A tiny semiconductor diode which emits light when an electric current is passed through it. It is used in electronic calculator displays and digital watch read-outs, where the digits are made up from the diodes. The colour of the light emitted depends on the material of the crystal.
A light-emitting diode (LED) is a semiconductor device that emits incoherent narrow-spectrum light when electrically biased in the forward direction.
LED technology
Physical function
An LED is a unique type of semiconductor diode. The materials used for an LED have a direct band gap with energies corresponding to near-infrared, visible or near-ultraviolet light.
LEDs are usually constantly illuminated when a current passes through them, but flashing LEDs are also available. Flashing LEDs resemble standard LEDs but they contain a small chip inside which causes the LED to flash with a typical period of one second. This type of LED comes most commonly as red, yellow, or green. Most flashing LEDs emit light of a single wavelength, but multicolored flashing LEDs are available too.
LED development began with infrared and red devices made with gallium arsenide.
LEDs are usually built on an n-type substrate, with electrode attached to the p-type layer deposited on its surface. Substrates that are transparent to the emitted wavelength, and backed by a reflective layer, increase the LED efficiency.
Conventional LEDs are made from a variety of inorganic semiconductor materials, producing the following colors:
aluminum gallium arsenide (AlGaAs) - red and infrared aluminum gallium phosphide (AlGaP) - green aluminum gallium indium phosphide (AlGaInP) - high-brightness orange-red, orange, yellow, and green gallium arsenide phosphide (GaAsP) - red, orange-red, orange, and yellow gallium phosphide (GaP) - red, yellow and green gallium nitride (GaN) - green, pure green (or emerald green), and blue also white (if it has an AlGaN Quantum Barrier) indium gallium nitride (InGaN) - near ultraviolet, bluish-green and blue silicon carbide (SiC) as substrate — blue silicon (Si) as substrate — blue (under development) sapphire (Al2O3) as substrate — blue zinc selenide (ZnSe) - blue diamond (C) - ultraviolet aluminum nitride (AlN), aluminum gallium nitride (AlGaN) - near to far ultravioletUltraviolet, Blue and white LEDs
Commercially viable blue LEDs based on the wide band gap semiconductors GaN (gallium nitride) and InGaN (indium gallium nitride) were invented by Shuji Nakamura while working in Japan at Nichia Corporation in 1993 and became widely available in the late 1990s. They can be added to existing red and green LEDs to produce white light, though white LEDs today rarely use this principle.
The blue LEDs have an active region consisting of one or more InGaN quantum wells sandwiched between thicker layers of GaN, called cladding layers. AlGaN aluminum gallium nitride of varying AlN fraction can be used to manufacture the cladding and quanutm well layers for ultraviolet LEDs, but these devices have not yet reached the level of efficiency and technological maturity of the InGaN-GaN blue/green devices. Green LEDs manufacture from the InGaN-GaN system are far more efficient and bright then green LEDs produced with non-nitride material systems.
Most "white" LEDs in production today are based on an InGaN-GaN structure, and emit blue light of wavelengths between 450 nm – 470 nm blue GaN. Since yellow light stimulates the red and green receptors of the eye, the resulting mix of blue and yellow light gives the appearance of white, the resulting shade often called "lunar white". This approach was developed by Nichia and was used by them from 1996 for manufacturing of white LEDs. Due to the spectral characteristics of the diode, the red and green colors of objects in its blue yellow light are not as vivid as in broad-spectrum light. Manufacturing variations and varying thicknesses in the phosphor make the LEDs produce light with different color temperatures, from warm yellowish to cold bluish; Philips Lumileds patented conformal coating process addresses the issue of varying phosphor thickness, giving the white LEDs a more consistent spectrum of white light.
White LEDs can also be made by coating near ultraviolet (NUV) emitting LEDs with a mixture of high efficiency europium based red and blue emitting phosphors plus green emitting copper and aluminum doped zinc sulfide (ZnS:Cu,Al). However the ultraviolet light causes photodegradation to the epoxy resin and many other materials used in LED packaging, causing manufacturing challenges and shorter lifetimes. This method is less efficient than the blue LED with YAG:Ce phosphor, as the Stokes shift is larger and more energy is therefore converted to heat, but yields light with better spectral characteristics, which render color better. Due to the higher radiative output of the ultraviolet LEDs than of the blue ones, both approaches offer comparable brightness.
The newest method used to produce white light LEDs uses no phosphors at all and is based on homoepitaxially grown zinc selenide (ZnSe) on a ZnSe substrate which simultaneously emits blue light from its active region and yellow light from the substrate.
A new technique just developed by Michael Bowers, a graduate student at Vanderbilt University in Nashville, involves coating a blue LED with quantum dots that glow white in response to the blue light from the LED. They consist of one or two phosphor layers over a blue LED chip. The first phosphor layer of a pink LED is a yellow glowing one, and the second phosphor layer is either red or orange glowing. Purple LEDs are blue LEDs with an orange glowing phosphor over the chip. For example, some are blue LEDs painted with fluorescent paint or fingernail polish that can wear off, and some are white LEDs with a pink phosphor or dye that unfortunately fades after a short time.
