A light-emitting diode ( LED ) is a two-way semiconductor light source. This is a p-n connection diode that emits light when activated. When a suitable current is applied to the lead, the electrons can recombine with the electron hole inside the device, releasing energy in the form of photons. This effect is called electroluminescence, and the color of light (corresponding to photon energy) is determined by the bandgap of the semiconductor energy. LEDs are usually small (less than 1 mm 2 ) and integrated optical components can be used to form radiation patterns.
Appearing as a practical electronic component in 1962, the earliest LEDs emit low-intensity infrared light. Infrared LEDs are still often used as transmitting elements in remote control circuits, such as those in remote control for a wide range of consumer electronics. The first LED lights that look have low intensity and are limited to red. Modern LEDs are available along the visible, ultraviolet, and infrared wavelengths, with ultra brightness.
Early LEDs are often used as indicator lights for electronic devices, replacing small incandescent lamps. They are immediately packed into numerical readings in the form of seven segment displays and are generally seen in digital clocks. Recent developments have produced LEDs suitable for environmental and task lighting. LEDs have led to new displays and sensors, while high switching rates are useful in advanced communications technology.
LEDs have many advantages over incandescent light sources, including lower energy consumption, longer lifetime, better physical resistance, smaller size, and faster switching. The light-emitting diodes are used in a variety of applications such as aviation lighting, automotive headlamps, advertisements, general lighting, traffic signals, camera flash, lighted wallpapers and medical devices. They are also significantly more energy efficient and, arguably, have fewer environmental problems associated with their disposal.
Unlike lasers, the light emitted from the LED is not coherent or monochromatic, but the spectrum is narrow with respect to human vision, and for most light purposes of a simple diode element can be regarded as functional monochromatic.
Video Light-emitting diode
History
Findings and startup
Electroluminescence as a phenomenon was discovered in 1907 by the British experiment H. J. Round of Marconi Labs, using crystalline silicon carbide and cat whisker detectors. Russian inventor Oleg Losev reported the first LED manufacture in 1927. His research was distributed in Soviet, German and British scientific journals, but no practical use has been made of the invention for decades.
In 1936, Georges Destriau observed that Electroluminescence could be produced when the zinc sulphide powder (ZnS) was suspended in the insulator and an alternating electric field was applied to it. In its publication, Destriau is often called luminescence as Losev-Light. Destriau works in the laboratory of Madame Marie Curie, also an early pioneer in the field of luminescence with radium research.
Kurt Lehovec, Carl Accardo, and Edward Jamgochian described the first light-emitting diodes in 1951 using a tool that uses SiC crystals with a battery current source or pulse generator and in comparison to a pure, crystal, variant in 1953.
Rubin Braunstein of Radio Corporation of America reported infrared emissions from gallium arsenide (GaAs) and other semiconductor alloys in 1955. Braunstein observed infrared emissions generated by simple diode structures using gallium antimonide (GaSb), GaAs, indium phosphide (InP), and silicon-germanium (SiGe) alloys at room temperature and at 77 Kelvin.
In 1957, Braunstein further pointed out that imperfect devices could be used for non-radio communication at short distances. As expressed by Kroemer Braunstein "... has attached a simple optical communication link: The music emerging from the recording player is used by an appropriate electronics to modulate the forward current from the GaAs diode." The emitted light is detected by the PbS diode some distance. audio amplifiers and rotated by loudspeakers.Cutting blocks stops music.We love playing with these settings. "This setting practices the use of LEDs for optical communication applications.
In September 1961, while working at Texas Instruments in Dallas, Texas, James R. Biard and Gary Pittman discovered near-infrared light emission (900Ã, nm) of the tunnel diodes they had built on the GaAs substrate. In October 1961, they have demonstrated efficient light emission and coupling signals between GaAs p-n GaAs and electrically isolated semiconductor photodetectors. On August 8, 1962, Biard and Pittman filed a patent titled "Semiconductor Radiant Diode" based on their findings, describing diffuse zinc p-n junction LEDs with cathode spacing contacts to enable efficient infrared light emission under forward bias. Having set their work priorities based on notebook engineering that precedes the delivery of G.E. Labs, RCA Research Labs, IBM Research Labs, Bell Labs, and Lincoln Lab at MIT, the US patent office issued two patent inventors for the Gaas infrared emitting diode (US Patent US3293513), the first practical LED. Immediately after filing a patent, Texas Instruments (TI) started the project to produce infrared diodes. In October 1962, TI announced its first commercial LED product (SNX-100), which uses pure GaAs crystals to emit 890nm light output. In October 1963, TI announced the first commercial hemispherical LED, the SNX-110.
The first visible-spectrum (red) LED was developed in 1962 by Nick Holonyak, Jr. while working at General Electric. Holonyak first reported its LEDs in the Applied Physics Letters journal on December 1, 1962. M. George Craford, a former Holonyak graduate student, invented the first yellow LED and increased the brightness of red and red-orange LEDs by a factor ten in 1972. In 1976, TP Pearsall created LEDs with the first high-brightness and high efficiency for fiber-optic telecommunications by creating new semiconductor materials that are specifically tailored to the wavelength of fiber optic transmission.
