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A light-emitting diode (LED) is a semiconductor device that emits incoherent narrow-spectrum light when electrically biased in the forward direction. This effect is a form of electroluminescence. The color of the emitted light depends on the chemical composition of the semiconducting material used, and can be near-ultraviolet, visible or infrared. Rubin Braunstein. (born 1922) of the Radio Corporation of America first reported on infrared emission from GaAs and other semiconductor alloys in 1955. Nick Holonyak Jr. (born 1928) of the General Electric Company developed the first practical visible-spectrum LED in 1962.

LED technology

Physical function

An LED is a special type of semiconductor diode. Like a normal diode, it consists of a chip of semiconducting material impregnated, or doped, with impurities to create a structure called a p-n junction. As in other diodes, current flows easily from the p-side, or anode to the n-side, or cathode, but not in the reverse direction. Charge-carriers - electrons and holes flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon as it does so.

The wavelength of the light emitted, and therefore its color, depends on the bandgap energy of the materials forming the p-n junction. In silicon or germanium diodes, the electrons and holes recombine by a non-radiative transition which produces no optical emission, because these are indirect bandgap materials. The materials used for an LED have a direct bandgap with energies corresponding to near-infrared, visible or near-ultraviolet light.

LED development began with infrared and red devices made with gallium arsenide. Advances in materials science have made possible the production of devices with ever shorter wavelengths, producing light in a variety of colors.

Conventional LEDs are made from a variety of inorganic semiconductor materials, producing the following colors:

• aluminium gallium arsenide (AlGaAs) - red and infrared
• aluminium gallium phosphide (AlGaP) - green
• aluminium 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
• 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
• aluminium nitride (AlN), aluminium gallium nitride (AlGaN) - near to far ultraviolet

Blue and white LEDs

Commercially viable blue LEDs based on the wide bandgap semiconductor gallium nitride and 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.

Most "white" LEDs in production today use a 450 nm – 470 nm blue GaN (gallium nitride) LED covered by a yellowish phosphor coating usually made of cerium-doped yttrium aluminium garnet (Ce:YAG) crystals which have been powdered and bound in a type of viscous adhesive. The LED chip emits blue light, part of which is efficiently converted to a broad spectrum centered at about 580 nm (yellow) by the Ce:YAG. The single crystal form of Ce:YAG is actually considered a scintillator rather than a phosphor. 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.

The pale yellow emission of the Ce:YAG can be tuned by substituting the cerium with other rare earth elements such as terbium and gadolinium and can even be further adjusted by substituting some or all of the aluminium in the YAG with gallium. 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 make the LEDs produce light with different color temperatures, from warm yellowish to cold bluish; the LEDs have to be sorted during manufacture by their actual characteristics.

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 aluminium doped zinc sulfide (ZnS:Cu,Al). This is a method analogous to the way fluorescent lamps work. 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 renders colors 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. This technique produces a warm, yellowish-white light similar to that produced by incandescent bulbs.

Other colors

Recent color developments include pink and purple. 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. Some pink LEDs have run into issues. 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 infrareds and are thus less commonly used in commercial applications.

The semiconducting chip is encased in a solid plastic lens, which is much tougher than the glass envelope of a traditional light bulb or tube. The plastic may be colored, but this is only for cosmetic reasons or to improve the contrast ratio; the color of the packaging does not substantially affect the color of the light emitted.

Organic light-emitting diodes (OLEDs)

If the emissive layer material of an LED is an organic compound, it is known as an Organic Light Emitting Diode (OLED). To function as a semiconductor, the organic emissive material must have conjugated pi bonds. The emissive material can be a small organic molecule in a crystalline phase, or a polymer. Polymer materials can be flexible; such LEDs are known as PLEDs or FLEDs.

Compared with regular LEDs, OLEDs are lighter and polymer LEDs can have the added benefit of being flexible. Some possible future applications of OLEDs could be:

• Inexpensive, flexible displays
• Light sources
• Wall decorations
• Luminous cloth

Operational parameters and efficiency

Most typical LEDs are designed to operate with no more than 30-60 milliwatts of electrical power. Around 1999, commercial LEDs capable of continuous use at one watt of input power were introduced. These LEDs used much larger semiconductor die sizes to handle the large power input. As well, the semiconductor dies were mounted to metal slugs to allow for heat removal from the LED die. In 2002, 5-watt LEDs were available with efficiencies of 18-22 lumens per watt. It is projected that by 2005, 10-watt units will be available with efficiencies of 60 lumens per watt. These devices will produce about as much light as a common 50-watt incandescent bulb, and will facilitate use of LEDs for general illumination needs.

In September 2003 a new type of blue LED was demonstrated by the company Cree, Inc. to have 35% efficiency at 20 mA. 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 2005 they have demonstrated a prototype with a record white LED efficiency of 70 lumens per watt at 350 mA .

Today, OLEDs operate at substantially lower efficiency than inorganic (crystaline) LEDs. The best efficiency of an OLED so far is about 10%. 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.

