When DC voltage is applied, LED can emit monochromatic and discontinuous light, which is one of the electroluminescent effects. Changing the chemical composition of semiconductor materials can make light-emitting diodes emit near ultraviolet, visible or infrared light.
1955, Rubin Blancstein of American radio company (born in 1922) first discovered the infrared radiation of gallium arsenide (GaAs) and other semiconductor alloys. 1962, Nick Holonyak Jr. of General Electric Company (born in 1928) developed the first practical visible light emitting diode.
catalogue
1 LED technology
1. 1 principle
1.2 blue and white LED
1.3 Other colors
1.4 organic light emitting diode, organic light emitting diode
1.5 operating parameters and efficiency
1.6 Several wrong trial methods
Two trade-offs when using led
3LED application
3. 1 List of known LED applications
3.2 Lighting application
3.3LED display kanban
3.4 Multi-touch induction
4 Related references
Related. See Synonyms at related
6 related resources
7 External connection
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Light emitting diode technology
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principle
Light-emitting diode is a special kind of diode. Light-emitting diodes, like ordinary diodes, are composed of semiconductor wafers, and these semiconductor materials will be pre-injected or doped to produce pn junction structure. Like other diodes, the current in light-emitting diodes can easily flow from the P pole (anode) to the N pole (cathode), but it cannot flow in the opposite direction. Two different carriers, holes and electrons, flow from the electrode to the pn junction at different electrode voltages. When holes and electrons meet and recombine, electrons will drop to a lower energy level and release energy in the form of photons.
The wavelength and color of the light it emits are determined by the band gap energy of the semiconductor material that constitutes the pn junction. Because silicon and germanium are indirect band gap materials, the recombination of electrons and holes in these materials is a non-radiative transition, which does not release photons, so silicon and germanium diodes cannot emit light. The materials used in light emitting diodes are all direct band gaps, and these band gaps correspond to light energy in near infrared, visible light or near ultraviolet bands.
In the early stage of development, light emitting diodes using gallium arsenide (GaAs) can only emit infrared light or red light. With the progress of materials science, people have made light-emitting diodes with shorter wavelength and various colors.
The following are inorganic semiconductor materials used in traditional light emitting diodes and their colors:
Alga as)- red and infrared
AlGaP)- green
AlgainP)- High brightness orange, orange, yellow and green.
GaAsP)- Red, orange, yellow
GaP)- red, yellow, green
Gallium nitride (GaN)- green, turquoise, blue
Indium nitride (InGaN)- near ultraviolet, turquoise, blue
Silicon carbide (SiC) (used as substrate)-Blue
Silicon (Si) (used as substrate)-blue (under development)
Sapphire (Al2O3) (used as substrate)-blue
Zinc selenide blue
Diamond (C)- ultraviolet
Aluminum nitride (AlN), aluminum gallium nitride (AlGaN)-ultraviolet rays with far to near wavelengths.
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Blue and white LED
GaN ultraviolet LED 1993, Shuji Nakamura, who worked for Nichia company in Japan at that time, invented a blue LED with commercial application value based on wide band gap semiconductor materials GaN and InGaN, which was widely used in the late 1990' s ... Theoretically, blue LED can produce white light by combining the original red LED with green LED, but now white LED is rarely made in this way.
At present, most white LEDs are made by covering blue LEDs (wavelength 450 nm to 470 nm) with a layer of pale yellow phosphor, which is usually made by grinding cerium-doped yttrium aluminum garnet (Ce3+:YAG) crystals into powder and mixing them in a dense binder. When the LED chip emits blue light, part of the blue light will be effectively converted into yellow light with broad spectrum (spectrum center is about 580nm) by the crystal. (In fact, single crystal Ce-doped YAG is more used as scintillator than phosphor. Because yellow light can stimulate the red and green receptors of the naked eye, and then mix the blue light of the LED itself, it looks like white light, and its color is often called "moonlight white". This method of manufacturing white LED was developed by Nichia Corporation and has been used in the production of white LED since 1996. In order to adjust the color of light yellow light, other rare earth metals, such as terbium or gadolinium, can be used instead of Ce doped in ce3+:YAG, or even part or all of aluminum in YAG.