Initial commercial development
The first commercial LEDs are generally used in place of fluorescent and fluorescent indicator lights, and in seven segment displays, first in expensive equipment such as laboratory and electronic test equipment, then in devices such as TVs, radios, telephones, calculators, as well as watches (see usage list signal). Until 1968, visible and infrared LEDs were very expensive, in the order of US $ 200 per unit, and so little practical use. The Monsanto Company was the first organization to mass-produce visible LEDs, using gallium arsenide phosphide (GaAsP) in 1968 to produce red LEDs suitable for indicators. Hewlett-Packard (HP) introduced LEDs in 1968, initially using GaAsP supplied by Monsanto. The red LED is bright enough just to be used as an indicator, because the light output is insufficient to illuminate an area. The readings in the calculator are so small that the plastic lenses are built on each digit to make them legible. Then, other colors become widely available and appear in equipment and equipment. In the 1970s a commercially successful LED device with less than five cents each was produced by Fairchild Optoelectronics. This device uses a mixed semiconductor chip made with a planar process invented by Dr. Jean Hoerni at Fairchild Semiconductor. The combination of planar processing for chip manufacturing and innovative packaging methods enabled the team at Fairchild to be led by Thomas Brandt's optoelectronic pioneer to achieve the required cost reductions. LED manufacturers continue to use this method.
Most LEDs are made in very common T1Ã,¾ and T1Ã, mm 3 3 3 mm packages, but with increased power output, it has been increasingly required to release excess heat to maintain reliability, so that more complex packages have been adapted for efficient heat dissipation. Packages for high-powered LEDs are commonplace slightly similar to the early LEDs.
Blue LED
Blue LED was first developed by Herbert Paul Maruska at RCA in 1972 using gallium nitride (GaN) on sapphire substrates. The SiC-type was first sold commercially in the United States by Cree in 1989. However, none of these early blue LEDs is so bright.
The first high brightness blue LED was demonstrated by Shuji Nakamura of Nichia Corporation in 1994 and is based on InGaN. In parallel, Isamu Akasaki and Hiroshi Amano in Nagoya are working on developing an important GaN nucleation on sapphire substrates and a GaN doping p-type demonstration. Nakamura, Akasaki, and Amano were awarded the 2014 Nobel Prize in physics for their work. In 1995, Alberto Barbieri at Cardiff University Laboratory (GB) investigated the efficiency and reliability of LEDs with high brightness and demonstrated "transparent contact" LEDs using indium tin oxide (ITO) at (AlGaInP/GaAs).
In 2001 and 2002, the process for growth of gallium nitride (GaN) LEDs on silicon was successfully demonstrated. In January 2012, Osram demonstrated high powered InGaN LEDs grown on commercial silicon substrates, and GaN-on-silicon LEDs in production at Plessey Semiconductors. By 2017, some manufacturers use SiC as a substrate for LED production, but sapphires are more common.
White LED and illumination illumination
The achievement of high efficiency in blue LED is quickly followed by the development of the first white LED. In this device, Y
3 Al
5 O
12 : Ce (known as" YAG ") the phosphor layer on the emitter absorbs some blue color emissions and produces yellow light through fluorescence. The combination of yellow color with the rest of the blue light was white for the eyes. However, using different phosphorus (fluorescent material) it also becomes possible to produce green and red light through fluorescence. Mixtures resulting from red, green and blue are not only perceived by humans as white light but are superior to lighting in color rendering, while one can not appreciate the color of red or green objects illuminated only by yellow (and blue remains) wavelength of YAG phosphorus.
The first white LEDs are expensive and inefficient. However, the output of LED light has increased exponentially, with doubling occurring approximately every 36 months since the 1960s (similar to Moore's law). Recent research and development has been disseminated by Japanese manufacturers such as Panasonic, Nichia, etc. And then by Korean and Chinese manufacturers and investments like: Samsung, Solstice, Kingsun, and many others. This trend is generally associated with the development of other parallel semiconductor technologies and advancements in optical and material sciences and has been called the law of Haiti after Dr. Roland Haitz.
The light output and efficiency of blue and near-ultraviolet LEDs increases as the cost of a reliable device falls. This causes a relatively high-energy white light for lighting, which replaces incandescent and fluorescent lamps.
Experimental white LEDs have been shown to produce more than 300 lumens per watt of electricity; some can last up to 100,000 hours. Compared to incandescent lights, this is not only a major increase in electrical efficiency but - over time - the same or lower cost per bulb.
Maps Light-emitting diode
Working principle
The P-N connection can convert the light energy absorbed into a proportional electric current. The same process is reversed here (ie the P-N junction emits light when electrical energy is applied to it). This phenomenon is generally called electroluminescence, which can be defined as the emission of light from a semiconductor under the influence of an electric field. The charge carrier rejoins the P-N junction forward as the electrons cross from the N-region and rejoin the existing hole in the P-region. The free electrons are in the energy level conduction band, while the holes are in the valence energy band. So the energy level of the hole is smaller than the energy level of the electron. Some parts of the energy must be dissipated to recombine with electrons and holes. This energy is emitted in the form of heat and light.
Electrons dissipate energy in heat for silicon and germanium diodes but in gallium arsenide phosphide (GaAsP) and semiconductor gallium phosphide (GaP), electrons dissipate energy by emitting photons. If the semiconductor is transparent, the connection becomes the light source when it is emitted, so it becomes a light-emitting diode. However, when the intersection turns biased, the LED does not produce light and - if its potential is large enough, the device is damaged.
Technology
Physics
The LED consists of semiconductor material chips that are processed with impurities to create a p-n junction . As with other diodes, the current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the opposite direction. Cargo operators - electrons and holes - flows to the intersections of electrodes with different voltages. When an electron meets the hole, it falls to a lower energy level and releases energy in the form of a photon.