Considerations in use

Unlike incandescent light bulbs, which light up regardless of the electrical polarity, LEDs will only light with positive electrical polarity. When the voltage across the pn junction is in the correct direction, a significant current flows and the device is said to be forward-biased. If the voltage is of the wrong polarity, the device is said to be reverse biased, very little current flows, and no light is emitted. LEDs can be operated on an AC 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:

It should be noted that looking at the inside of the LED is not an accurate way of determining polarity. While in most LEDs the large part is the "-", in some it is the + terminal. The flat tab or the short pin are more accurate ways of determining polarity.

Because the voltage versus current characteristics of an LED are much like any diode, they can be destroyed by connecting them to a voltage source higher than their turn-on voltage. Most LEDs have low reverse breakdown voltage ratings, so they will also so be damaged by an applied reverse voltage of more than a few volts. Since some manufacuters 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.

Because of the risk of excess voltage damaging the device, a good LED driver circuit is a constant current source. If high efficiency is not required, an approximation to a current source made by connecting the LED in series with a current limiting resistor to a voltage source may be substituted. To increase efficiency, the power may be applied periodically or intermittently; so long as the flicker rate is greater than the human flicker fusion threshold, the LED will appear to be continuously lit.

Parallel operation is generally problematic. The LEDs have to be of the same type in order to have a similar forward voltage. Even then, variations in the manufacturing process can make the odds of satisfactory operation low. For more information see Nichia Application Note.

Some 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. These can be driven to different colors without reversing the polarity.

LED units may have an integrated multivibrator circuit that makes the LED flash.

Advantages of using LEDs

• LEDs are capable of emitting light of an intended color without the use of color filters that traditional lighting methods require.
• The shape of the LED package allows light to be focused. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a useable manner.
• LEDs are insensitive to vibration and shocks, unlike incandescent and discharge sources.
• LEDs are built inside solid cases that protect them, making them hard to break and extremely durable.
• LEDs have an extremely long life span: typically ten years, twice as long as the best fluorescent bulbs and twenty times longer than the best incandescent bulbs.
• Further, LEDs fail by dimming over time, rather than the abrupt burn-out of incandescent bulbs.
• LEDs give off less heat than incandescent light bulbs with similar light output.
• LEDs light up very quickly. An illumination LED will achieve full brightness in approximately 0.01 seconds, 10 times faster than an incandescent light bulb (0.1 second), and many times faster than a compact fluorescent lamp, which starts to come on after 0.5 seconds or 1 second, but does not achieve full brightness for 30 seconds or more. A typical red indicator LED will achieve full brightness in microseconds, or possibly less if it's used for communication devices.

Disadvantages of using LEDs

• LEDs are currently more expensive than more conventional lighting technologies. The additional expense partially stems from the relatively low lumen output (requiring more light sources) and drive circuitry/power supplies needed. A good measure to compare lighting technologies is lumen/dollar.
• LED performance largely depends on the ambient temperature of the operating environment. "Driving" an LED 'hard' in high ambient temperatures may result in overheating of the LED package, eventually to device failure. Adequate heat-sinking is required to maintain long life. This is especially important when considering automotive/military applications where the device must operate over a large range of temperatures, with government-regulated output.

LED applications

List of known applications for LEDs

Some of these applications are further elaborated upon in the following text.

LEDs are used as informative indicators in various types of embedded systems:

• Status indicators, e.g. on/off lights on professional instruments and consumers audio/video equipment.
• In toys, especially as light-up "eyes" of robot toys.
• continuity indicators
• Seven segment displays, in calculators and measurement instruments, although now mostly replaced by liquid crystal displays.
• Thin, lightweight message displays, e.g. in public information signs (at airports and railway stations and as destination displays for trains, buses, trams and ferries).
• Red or yellow LEDs are used in indicator and numeric displays in environments where night vision must be retained: aircraft cockpits, submarine and ship bridges, astronomy observatories, and in the field, e.g. night time animal watching and military field use.

LEDs may also be used to transmit digital information:

• Remote controls for TVs, VCRs, etc, using Infrared LEDs.
• In fiber optic communications.
• In dot matrix arrangements for displaying messages.

LEDs find further application in safety devices, where high brightness and reliability are critical:

• In traffic signals, LED clusters are replacing colored incandescent bulbs.
• In level crossing lights, red LEDs have been used to replace incandescant bulbs.
• In car brake and indicator lights, where the quick-on characteristic of LEDs enhances safety.
• In bicycle lighting; also for pedestrians to be seen by car traffic.
• Signaling and emergency beacons or strobes.
• Navigation lights on boats which are red, green, and white and shine in specific directions. Boats use direct current batteries to power their lights, so not only does that match the requrements of LEDs, but the efficiency of colored LEDs is a big advantage.

LEDs are also used for illumination:

• In photographic darkrooms, red or yellow LEDs are also used for providing lighting which does not lead to unwanted exposure of the film.
• In flashlights (US) / torches (UK), and backlights for LCD screens.
• As a replacement for incandescent and fluorescent bulbs in home and office lighting, an application known as Solid State Lighting (SSL).
• As disco/club lighting products.
• In projectors. LED projectors are smaller, lighter, and produce much less heat than incandescent technology.