Based on their spectral characteristics, red and green objects do not look as bright as those illuminated by broad-spectrum light sources.
In addition, due to the change of production conditions, the color temperature of LED products is not uniform, ranging from warm yellow to cool blue, so it will be distinguished according to its characteristics in the production process.
Another way to make white LEDs is a bit like fluorescent lamps. The LED emitting near ultraviolet light will be coated with a mixture of two phosphors, one is europium emitting red and blue light, and the other is copper and aluminum emitting green light, doped with zinc sulfide (ZnS). However, due to ultraviolet rays, the quality of epoxy resin in adhesive will deteriorate, so it is difficult to produce and has a short life. Compared with the first method, it has lower efficiency and generates more heat (because of the large Stokes shift), but its advantages are better spectral characteristics and more beautiful light. Because the LED power of ultraviolet light is higher, its efficiency is lower than that of the first method, but its brightness is similar.
The latest method of manufacturing white LED does not use fluorescent powder. A new method is to grow zinc selenide epitaxial layer on zinc selenide (ZnSe) substrate. When energized, the active region will emit blue light, and the substrate will emit yellow light, which is white light when mixed.
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Other colors
Recently developed LED colors are pink and purple, which are all caused by covering one or two layers of phosphor on the blue LED. The first layer of phosphor used for pink LED can emit yellow light, while the second layer emits orange or red light. While phosphors for purple LEDs emit orange light. Other manufacturing methods of pink led have some problems. For example, some pink LEDs are coated with fluorescent paint or nail polish on blue LEDs, but they may peel off; Others use white led with pink fluorescent powder or dye, but the color will fade in a short time.
In terms of price, ultraviolet, blue, pure green, white, pink and purple LEDs are more expensive than red, orange, green, yellow and infrared LEDs, so the former is inferior in business.
Light-emitting diodes are encapsulated in plastic lenses, which are stronger than glass bulbs or fluorescent lamps. Sometimes these packages are colored, but this is just for decoration or comparison, and it can't actually change the color of LED.
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Organic light emitting diode
Combined with the spectral characteristic curves of blue, yellow-green (grass green) and high brightness red LED, the bandwidth of the three primary colors in FWHM spectrum is about 24 nm-27 nm. Main products: organic light-emitting semiconductors.
The materials used in organic light-emitting diodes are crystalline organic molecules or polymer materials, and LEDs made of the latter have flexible characteristics. Compared with traditional light-emitting diodes, organic light-emitting diodes have higher brightness and are expected to be used to manufacture cheap flexible display screens, lighting equipment, luminous clothes or decorative walls in the future. Since 2004, organic light emitting diodes have been widely used in portable MP3 players.
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Operating parameters and efficiency
Generally speaking, the most common LED operating power is set below 30 to 60 milliwatts. 1999 introduced commercial-grade LED, which can be used continuously under the power input of1w. These LEDs use oversized semiconductor chips to deal with the problem of high power input, and the semiconductor chips are fixed on metal iron sheets to help dissipate heat. In 2002, 5-watt LED appeared on the market, and the efficiency was about 18 to 22 lumens per watt.
In September 2003, Cree, Inc. demonstrated its new blue LED, which achieved 35% lighting efficiency at 20 mA. They also made a white LED product with 65 lumens per watt, which was the brightest white LED on the market at that time. In 2005, they demonstrated the prototype of white LED, which achieved a recording efficiency of 70 lumens per watt in a working environment of 350 mA. [ 1]
Nowadays, the working efficiency of organic light-emitting diodes is much lower than that of ordinary LEDs, and the highest is only about 10%. However, the production cost of organic light emitting diodes is much lower. For example, a large array of organic light-emitting diodes can be placed on a screen by a simple printing method to manufacture a color display screen.