The wavelength of the transmitted light, and thus its color, depends on the bandgap energy of the material forming the p-n junction. In silicon or germanium diodes, electrons and holes are usually rejoined with non-radiative transition, which does not produce optical emissions, since this is an indirect band gap material. The material used for LEDs has a direct band gap with energy associated with near infrared light, visible light, or almost ultraviolet.
LED development begins with infrared and red devices made with gallium arsenide. Advances in materials science have enabled the manufacture of devices with shorter wavelengths, emitting light in various colors.
LEDs are usually built on an n-type substrate, with electrodes attached to p-type layers stored on the surface. The P-type substrate, while less common, occurs as well. Many commercial LEDs, especially GaN/InGaN, also use sapphire substrates.
Refractive index
Bare semiconductors such as silicon exhibit a very high refractive index relative to open air, which prevents passage of photons arriving at sharp angles relative to the air-contact surface of semiconductors due to total internal reflection. These properties affect the efficiency of LED-light emissions as well as the light absorption efficiency of photovoltaic cells. The refractive index of silicon is set at 3.96 (at 590 nm), while the air refractive index is set at 1.0002926.
In general, a flat, non-surface LED semiconductor chip emits light only perpendicular to the surface of the semiconductor, and some degree to the side, in the cone shape referred to as cone light , light cone , or escape cone . The maximum occurrence angle is referred to as the critical angle. When the critical angle is exceeded, the photon no longer escapes the semiconductor but, on the contrary, is reflected internally inside the semiconductor crystal as if it were a mirror.
Internal reflection can pass through other crystal faces if the angle is low enough and the crystal is transparent enough not to reabsorb the photon emissions. But for a simple square LED with a 90 degree angled surface on all sides, all faces act as mirrors of the same angle. In this case, most of the light can not escape and disappear as waste heat in the crystal.
The tortuous surface of the chip with its beveled side similar to a jewel or fresnel lens can increase the light output by distributing the light perpendicular to the chip's surface and away to the side of the photon emission point.
An ideal semiconductor shape with maximum light output would be a microsphere with exact photon emissions at the center, with electrodes penetrating into the center to contact at the point of emission. All rays of light coming from the center will be perpendicular to the entire surface of the sphere, so there is no internal reflection. A half-spherical semiconductor will also work, with a flat rear surface functioning as a mirror for photons that are scattered behind.
Transition layer
After doping the wafer, it is usually cut apart into individual die. Each dice is commonly called a chip.
Many LED semiconductor chips are packaged or pots in clear or colored plastic shells. The plastic template has three objectives:
- Installing semiconductor chips in a device is easier to do.
- Very fragile electrical cables are physically supported and protected from damage.
- Plastics act as refractive intermediaries between relatively high index semiconductors and low index open air.
The third feature helps to increase the emission of light from semiconductors by acting as a diffuse lens, emitting light at a much higher angle of view than a light cone than an empty chip will be alone.
Operational efficiency and parameters
Typical LED indicators are designed to operate with no more than 30-60 milliwatt (mW) of electrical power. Around 1999, Philips Lumileds introduced a power LED that could be used continuously in one watt. This LED uses a much larger semiconductor die size to handle large power inputs. Also, the die semiconductor is fitted to a metal slug to allow for greater heat dissipation of the LED die.
One of the key advantages of LED-based lighting sources is high luminous efficacy. White LEDs quickly match and exceed the efficacy of standard incandescent lighting systems. In 2002, Lumileds made a five watt LED available with a luminous efficacy of 18-22 lumens per watt (lm/W). By comparison, conventional 60-100 watt incandescent light emits about 15 lm/W, and standard fluorescent lamps emit up to 100 lm/W.
In 2012, Philips has achieved the following efficacy for each color. The value of efficiency indicates physics - the power of outgoing light per incoming electrical power. The lumen-per-watt efficacy value includes the characteristics of the human eye and is derived using the luminosity function.
In September 2003, a new type of blue LED was shown by Cree. This produces a commercially packed white light giving 65 μm/W at 20 mA, being the brightest white LED available commercially at the time, and more than four times more efficient than standard incandescents. In 2006, they demonstrated a prototype with a glowing white light efficacy record of 131 lm/W at 20 mA. Nichia Corporation has developed a white LED with a luminous efficacy of 150 μm/W at a forward current of 20 mA. Cree's XLamp XM-L LED, commercially available in 2011, generates 100 μm/W at full power of 10 WW, and up to 160 Âμm/W at about 2 W W input power. In 2012, Cree announced a white LED delivers 254 μm/W, and 303 μm/W in March 2014. General practical lighting requires high-power LEDs, of one watt or more. The typical operating current for the device starts at 350 mA.
This efficiency is only for light-emitting diodes, which are held at low temperatures in the laboratory. Because LEDs installed in real equipment operate at higher temperatures and with motorist losses, real world efficiency is much lower. The United States Department of Energy (DOE) commercial LED light testing designed to replace incandescent or CFL shows that the average efficacy is still around 46 lm/W in 2009 (tested performance ranged from 17 lm/W to 79 Âμm/W).
Efficiency droop
Droop efficiency is a decrease in luminous LED efficiency when electric current rises above tens of milliamperes.
This effect was originally thought to be associated with high temperatures. Scientists prove the opposite is true: even though the LED lifespan is shortened, the efficiency droops less severe at elevated temperatures. The mechanism that led to dull efficiency was identified in 2007 as Auger recombination, which was taken with mixed reactions. In 2013, a study confirms Auger's recombination as the cause of drooping efficiency.
