Light-emitting diodes (LEDs)

HeNe laser
Laser Diodes
Optical spectrum

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 .

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 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.

Light emission

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 materials

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:

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.

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 and does not affect the color of the light emitted.

LED applications

Here is a list of known applications for LEDs, some of which are further elaborated upon in the following text:

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.

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