Finally, LEDs have additional applications not categorized above:

• Movement sensors, for example, in mechanical and optical computer mice and trackballs.
• In pulse oximeters, both a red and an infra-red LED are used.
• In LED printers such as high-end color printers.
• In phototherapy, the use of light for healing purposes.
• More recently, LEDs have been used as a replacement to incandescent bulbs for Christmas lights.

Illumination applications

LEDs used as a replacement for incandescent bulbs and fluorescent lamps are known as Solid State Lighting (SSL) LEDs. SSL LEDs are packaged as a cluster of white LEDs grouped together to form a light source. LEDs are moderately efficient: the average commercial LED currently outputs 32 lumens per watt (lm/W), and new technologies promise to deliver up to 80 lm/W. They are also more mechanically robust than incandescent light bulbs and fluorescent tubes. LEDs today are not sold in many places, require power source conversion in household applications, and are relatively expensive, although their costs are decreasing.

Incandescent bulbs are much less expensive but also less efficient, generating from about 16 lm/W for a domestic tungsten bulb to 22 lm/W for a halogen bulb. Fluorescent tubes are more efficient, providing 50 to 100 lm/W for domestic tubes (average 60 lm/W), but are bulky and fragile and require starter or ballast circuits that sometimes buzz audibly. Compact fluorescent light bulbs, which include a quiet integrated ballast, are relatively robust and efficient, fit in standard light bulb sockets, and are currently the best choice for efficient household lighting.

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. One of the most recently introduced colors is the emerald green (bluish green, about 500 nm) that meets the legal requirements for traffic signals and navigation lights.

There are applications that specifically require light without any blue component. Examples are photographic darkroom safe lights, illumination in laboratories where certain photo-sensitive chemicals are used, and situations where dark adaptation (night vision) must be preserved, such as cockpit and bridge illumination, observatories, etc. 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.

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. A cluster of red, green, and blue diodes is driven together to form a full-color pixel, usually square in shape. These pixels are spaced evenly apart and are measured from center to center for absolute pixel resolution. The largest LED display in the world is over 1,500 feet long and is located in Las Vegas, Nevada covering the Fremont Street Experience.

Most indoor screens on the market are built using SMD technology — a trend that is now extending to the outdoor market. An SMD pixel consists of red, green, and blue diodes mounted on a chipset, which is then mounted on the driver PC board. The individual diodes are smaller than a pinhead and are set very close together. The difference is that minimum viewing distance is reduced by 25% from the discrete diode screen with the same resolution.

Indoor use generally requires a screen that is based on SMD technology and has a minimum brightness of 600 candelas per square metre (unofficially called nits). This will usually be more than sufficient for corporate and retail applications, but under high ambient-brightness conditions, higher brightness may be required for visibility. Fashion and auto shows are two examples of high-brightness stage lighting that may require higher LED brightness. Conversely, when a screen may appear in a shot on a television show, the requirement will often be for lower brightness levels with lower color temperatures (common displays have a white point of 6500-9000K, which is much bluer than the common lighting on a television production set).

For outdoor use, at least 2,000 nits are required for most situations, whereas higher brightness types of up to 5,000 nits cope even better with direct sunlight on the screen. Until recently, only discrete diode screens could achieve that brightness level. (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 sight lines, 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.

Multi-Touch Sensing

Since LEDs share some basic physical properties with photodiodes, which also use p-n junctions with bandgap energies in the visible light wavelengths, they can also be used for photodetection. These properties have been known for some time, but more recently so-called bidirectional LED matrices have been proposed as a method of touch-sensing. In 2003, Dietz, Yerazunis, and Leigh published a paper describing the use of LEDs as cheap sensor devices.

In this usage, the LED matrix quickly switches itself on and off. When the LEDs are on, they shine light onto a user's fingers or a stylus. When the LEDs switch off again, they 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. The website of Jeff Han features a video demonstrating one such implementation of an LED matrix multi-touch sensor.

References

• Mills, Evan (2005). "The Specter of Fuel-Based Lighting". Science. 308: 1274–1278.
• Salisbury, David F., Quantum dots that produce white light could be the light bulb’s successor Exploration - The Online Research Journal of Vanderbilt University October 20, 2005 (more details regarding the use of quantum dots as a phosophor for white LEDs).

See also

• Laser diode, a coherent solid-state light source
• Nixie tube

Resources

• Chart showing voltage drops of various colors
• Light Resource Center: SPE method, which results in 80l/w (vs 60l/w for fluorescent lighting)
• Light Resource Center: Information on developing a standard lifetime measurement for LEDs

External links

• How LEDs work - from Howstuffworks.com
• The LED FAQ Pages - A compilation of questions and answers about light-emitting diodes and infrared emitters
• LED Center - Information for hobbyists and articles on solid state lighting
• Free IC DataSheet Search Site : http://www.Datasheet4U.com

• Display technology
• Optical diodes
• Lighting