In addition to being less efficient, the operation of LEDs at higher electrical currents creates higher heat levels, which can harm LED life. Due to this increase in heat at higher currents, high brightness LEDs have an industry standard operating only 350 mA, which is a compromise between light output, efficiency, and long life.
Possible solutions
Instead of increasing the current level, luminance is usually enhanced by combining multiple LEDs in a single bulb. Solving the problem of droop efficiency means that a household LED bulb will require fewer LEDs, which will significantly reduce costs.
Researchers at the US Naval Research Laboratory have found a way to reduce the vacuum of efficiency. They found that the droop emerged from non-radiation auger recombination of the injected carrier. They created a quantum well with a soft confinement potential to reduce non-radiation auger processes.
Researchers at National Central University of Taiwan and Epistar Corp are developing ways to reduce drooping efficiency by using aluminum nitride ceramic substrate (AlN), which is more thermally conductive than the commercially used sapphire. Higher thermal conductivity reduces its own heating effect.
Lifetime and failure
Solid-state devices such as LEDs are subject to very limited usage when operated at low currents and at low temperatures. The typical resistance quoted is 25,000 to 100,000 hours, but heat and current settings can extend or shorten this time significantly. One way to cool the LEDs and use energy efficiently, is to wink the LEDs to get biomass for example in microalgae cultivation. It is important to note that these projections are based on standardized tests that may not speed up all potential mechanisms that may cause failure in LEDs.
The most common symptoms of LED failure (and laser diode) are the gradual decrease in light output and loss of efficiency. Sudden failure, though rare, can also occur. Early red LEDs are well known for their short service life. With the development of high power LEDs, the device is subjected to higher connection temperatures and higher current density than traditional devices. This causes pressure on the material and may cause early light degradation. To quantify useful life in a standard way, some suggest using L70 or L50, which is a runtime (usually within thousands of hours) where a particular LED reaches 70% and 50% of the initial light output, respectively.
Whereas in most of the previous light sources (incandescent, discharge lamps, and burning combustible fuels, eg candles and oil lamps) of light from heat, LEDs only operate if they remain cool enough. Manufacturers generally set a maximum connection temperature of 125 or 150 Â ° C, and lower temperatures are recommended for longevity purposes. At this temperature, relatively little heat is lost by radiation, which means that the rays generated by the LED are cool.
Waste heat in high-power LEDs (which in 2015 can be less than half the power consumed) is delivered by conduction through the substrate and packs the LEDs to the heat sink, which provides heat to the ambient air by convection. Therefore, careful thermal design is essential, taking into consideration the thermal durability of LED packs, heat sinks, and interfaces between the two. Medium power LEDs are often designed to be soldered directly to printed circuit boards that contain a thermally conductive metal layer. High-power LEDs are packaged in large-area ceramic packs attached to metal-interface heat sinks to materials with high thermal conductivity (thermal grease, phase change materials, thermal conductive pads, or thermal adhesives).
If a LED-based lamp is installed in an unventilated luminaire, or the luminaire is located in an environment with no free air circulation, the LEDs tend to overheat, thus reducing the age or initial failure of the disaster. Thermal designs are often based on temperatures of about 25Ã, Â ° C (77Ã, Â ° F). LEDs used in outdoor applications, such as traffic signals or signal lights on the sidewalk, and in climates where temperatures in the lamps become very high, may decrease output or even failure.
Because the efficacy of LEDs is higher at low temperatures, LED technology is perfect for supermarket freezer lighting. Since the LEDs produce less heat than the incandescent bulbs, the use of a freezer tube lamp can save on cooling costs as well. However, they may be more susceptible to accumulation of ice and snow than incandescent bulbs, so some LED lighting systems have been designed with additional heating circuits. In addition, research has developed a heat sink technology that transfers the resulting heat at the junction to the corresponding area of ​​the lamp.
Color and materials
Conventional LEDs are made of various inorganic semiconducting materials. The following table shows available colors with a range of wavelengths, voltage drops, and materials:
Biru dan ultraviolet
The first blue-violet LEDs using magnesium-doped gallium nitride were made at Stanford University in 1972 by Herb Maruska and Wally Rhines, doctoral students in materials science and engineering. At that time Maruska was on leave from RCA Laboratories, where he worked with Jacques Pankove for related work. In 1971, a year after Maruska left for Stanford, his RCA counterparts, Pankove, and Ed Miller demonstrated the first blue electroluminescence of zinc-doped gallium nitride, although the next built Pankove and Miller device, which first used gallium nitride light-emitting diode. green light. In 1974, the US Patent Office awarded Maruska, Rhines and Stanford, professor David Stevenson, a patent for their work in 1972 (US Patent US3819974 A) and today, magnesium-doping gallium nitride remains the basis for all commercial blue LEDs and laser diodes. In the early 1970s, the device was too dim for practical use, and research into the gallium nitride device slowed down. In August 1989, Cree introduced the first commercially available blue LED based on indirect tape semiconductors, silicon carbide (SiC). SiC LEDs have very low efficiency, no more than about 0.03%, but emit in the blue part of the visible light spectrum.
In the late 1980s, important breakthroughs in the growth of epitaxial GaN and p-doping types ushered in the modern era of GaN-based optoelectronic devices. Built on this foundation, Theodore Moustakas at Boston University patented a method for producing high brightness blue LEDs using a new two-step process. Two years later, in 1993, blue LEDs with high brightness were reenacted by Shuji Nakamura of Nichia Corporation using a gallium nitride growth process similar to Moustakas's. Both Moustakas and Nakamura issued a separate patent, which puzzled the question of who the original inventor was (partly because although Moustakas found the first, Nakamura filed it first). This new development revolutionizes LED lighting, making high-power blue light sources practical, leading to the development of technologies such as Blu-ray, as well as enabling the bright high-resolution screen of tablets and modern mobile phones.