Ultraviolet, blue, pure green, white, pink and purple LEDs are relatively expensive compared to the more common reds, oranges, greens, yellows and infrared and are thus less commonly used in commercial applications. However, as of 2006, the "Chernobyl blue" light from blue LEDs has a certain commercial cachet and is used as a styling element in many products such as mobile phones and thus the price has dropped significantly.
Organic light-emitting diodes (OLEDs)
If the emitting layer material of an LED is an organic compound, it is known as an Organic Light Emitting Diode (OLED).
Compared with regular LEDs, OLEDs are lighter, and polymer LEDs can have the added benefit of being flexible.
Operational parameters and efficiency
Most typical LEDs are designed to operate with no more than 30-60 milliwatts of electrical power. These LEDs used much larger semiconductor die sizes to handle the large power input.
In September 2003 a new type of blue LED was demonstrated by the company Cree, Inc. This produced a commercially packaged white light having 65 lumens per watt at 20 mA, becoming the brightest white LED commercially available at the time. In 2006 they have demonstrated a prototype with a record white LED efficiency of 131 lumens per watt at 20 mA .
Today, OLEDs operate at substantially lower efficiency than inorganic (crystalline) LEDs. These promise to be much cheaper to fabricate than inorganic LEDs, and large arrays of them can be deposited on a screen using simple printing methods to create a color graphic display.
Failure modes
The most common way for LEDs (and diode lasers) to fail is the gradual lowering of light output and loss of efficiency.
White LEDs often use one or more phosphors.
High-power LEDs are susceptible to current crowding, nonhomogenous distribution of the current density over the junction. Thermal runaway is a common cause of LED failures.
Considerations in use
Unlike incandescent light bulbs, which light up regardless of the electrical polarity, LEDs will only light with positive electrical polarity. LEDs can be operated on an Alternating current voltage, but they will only light with positive voltage, causing the LED to turn on and off at the frequency of the AC supply.
The correct polarity of an LED can usually be determined as follows:
| sign: | + | − |
| polarity: | positive | negative |
| terminal: | anode | cathode |
| wiring: | red | black |
| leads: | long | short |
| marking: | none | stripe |
| pin: | 1 | 2 |
| PCB: | square | round |
| interior: | small | large |
| exterior: | round | flat |
NOTE: Neither the interior nor exterior method of determing an LED's polarity is 100% accurate.
Because the voltage versus current characteristics of an LED are much like any diode (that is approximately exponential), a small voltage change results in a huge change in current. Added to deviations in the process this means that a voltage source may barely make one LED light while taking another of the same type beyond its maximum ratings and potentially destroying it.
Since the voltage is logarithmically related to the current it can be considered to remain largely constant over the LEDs operating range. To try and keep power close to constant across variations in supply and LED characteristics the power supply should be a current source. in most indicator applications), an approximation to a current source made by connecting the LED in series with a current limiting resistor to a voltage source is generally used.
Most LEDs have low reverse breakdown voltage ratings, so they will also be damaged by an applied reverse voltage of more than a few volts. Since some manufacturers don't follow the indicator standards above, if possible the data sheet should be consulted before hooking up an LED, or the LED may be tested in series with a resistor on a sufficiently low voltage supply to avoid the reverse breakdown. If it is desired to drive an LED direct from an AC supply of more than the reverse breakdown voltage then it may be protected by placing a diode (or another LED) in inverse parallel.
LEDs can be purchased with built in series resistors. However the resistor value is set at the time of manufacture, removing one of the key methods of setting the LEDs intensity. so long as the flicker rate is greater than the human flicker fusion threshold, the LED will appear to be continuously lit.
Provided there is sufficient voltage available, multiple LEDs can be connected in series with a single current limiting resistor. The LEDs have to be of the same type in order to have a similar forward voltage.
Bicolor LED units contain two diodes, one in each direction (that is, two diodes in inverse parallel) and each a different color (typically red and green), allowing two-color operation or a range of apparent colors to be created by altering the percentage of time the voltage is in each polarity. Other LED units contain two or more diodes (of different colors) arranged in either a common anode or common cathode configuration.
LED units may have an integrated multivibrator circuit that makes the LED flash.
Advantages of using LEDs
LEDs produce somewhat more light per Watt than do incandescent bulbs; LEDs can emit light of an intended color without the use of color filters that traditional lighting methods require. The solid package of an LED can be designed to focus its light. When used in applications where dimming is required, LEDs do not change their colour tint as the current passing through them is lowered, unlike incandescent lamps, which yellow. LEDs are built inside solid cases that protect them, unlike incandescent and discharge sources, making them extremely durable. LEDs have an extremely long life span: upwards of 100,000 hours, twice as long as the best fluorescent bulbs and twenty times longer than the best incandescent bulbs. LEDs have a long life when operated at their rated power.) Further, LEDs mostly fail by dimming over time, rather than the abrupt burn-out of incandescent bulbs. LEDs light up very quickly. A typical red indicator LED will achieve full brightness in microseconds; LEDs used in communications devices can have even faster response times. For instance, purple plastic is often used for infrared LEDs, and most blue devices have clear housings.Disadvantages of using LEDs
LEDs are currently more expensive, in lumens per dollar, than more conventional lighting technologies. "Driving" an LED "hard" in high ambient temperatures may result in overheating of the LED package, eventually leading to device failure. LEDs require complex power supply setups to be efficiently driven.LED applications
List of LED applications
Some of these applications are further elaborated upon in the following text.
