Light-emitting diodes (LEDs)Laser
A light -emitting diode (LED) is a semiconductor device that emits incoherent monochromatic light when electrically biased in the forward direction. This effect is a form of electroluminescence . The color depends on the semiconducting material used, and can be near- ultraviolet , visible or infrared .
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 pn junction. Charge-carriers (electrons and holes) are created by an electric current passing through the junction. 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 pn junction. A normal diode, typically made of silicon or germanium, emits invisible far-infrared light, but the materials used for an LED have bandgap energies corresponding to near-infrared, visible or near-ultraviolet light
Considerations in use
Unlike incandescent light bulbs, which can operate with either AC or DC, LEDs require a DC supply of the correct 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. The voltage across the LED in this case is fixed for a given LED and is proportional to the energy of the emitted photons. 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.
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 much higher than their turn on voltage. A good LED driver circuit is either a constant current source or an approximation to a current source made by connecting the LED in series with a current limiting resistor to a voltage source. The voltage drop across a forward biased LED increases as the amount of light emitted increases because of the optical power being radiated. One consequence is that LEDs of the same type can be readily operated in parallel. The turn-on voltage of an LED is a function of the color, a higher forward drop is associated with emitting higher energy (bluer) photons. The reverse voltage that most LEDs can sustain without damage is usually only a few volts.
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 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 minerals, producing the following colors:
- aluminium gallium arsenide (AlGaAs) - red and infrared
- gallium aluminium phosphide - green
- gallium arsenide/phosphide (GaAsP) - red, orange-red, orange, and yellow
- gallium nitride (GaN) - green, pure green (or emerald green), and blue
- gallium phosphide (GaP) - red, yellow and green
- zinc selenide (ZnSe) - blue
- indium gallium nitride (InGaN) - bluish-green and blue
- indium gallium aluminium phosphide - orange-red, orange, yellow, and green
- silicon carbide (SiC) - blue
- diamond (C) - ultraviolet
- silicon (Si) - under development
|Color Name||LED Dye Material|
|940||Infrared||GaAIAs/GaAs -- Gallium Aluminum Arsenide/Gallium Arsenide|
|880||Infrared||GaAIAs/GaAs -- Gallium Aluminum Arsenide/Gallium Arsenide|
|850||Infrared||GaAIAs/GaAs -- Gallium Aluminum Arsenide/Gallium Arsenide|
|660||Ultra Red||GaAIAs/GaAs -- Gallium Aluminum Arsenide/Gallium Arsenide|
|635||High Eff. Red||GaAsP/GaP - Gallium Arsenic Phosphide / Gallium Phosphide|
|633||Super Red||InGaAIP - Indium Gallium Aluminum Phosphide|
|620||Super Orange||InGaAIP - Indium Gallium Aluminum Phosphide|
|612||Super Orange||InGaAIP - Indium Gallium Aluminum Phosphide|
|605||Orange||GaAsP/GaP - Gallium Arsenic Phosphide / Gallium Phosphide|
|595||Super Yellow||InGaAIP - Indium Gallium Aluminum Phosphide|
|592||Super Pure Yellow||InGaAIP - Indium Gallium Aluminum Phosphide|
|585||Yellow||GaAsP/GaP - Gallium Arsenic Phosphide / Gallium Phosphide|
|574||Super Lime Yellow||InGaAIP - Indium Gallium Aluminum Phosphide|
|570||Super Lime Green||InGaAIP - Indium Gallium Aluminum Phosphide|
|565||High Efficiency Green||GaP/GaP - Gallium Phosphide/Gallium Phosphide|
|560||Super Pure Green||InGaAIP - Indium Gallium Aluminum Phosphide|
|555||Pure Green||GaP/GaP - Gallium Phosphide/ Gallium Phosphide|
|525||Aqua Green||SiC/GaN - Silicon Carbide / Gallium Nitride|
|505||Blue Green||SiC/GaN - Silicon Carbide / Gallium Nitride|
|470||Super Blue||SiC/GaN - Silicon Carbide / Gallium Nitride|
|430||Ultra Blue||SiC/GaN - Silicon Carbide / Gallium Nitride|
Blue and white LEDs
Commercially viable blue LEDs based on the wide bandgap semiconductor 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.
Most "white" LEDs in production today use a 450nm - 470nm blue GaN (gallium nitride) LED covered by a yellowish phosphor coating usually made of cerium doped yttrium aluminium garnet (YAG:Ce) crystals which have been powdered and bound in a type of viscous adhesive. The LED chip emits blue light, part of which is converted to yellow by the YAG:Ce. The single crystal form of YAG:Ce 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.
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 lights work.
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.
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 and does not affect the color of the light emitted.
Here is a list of known applications for LEDs, some of which are further elaborated upon in the following text:
- in general, commonly used as information indicators in various types of embedded systems (many of which are listed below)
- thin, lightweight message displays, e.g. in public information signs (at airports and railway stations, among other places)
- status indicators, e.g. on/off lights on professional instruments and consumers audio/video equipment
- infrared LEDs in remote controls (for TVs, VCRs, etc)
- clusters in traffic signals, replacing ordinary bulbs behind colored glass
- car indicator lights and bicycle lighting; also for pedestrians to be seen by car traffic
- calculator and measurement instrument displays (seven segment displays), although now mostly replaced by LCDs
- red or yellow LEDs are used in indicator and [alpha]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
- red or yellow LEDs are also used in photographic darkrooms, for providing lighting which does not lead to unwanted exposure of the film
- illumination, e.g. flashlights (US) / torches (UK), and backlights for LCD screens
- signaling/emergency beacons and strobes
- movement sensors, e.g. in mechanical and optical computer mice and trackballs
- in LED printers, e.g. high-end color printers
- phototherapy, the concept of using light for healing purposes
LEDs offer benefits in terms of maintenance and safety. The typical working lifetime of a device, including the bulb, is ten years, which is much longer than the lifetimes of most other light sources. 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 and are less fragile than fluorescent lamps. Since an individual device is smaller than a centimetre in length, LED-based light sources used for illumination and outdoor signals are built using clusters of tens of devices.
From Wikipedia, the free encyclopedia http://en.wikipedia.org