Nakamura was awarded the 2006 Millennium Technology Award for his invention. Nakamura, Hiroshi Amano and Isamu Akasaki were awarded the Nobel Prize in Physics in 2014 for the discovery of blue LEDs. In 2015, a US court ruled that three companies (ie previously untitled litigants) who had licensed Nakamura patents for production in the United States had breached previous patents of Moustakas, and ordered them to pay a license fee of not less than 13 million USD.
In the late 1990s, blue LEDs became widely available. They have an active region consisting of one or more InGaN quantum wells flanked between layers of thick GaN, called a cladding layer. By varying the relative In/Ga fraction in InGaN quantum wells, the emission of light can theoretically vary from purple to yellow. Aluminum gallium nitride (AlGaN) from various Al/Ga fractions can be used to produce cladding and a good quantum layer for ultraviolet LEDs, but this device has not yet reached the level of efficiency and technological maturity of InGaN/GaN blue/green devices. If un-alloyed GaN is used in this case to form an active quantum layer, the device emits ultraviolet light close to the peak wavelength centered around 365Ã, nm. The green LEDs manufactured from the InGaN/GaN system are much more efficient and lighter than the green LEDs that are manufactured with non-nitride material systems, but practical devices still show too low efficiency for high-brightness applications.
With aluminum containing nitrides, most often AlGaN and AlGaInN, even shorter wavelengths can be achieved. Ultraviolet LEDs in the wavelength range become available on the market. Near-UV emitters at wavelengths of about 375-395 nm are inexpensive and often encountered, for example, in lieu of black lights for UV waterproof anti-counterfeiting checks on several documents and paper currency. Shorter wavelength diodes, though much more expensive, are commercially available for wavelengths up to 240 nm. Because the photosensitivity of microorganisms is more or less the same as the DNA absorption spectrum, with a peak of about 260 m, UV LEDs emitting at 250-270 nm are expected in prospective disinfection and sterilization. Recent studies have shown that commercially available UVA LEDs (365Ã, nm) are already effective disinfection and sterilization equipment. UV-C wavelengths were obtained in the laboratory using aluminum nitride (210Ã,nm), boron nitride (215 nm) and diamond (235 nm).
RGB
RGB LEDs consist of one red LED, one green, and one blue LED. By independently adjusting each of the three, the RGB LED is capable of producing a wide color gamut. Unlike special colored LEDs, however, this obviously does not produce pure wavelengths. In addition, commercially available modules are often not optimized for seamless color mixing.
White
There are two main ways of producing white light-emitting diodes (WLEDs), LEDs that produce high-intensity white light. One is to use individual LEDs that emit the three main colors - red, green, and blue - and then mix all the colors to form white light. The other is to use a phosphor material to convert monochromatic light from a blue LED or UV into a broad-spectrum white light, just like a working fluorescent light bulb. The 'white' light produced is essentially engineered to fit the human eye.
There are three main methods of mixing colors to produce white light from LEDs:
- Blue LED green LED red LED (color mixing; can be used as backlight for display, very bad for lighting due to gap in spectrum)
- near-UV or UV LED phosphorus RGB (LED lamps produced with shorter wavelengths than blue are used to generate RGB phosphorus)
- Yellow phosphor blue LED (two complementary colors combine to form white light, more efficient than the first two and more commonly used)
Because of metamerism, it is possible to have a very different spectrum that looks white. However, the appearance of the object illuminated by the light can vary because the spectrum varies, this is a matter of color appearance, quite apart from the color temperature, where objects that are really orange or cyan can appear with the wrong color and are much darker like LEDs or the phosphor does not emit the reflected wavelength. The best color appearance of CFLs and LEDs uses a mixture of phosphorus, resulting in less efficiency but better light quality. Although incandescent halogen lights have more orange color temperatures, these lights are still the easiest artificial light source available at best in terms of color appearance.
RGB System
White light can be formed by mixing different color of lamps; the most common method is to use red, green, and blue (RGB). Therefore this method is called multicolor white LED (sometimes referred to as RGB LED). Because this requires electronic circuits to control the mixing and diffusion of different colors, and since individual color LEDs usually have slightly different emission patterns (leading to color variations depending on direction) even if they are made as one unit, these are rarely used to produce white lighting. Nevertheless, this method has many applications due to the flexibility of different color mixing, and in principle, this mechanism also has a higher quantum efficiency in generating white light.
There are several types of white multicolor LEDs: di-, tri-, and tetrachromatic white LEDs. Some of the key factors that play between these different methods include color stability, color rendering ability, and glowing success. Often, higher efficiency means lower color rendering, presenting a trade-off between luminous efficacy and color rendering. For example, a chromatic white LED has the best luminous efficacy (120 lm/W), but the lowest color rendering capability. However, although tetrachromatic white LEDs have excellent color rendering capabilities, they often have poor luminous success. Trichromatic white LEDs are in between, have both a luminous efficacy (& gt; 70 lm/W) and a fair color rendering capability.
One of the challenges is the development of green LEDs that are more efficient. Theoretical maximum for green LED is 683 lumens per watt but in 2010 some green LEDs even exceed 100 lumens per watt. Blue and red LEDs approach their theoretical limits.