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Several wrong attempts
The most common failure mode of led (and diode laser) is to gradually reduce the light output efficiency and loss. However, when the active region is well degraded, sudden failure may occur. Mechanism, radioactive recombination occurs, dislocation participates in nucleation and growth; This requires the presence of existing defects in the crystal and is accelerated by heat, high current density and emitted light. Gallium arsenide and aluminum gallium arsenide are more susceptible to this mechanism than gallium arsenide phosphide, indium gallium arsenide phosphide and indium phosphide. Because the products of active region, gallium nitride and indium gallium nitride are different, it is shameless to this defect; However, high current density may lead to the electromigration of atoms in the active region, and dislocations and point defects act as non-radiative recombination centers, leading to the substitution of light by external heat. Ionizing radiation may lead to this kind of creative defect, which leads to the problem that the radiation reinforced circuit contains led (that is, in the optical isolator). Early red light-emitting diodes are usually known for their short life. White led uses one or more yellow phosphorus. Yellow phosphorus is often degraded by heating and aging, losing its efficiency and causing the color of the obtained light to change. High current at elevated temperature may cause metal atoms to diffuse from the electrode to the active region. Some materials, especially indium tin oxide and silver, rely on electromigration. In some cases, especially for GaN/InGaN diodes, the use of barrier metal layers will hinder electromigration. Mechanical stress, high tide and corrosive environment may lead to the formation of whiskers, which may lead to short circuit. High-power light-emitting diodes are easily affected by current congestion, and the current density at the connection point is uneven. This is also the generation of local hot spots, which forms the risk of heat escape. The inhomogeneity in the matrix leads to the localized loss of thermal conductivity, which aggravates this situation. The most * * * is that the gap is caused by electromigration, Kirkendall is not completely welded, or. Heat dissipation is LED fault. The same laser channel of the laser diode will also be turned off. O depends on catastrophic optical damage, when the light output exceeds an important level and the molten plastic wraps the facet. Some materials are often yellow, which is caused by heat, resulting in partial efficiency absorption (and thus loss). Wavelength. Sudden failures are usually caused by thermal stress. Danghuan
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Advantages and disadvantages of using led
Looking closely at a typical LED, we can see its internal structure. Unlike incandescent bulbs, regardless of the electronic polarity, LEDs will only light up when the electronic polarity is positive. When the voltage direction at both ends of the pn junction is correct, there will be significant power flow, and the equipment is regarded as forward bias. If the voltage polarity is wrong, it is considered that the device is eccentric in reverse, the current is small and it does not emit light. Led can work under AC voltage, but they can only light up under the front voltage. Causing the LED to rotate intermittently. The correct polarity of LED is usually determined by AC frequency. The following: symbol:+-polarity: positive polarity terminal: anonymous wiring: red and black pin arrangement is not an accurate method to determine the polarity when looking inside the LED. : longshortentinor: small large shape: round flat marking: none stripe it should be noted. When most LEDs are "-",this is a "+"terminal in some cases. Flat tabs or short pins are a more accurate way to determine the current characteristics of the voltage on the LED, which is very similar to polarity. Because any diode (approximately exponential). Small voltage changes will lead to great changes in the trend. The deviation in the increase process means that the voltage source is also trapped in X, which hardly causes the LED to take out another type from its maximum rating and may destroy it. Because the logarithm of voltage is related to the trend that can be considered to be basically constant within the working range of light emitting diodes. Therefore, it can be considered that power is almost proportional to the trend. When the power supply and LED characteristic power supply change, try to keep the power close to constant, and the power supply should be a current source. If high efficiency is not required (that is, in most display applications), the current source is connected in series with the current limiting resistor as an LED to the voltage source. Most LEDs generally have low reverse breakdown voltage, so they will be damaged by reverse voltage rather than a few volts. Because some manufacturers don't follow the display standard, they should consult the data sheet as much as possible before connecting the LED, or conduct Q test on the LED in series with a resistor at a sufficiently low power supply voltage to avoid reverse failure. If it wants to drive the LED directly from more AC power than it does, then protect the diode from reverse breakdown voltage (or other L