Architectural lighting Status indicators on all sorts of equipment Traffic lights and signals Exit signs Bicycle lights Toys and recreational sporting goods, such as the Flashflight Railroad crossing signals Continuity indicators Flashlights. Red or yellow LEDs are used in indicator and alphanumeric displays in environments where night vision must be retained: aircraft cockpits, submarine and ship bridges, astronomy observatories, and in the field, e.g. Red, yellow, green, and blue LEDs can be used for model railroading applications Remote controls, such as for TVs and VCRs, often use infrared LEDs. Movement sensors, for example in optical computer mice Because of their long life and fast switching times, LEDs have been used for automotive high-mounted brake lights and truck and bus brake lights and turn signals for some time, but many high-end vehicles are now starting to use LEDs for their entire rear light clusters. Besides the gain in reliability, this has styling advantages because LEDs are capable of forming much thinner lights than incandescent lamps with parabolic reflectors. The availability of LEDs in specific colors (RGB) enables a full-spectrum light source which expands the color gamut by as much as 45%. New stage lighting equipment is being developed with LED sources in primary red-green-blue arrangements. For example, a set of 50 multi-colored incandescent Christmas lights might cost $2.00 USD, while a similar set of 50 multi-colored LED Christmas lights might cost $10.00 USD. Regardless of the higher initial purchase price, the total cost of ownership for LED Christmas lights would eventually be lower than the TCO for similar incandescent Christmas lights since an LED requires less power to output the same amount of light as a similar incandescent bulb. LED phototherapy for acne using blue or red LEDs has been proven to significantly reduce acne over a 3 month period.Illumination applications
LEDs used as a replacement for incandescent light bulbs and fluorescent lamps are known as solid-state lighting (SSL) - packaged as a cluster of white LEDs grouped together to form a light source (pictured). Recently a number of manufacturers have started marketing ultra-compact LCD video projectors that use high-powered white LEDS for the light source. Another alternative design is to use red, green, and blue LEDs in a sequential DLP design.
Proponents of LEDs expect that technological advances will reduce costs such that SSL can be introduced into most homes by 2020. However, they are still not commercially viable for general lighting applications, and so LEDs are found today in illumination applications where their special characteristics provide a distinct advantage.
Due to their monochromatic nature, LED lights have great power advantages over white lights when a specific color is required. Unlike traditional white lights, the LED does not need a coating or diffuser that can absorb much of the emitted light. LED lights are inherently colored, and are available in a wide range of colors. Yellow LED lights are a good choice to meet these special requirements because the human eye is more sensitive to yellow light (about 500 lm/watt emitted) than that emitted by the other LEDs.
The first residence lit solely by LED's was the "Vos Pad" in London. The entire flat is lit by a combination of white and RGB (colour changing) LED's.
LED display panels
There are two types of LED panels: conventional, using discrete LEDs, and Surface Mounted Device (SMD) panels. Most outdoor screens and some indoor screens are built around discrete LEDs, also known as individually mounted LEDs. Fashion and auto shows are two examples of high-brightness stage lighting that may require higher LED brightness. (The brightness of LED panels can be reduced from the designed maximum, if required.)
Suitable locations for large display panels are identified by factors such as line of sight, local authority planning requirements (if the installation is to become semi-permanent), vehicular access (trucks carrying the screen, truck-mounted screens, or cranes), cable runs for power and video (accounting for both distance and health and safety requirements), power, suitability of the ground for the location of the screen (check to make sure there are no pipes, shallow drains, caves, or tunnels that may not be able to support heavy loads), and overhead obstructions.
Early LED Flat Panel TV History
Perhaps the first recorded flat Light-emitting diode display LED television screen prototype to be developed was by James P Mitchell in 1977. The LED flat panel TV display received special recognition by NASA, General Motors Corporation, and area Universities including The University of California Irvine, Robert M Saunders Prof. Additionally, technology business represetatives from the U.S. and overseas witnessed operation of the monochomatic LED flat panel television display. The blue LED did not emerge until the mid-1980s, completing the RGB color triad.
Multi-touch sensing
Since LEDs share some basic physical properties with photodiodes, which also use p-n junctions with band gap energies in the visible light wavelengths, they can also be used for photo detection.
In this usage, various LEDs in the matrix are quickly switched on and off. LEDs that are on shine light onto a user's fingers or a stylus. LEDs that are off function as photodiodes to detect reflected light from the fingers or stylus. The voltage thus induced in the reverse-biased LEDs can then be read by a microprocessor, which interprets the voltage peaks and then uses them elsewhere.
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