Multicolor LEDs offer not just another way to form white light but a new way to form light with different colors. The most understandable colors can be formed by mixing different amounts of the three primary colors. It allows precise dynamic color control. As more efforts are being devoted to investigating these methods, multicolor LEDs should have a major impact on the underlying methods we use to produce and control light colors. However, before this type of LED can play a role in the market, some technical issues have to be solved. This includes that this type of LED emission power decays exponentially with increasing temperature, resulting in major changes in color stability. Such problems inhibit and may hinder the use of industry. Thus, many new package designs aimed at solving this problem have been proposed and the results are now being reproduced by researchers and scientists. However, multicolor LEDs without phosphorus can never provide good quality lighting because each LED is a narrow band source (see graph). LEDs without phosphorus while worse solutions for general illumination are the best solutions for displays, either LCD backlights, or direct LED-based pixels.
The correlation of color temperature (CCT) dimming for LED technology is considered a difficult task since LED Binning, the age and drift effects of LEDs change the output of the actual color values. The feedback system is used for example with color sensors, to actively monitor and control the color output of many color mixing LEDs.
Phosphor-based LEDs
This method involves plating LEDs of a single color (mostly blue LEDs made of InGaN) with phosphors of different colors to form white light; The resulting LEDs are called white LEDs based on phosphorus or phosphor-converted (pcLEDs). A small part of blue light undergoes a Stokes shift, which changes it from shorter wavelengths to longer. Depending on the original LED color, various color phosphors are used. Multiple layers of different color phosphors broaden the emitted spectrum, effectively increasing the color rendering index (CRI).
LED-based phosphors have an efficiency loss due to heat loss from Stokes shift and also other phosphor-related problems. Their luminous effectiveness compared to normal LEDs depends on the distribution of the resulting light output spectrum and the original wavelength of the LED itself. For example, the luminous efficacy of yellow yag-based yellow phosphorus typically ranges from 3 to 5 times the luminous efficacy of the original blue LED because of greater human eye sensitivity to yellow than blue (as modeled in luminosity function). Due to the simplicity of manufacturing, the phosphorus method is still the most popular method of making white LEDs of high intensity. The design and production of light sources or lamps using monochrome emitters with phosphor conversion is simpler and less expensive than complex RGB systems, and most high-intensity white LEDs are currently produced using phosphor light conversion.
Among the challenges faced to improve the efficiency of LED-based white light sources is the development of more efficient phosphors. In 2010, the most efficient yellow phosphorus was still YAG phosphorous, with less than 10% of Stokes shift losses. Losses resulting from internal optical losses due to re-absorption in the LED chip and in the LED packing itself account for typically 10% to 30% of the efficiency loss. Currently, in the field of LED phosphorus development, much effort is spent on optimizing these devices to higher light output and higher operating temperatures. For example, efficiency can be improved by adapting to better package designs or by using more suitable phosphor types. Conformal coating process is often used to overcome various phosphor thickness problems.
Some white LED-based phosphors wrap InGaN blue LEDs inside a phosphor coated epoxy. Alternatively, LEDs can be paired with distant phosphorus, a preformed polycarbonate piece coated with phosphorus. Long distance phosphors provide more diffuse light, which is desirable for many applications. The design of long-range phosphor is also more tolerant to the variation of the LED emission spectrum. The common yellow phosphorus material is yttrium cerium aluminum garnet (Ce 3 : YAG).
White LEDs can also be made by coating the near-ultraviolet (NUV) LED with a mixture of high efficiency europium-based effector emitting red and blue, plus copper and aluminum-doped zinc sulfide (ZnS: Cu, Al) that emit green. This is a method analogous to the way fluorescent light works. This method is less efficient than blue LEDs with YAG: Ce phosphorus, because Stokes shift is larger, so more energy is converted to heat, but produces light with better spectral characteristics, which makes the color better. Because of the higher radiation output from ultraviolet LEDs than the blue ones, both methods offer a comparable brightness. The concern is that UV rays can leak from non-functioning light sources and cause damage to the eyes or human skin.
Other white LEDs
Another method used to produce LED experimental white light is not to use phosphorus at all and is based on homoepitaxially (ZnSe) selenide on the ZnSe substrate which simultaneously emits blue light from active and yellow light from the substrate.
The new wafer style consisting of gallium-nitride-on-silicon (GaN-on-Si) is being used to produce white LEDs using 200-mm silicon wafers. This avoids expensive sapphire substrates in relatively small 100- or 150-mm wafer sizes. Saphir apparatus must be paired with a mirror-like collector to reflect light that would otherwise be wasted. It is estimated that by 2020, 40% of all GaN LEDs will be made with GaN-on-Si. Producing large sapphire materials is difficult, while large silicon materials are cheaper and more abundant. LED companies shifting from using sapphires to silicon should be a minimal investment. Organic light-emitting diodes (OLED) Organic light-emitting diodes (OLED) Organic light-emitting diodes (OLED)
In an organic light-emitting diode (OLED), the electroluminescent material that constitutes the emissive layer of the diode is an organic compound. The organic material is electrically conductive because of the delocalization of pi electrons caused by the conjugation of all or part of the molecule, and therefore the material acts as an organic semiconductor. Organic materials can be small organic molecules in the crystalline phase, or polymers.
The potential advantages of OLED include thin, cheap screens with low driving voltages, wide viewing angles, and high contrast and gamut colors. Polymer LEDs have the added benefit of a printable and flexible display. OLEDs have been used to create visual displays for portable electronic devices such as mobile phones, digital cameras, and MP3 players while possible future uses include lighting and television.
