction. The LED developed at that time was made of GaASP material, and its luminous color was red. After nearly 30 years of development, the familiar LED has been able to emit red, orange, yellow, green, blue and other colors of light. However, white LED for lighting has only been developed in recent years. Here, the white LED for lighting is introduced to readers.
1. Relationship between visible light spectrum and LED white light. As we all know, the wavelength range of visible light spectrum is 380 nm ~ 760 nm, which is seven colors of light that human eyes can feel-red, orange, yellow, green, cyan, blue and purple, but these seven colors of light are monochromatic. For example, the peak wavelength of red light emitted by LED is 565 nm. There is no white light in the spectrum of visible light, because white light is not monochromatic light, but synthetic light composed of many monochromatic lights, just as sunlight is composed of seven monochromatic lights, and white light in color TV sets is also composed of three primary colors: red, green and blue. Therefore, in order to make LED emit white light, its spectral characteristics should include the whole visible spectral range. However, under the current process conditions, it is impossible to manufacture an LED with this performance. According to people's research on visible light, the white light that people can see needs at least a mixture of two kinds of light, that is, two-wavelength light (blue light+yellow light) or three-wavelength light (blue light+green light+red light). The above two modes of white light need blue light, so taking blue light has become the key technology to make white light, which is the "blue light technology" pursued by major LED manufacturing companies. At present, there are only a few manufacturers who have mastered "blue light technology" in the world, such as Japan's Nichia Chemical, Japan's Toyota Synthesis, American CREE, and Germany's Osram. Therefore, the promotion and application of white LED, especially the promotion of high brightness white LED in China, still has a process.
2. Technical structure of white LED and white light source. For general lighting, from the process structure, white LED is usually formed by two methods. The first method is to use "blue light technology" and phosphor to form white light; The second is a variety of monochromatic light mixing methods. These two methods have successfully manufactured white light devices. The system for generating white light by the first method is shown in figure 1. In the picture, the LED GaM chip emits blue light (λ p = 465 nm) and is encapsulated with YAG (yttrium aluminum garnet) phosphor. When the phosphor is excited by blue light, it emits yellow light. As a result, blue light and yellow light are mixed to form white light (the structure of LED is shown in Figure 2). The second method is to package chips with different colors of light and produce white light by mixing the colors of light.
3. The application prospect of white LED lighting new light source. In order to illustrate the characteristics of white LED, let's take a look at the current situation of lighting source. Incandescent lamps and tungsten halogen lamps have luminous efficiency of 12 ~ 24 lumens/watt; The luminous efficiency of fluorescent lamps and HID lamps is 50 ~ 120 lumens/watt. White LED: 1998, the light efficiency of white LED is only 5 lumens/watt, 1999, reaching 15 lumens/watt, which is similar to ordinary household incandescent lamps. In 2000, the luminous efficiency of white LED reached 25 lumens/watt, which was similar to that of halogen tungsten lamp. Some companies predict that by 2005, the light efficiency of LED will reach 50 lumens/watt, and by 20 15 years, the light efficiency of LED is expected to reach 150 ~ 200 lumens/watt. At that time, the working current of white LED can reach ampere level. It can be seen that the development of white LED lighting source for writers will become a possible reality.
Although incandescent lamps and tungsten halogen lamps for general lighting are cheap, they have low luminous efficiency (the thermal effect of lamps consumes electricity in vain), short service life and heavy maintenance workload. However, if white LEDs are used as lighting, they not only have high luminous efficiency, but also have a long service life (continuous working time is longer than 10000 hours) and hardly need maintenance. At present, German Hella Company has developed airplane reading lamp by using white LED. A street in Canberra, the capital of Australia, has used white LED as street lighting. China's urban traffic management lights are also replacing the early traffic order indicators with white LEDs. It can be predicted that in the near future, white LED will definitely enter the home to replace the existing lighting.
LED light source has the advantages of using low-voltage power supply, low energy consumption, strong applicability, high stability, short response time, no pollution to the environment, multi-color luminescence and so on. Although the price is more expensive than the existing lighting equipment, it is still considered that there will be existing lighting equipment.
Features and advantages of LED
The inherent characteristics of LED determine that it is the most ideal light source to replace the traditional light source, and it is widely used.
Small capacity
LED is basically a very small chip, encapsulated in epoxy resin, so it is very small and light.
low power consumption
The power consumption of LED is very low. Generally speaking, the working voltage of LED is 2-3.6V. The working current is 0.02-0.03A That is to say, the power consumption does not exceed 0.1w.
Long service life
Under proper current and voltage, the service life of LED can reach 654.38+ million hours.
High brightness, low heat
environmental protection
LED is made of non-toxic materials, unlike fluorescent lamps, which cause pollution and can be recycled.
sturdy and durable
LED is completely encapsulated in epoxy resin, which is stronger than light bulbs and fluorescent tubes. There is no loose part in the lamp body, which makes the LED not easy to be damaged.
Classification of light-emitting diodes
1. According to the luminous color of LED.
According to the luminous color of LED, it can be divided into red, orange, green (yellow-green, standard green and pure green) and blue light. In addition, some LEDs contain chips of two or three colors.
According to whether light-emitting diodes are doped with scatterers, colored or colorless, light-emitting diodes of the above colors can be divided into four types: colored transparent, colorless transparent, colored scattering and colorless scattering. Scattering LEDs and LEDs are used as indicator lights.
2. According to the characteristics of LED light-emitting surface.
According to the characteristics of light-emitting surface of LED, it can be divided into round lamp, square lamp, rectangle, surface LED, side tube and microtubule for surface mounting. The diameter of circular lamp is divided into φ2mm, φ4.4mm, φ5mm, φ8mm, φ 10mm and φ20mm. In foreign countries, light-emitting diodes with a diameter of 3 mm are usually marked as T-1; φ5mm is marked as t-1(3/4); Write φ4.4mm as T- 1( 1/4).
The angular distribution of circular light intensity can be estimated by half-value angle.
From the angle distribution diagram of luminous intensity, there are three types:
(1) high directivity. Generally, it is a pointed epoxy package or a package with a metal reflective cavity, and no scattering agent is added. The half-value angle is 5 ~ 20 or less, which has high directivity and can be used as a local lighting source or combined with a photodetector to form an automatic detection system.
(2) standard type. Usually used as an indicator light, its half-value angle is 20 ~ 45.
(3) Scattering type. This is an indicator lamp with a large viewing angle, with a half-value angle above 45 ~ 90, and a large number of scattering agents.
3. According to the structure of LED
According to the structure of LED, there are full epoxy package, metal-based epoxy package, ceramic-based epoxy package and glass package.
4. According to the luminous intensity and working current
According to the luminous intensity and working current, there are LEDs with ordinary brightness (luminous intensity100 MCD); Luminous intensity between 10 and 100 MCD is called high brightness led. Generally, the working current of LED is in the range of ten milliamps to tens of milliamps, while the working current of low-current LED is below 2mA (the brightness is the same as that of ordinary LED).
In addition to the above classification methods, there are also methods of classification by chip material and classification by function.