Quantum-dot LED
Quantum dots (QD) are semiconductor nanocrystals with optical properties that allow the color of their emissions to be tuned from the visible to the infrared spectrum. This allows dot quantum LEDs to create almost any color on the CIE diagram. It provides more color options and better color rendering than white LEDs because the emission spectrum is much narrower, quantum limited status characteristics.
There are two types of schemes for QD excitation. One uses photo excitation with the main LED light source (usually blue or UV LEDs are used). The other is the direct electrical excitation first shown by Alivisatos et al.
One example of photo-excitation schemes is a method developed by Michael Bowers, at Vanderbilt University in Nashville, which involves plating blue LEDs with quantum dots that glow white in response to blue light from LEDs. This method emits a warm, yellowish-white light similar to that made by an incandescent light bulb. Quantum dots are also considered for use in white light-emitting diodes in LCD televisions (liquid crystal displays).
In February 2011, scientists at PlasmaChem GmbH were able to synthesize quantum dots for LED applications and build light converters on their basis, which can efficiently convert light from blue to other colors for hundreds of hours. This kind of QD can be used to emit visible or near infrared light from any wavelengths attracted by light with shorter wavelengths.
The QD-LED structure used for electric excitation schemes is similar to OLED basic design. The layers of quantum dots are sandwiched between layers of electron transporting material and hole-carriers. The electric field used causes the electrons and holes to move to the dot layer of quantum and recombine to form excitons that excite QD. This scheme is generally studied for quantum dot display. The tunability of emission wavelengths and narrow bandwidth are also useful as an excitation source for fluorescence imaging. Near-field fluorescence optical microscopy scanning (NSOM) using integrated QD-LEDs has been demonstrated.
In February 2008, the luminous efficacy of 300 lumens of visible light per watt of radiation (not per watt of electricity) and the emission of warm light was achieved using nanocrystal.
Type
The main types of LEDs are miniature, high-powered devices, and special designs such as alphanumeric or multicolor.
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These are mostly die LEDs that are used as indicators, and they come in different sizes from 2 mm to 8 mm, through holes and surface mounting packages. They usually do not use separate heat sinks. Current rating ranges from about 1 mA to above 20 mA. Small size sets a natural upper limit on power consumption due to heat caused by high current densities and the need for heat sinks. Often daisy chained like used in LED tapes.
Common package shapes include rounded, with dome or flat top, rectangles with flat tops (as used in the bar graph display), and triangles or rectangles with flat tops. Encapsulation can also be clear or colored to improve contrast and viewing angle.
Researchers at the University of Washington have discovered the thinnest LEDs. It is made of two-dimensional flexible material (2-D). These are three thick atoms, which are 10 to 20 times thinner than three-dimensional (3-D) LEDs and are also 10,000 times smaller than the thickness of a human hair. 2-D This LED will make it possible to create smaller, more energy-efficient lighting, optical communications and nano lasers.
There are three main categories of single LED mini die:
- Low current
- Usually rated 2 mA about 2ÃV (consumption about 4 mW)
- Standard
- 20 mA LEDs (ranging from about 40 mW to 90 mW) around:
- 1.9 to 2.1Ã,V for traditional red, orange, yellow, and green
- 3.0 to 3.4Ã, V for pure green and blue
- 2.9 to 4.2Ã, V for violet, pink, purple and white
- Ultra-high-output
- 20 mA around 2 or 4-5 V, designed to be viewed in direct sunlight
5 V and 12 V LEDs are ordinary miniature LEDs that combine the appropriate series resistors for direct connections to V 5 or 12 V supply.
High power
The high power LED (HP-LED) or high output LED (HO-LED) can be driven at currents from hundreds of mA to more than one ampere, compared to tens of mA for other LEDs. Some can emit more than a thousand lumens. LED power density up to 300 W/cm 2 has been reached. Because overheating is damaging, the HP-LED must be mounted on the heat sink to allow heat dissipation. If the heat from the HP-LED is not removed, the device will fail in seconds. One HP-LED can often replace an incandescent bulb in a flashlight, or arranged in an array to form a powerful LED light.
Some of the well-known HP-LEDs in this category are the Nichia 19 series, Lumileds Rebel Led, Osram Opto Semiconductors Golden Dragon, and Cree X-lamps. As of September 2009, some HP-LEDs manufactured by Cree now exceed 105 Âμm/W.
An example for Haitz's law - which predicts an exponential increase in LED light output and efficacy over time - is the CREE XP-G series LED, which reaches 105 lm/W in 2009 and the Nichia 19 series with a typical 140 lm/W efficacy, was released in 2010.
AC-driven
LEDs developed by Seoul Semiconductor can operate on AC power without a DC converter. For every half cycle, part of the LED emits light and partly dark, and this is reversed during the next half cycle. The efficacy of this type of HP-LED is usually 40 lm/W. A large number of LED elements in series may be able to operate directly from the mains voltage. In 2009, Seoul Semiconductor released a high DC voltage LED, named as 'Acrich MJT', which is capable of being driven from AC power with a simple control circuit. The low power dissipation of these LEDs gives them more flexibility than the original AC LED design.
Application-specific variations
Blinking
Blinking LEDs are used as an attention-seeking indicator without the need for external electronics. LED Flashing resembles a standard LED but contains an integrated multivibrator circuit that causes the LED to flash with a typical period of one second. In a diffused lens LED, this circuit is seen as a small black dot. Most blinking LEDs emit light in one color, but more sophisticated devices can pass between several colors and even fade through the color sequence using RGB color mixing.
Bi-color
Bi-color LEDs contain two different LED emitters in one case. There are two types of this. One type consists of two dies connected to the same two leading antiparallel to each other. The current flow in one direction radiates one color, and the current in the opposite direction radiates another color. The other type consists of two dies with separate leads for both dies and other leads for common anodes or cathodes so that they can be independently controlled. The most common bi-color combinations are red/traditional green; however, other available combinations include traditional yellow/green, pure red/green, red/blue, and blue/green.
Tri-color
Tri-color LEDs contain three different LED emitters in one case. Each emitter is connected to separate leads so that they can be controlled independently. The order of four leads is typical with one common lead (anode or cathode) and additional instructions for each color.
RGB
RGB LEDs are tri-colored LEDs with red, green, and blue emitters, typically using a four-wire connection with a common lead (anode or cathode). These LEDs can have general positive leads in the case of common LED anodes, or common negative leads in the case of common cathode LEDs. However, others have only two prospects (positive and negative) and have an integrated electronic control unit.
Decorative-multicolor
Multicolor-decorative LEDs combine several different color emitters that are supplied by just two lead-out cables. The colors are internally shifted by varying the supply voltage.
Alphanumeric
Alphanumeric LEDs are available in seven-segment, starburst, and dot-matrix formats. The seven-segment display handles all numbers and a limited set of letters. The Starburst view can display all the letters. The dot-matrix display typically uses 5x7 pixels per character. The seven segment LED displays were used extensively in the 1970s and 1980s, but the increasing use of liquid crystal displays, with lower power requirements and greater display flexibility, has reduced the popularity of numerical and alphanumeric LED screens.
Digital-RGB
Digital RGB Addressable LEDs are RGB LEDs that contain their own "smart" electronics controls. In addition to power and ground, it provides connections for data-in, data-out, and sometimes clock or strobe signals. It is connected in a daisy chain, with data in the first LED sourced by the microprocessor, which can control the brightness and color of each LED independently of the others. They are used where minimum maximal and visible minimum control combinations are required such as strings for Christmas and LED matrices. Some even have a refresh rate in the kHz range, allowing for basic video applications. These devices are also known for their part numbers (WS2812 being the most common) or brand names like NeoPixel
Filament
The LED filaments consist of several LED chips that are connected in series on a common longitudinal substrate that forms thin rods that are reminiscent of traditional incandescent filaments. It is used as a low-cost decorative alternative to traditional light bulbs being removed in many countries. Filaments require a somewhat higher voltage to brightness up to nominal, allowing it to work efficiently and only with utility voltage. Often simple rectifiers and capacitive current limiters are used to make low cost reimbursements for traditional light bulbs without the hassle of low voltage, high current converters requiring one die LED. Typically, they are enclosed in enclosed enclosures with shapes similar to lamps designed to replace (eg bulbs) and filled with inert nitrogen or carbon dioxide gas to efficiently remove heat.
Considerations for using
Resources
The voltage characteristics of an LED are similar to other diodes, where the current depends exponentially on the voltage (see Shockley's diode equation). This means that small changes in voltage can cause major current changes. If the applied voltage exceeds the LED forward voltage reduction by a small amount, the current rating may be exceeded by a large amount, potentially damaging or destroying the LED. A common solution is to use a constant current power supply to keep current below the maximum LED current rating. Since most of the common resources (batteries, power sources) are constant voltage sources, most LED fixtures must include a power converter, at least a current-limiting resistor. However, the high resistance of a three-volt coin cell combined with the high differential resistance of a nitride-based LED makes it possible to light such LEDs from a coin cell without an external resistor.
Electrical polarity
Like all diodes, current flows easily from the p-type to the n-type. However, there is no current flow and no light is emitted if a small voltage is applied in the reverse direction. If the reverse voltage grows large enough to exceed the translucency voltage, the large current flows and the LED can be damaged. If the backflow is limited enough to avoid damage, the reverse-conduct LED is a useful noise diode.
Safety and health
Most LED-containing devices are "safe under all normal use conditions", and so are classified as "Class 1 LED products"/"Klasse 1 LEDs". Currently, only a few LEDs - very bright LEDs that also have 8 Â ° or less focused angles - can, in theory, cause temporary blindness, and so are classified as "Class 2". The opinion of the French Agency for Food, Environment and Health & amp; Security (ANSES) 2010, on LED-related health issues, recommends prohibiting the use of lights by the public in moderate Risk Group 2, especially those with high blue components, in places frequented by children.
In general, laser safety rules - and "Class 1", "Class 2" systems, etc. - also applies to LEDs.
While LEDs have advantages over fluorescent lamps that do not contain mercury, LEDs can contain other harmful metals such as lead and arsenic. Regarding LED toxicity when treated as waste, a study published in 2011 states: "According to federal standards, LEDs are not harmful except for low-intensity red LEDs, which melt Pb [lead] at levels that exceed the regulatory limit (186 mg/h ) Regulatory limits: 5) However, under California regulations, copper levels are excessive (up to 3892 mg/kg, limit: 2500), lead (up to 8103 mg/kg, limit: 1000), nickel (up to 4797 mg/kg; limit: 2000), or silver (up to 721 mg/kg, limit: 500) makes all but a low-intensity harmless yellow LED. "
Use of LEDs fall into four main categories:
- A visual signal in which light travels more or less directly from the source to the human eye, to convey a message or meaning
- Illumination in which light is reflected from an object to give a visual response of these objects
- Measure and interact with processes that do not involve human vision
- Narrow band light sensor in which the LED operates in reverse bias mode and responds to incident light,
Source of the article : Wikipedia
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