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The advantages of LEDs

A light diode (LED) is an electronic semi-conductor element, that converts electric current directly into light – when electric current is streaming through the diode in the direction of transmission, it emits light. LEDs consume little energy, do almost not generate heat, do not contain infrared heat or UV rays, they are shock resistant, do not have an incandescent filament, reach significantly shorter switch periods and have a very long life. Other than incandescent lamps, light diodes are not heat emitters. They emit light in a limited spectral segment, the light is almost monochromatic. They are therefore – compared to other light sources, where color filters have to filter out the major part of the spectrum – very effective for signal installations.
Light diodes do usually not break down they only become weaker. They do not have a hollow body which could implode. Their duration of life depends on the semi-conductor material and the operating conditions – high temperatures due to high current shortens their duration of life. Light diodes are usually operated by using a series resistance (rectifier) or a constant current source – the voltage of this must not be exceeded. The indicated lifetime ranges from some 1,000 hours at 5 W LEDs to more than 100,000 operating hours at LEDs, that operate with low power.

Configuration

The picture below left shows the configuration of a standard light diode. The chip is embedded in a reflecting tub. The picture to the right shows the reflector after the removal of the transparent plastic coating. The rectangular wire supporting the reflector, establishes the contact to the cathode and absorbs the lost heat. In the centre of the chip you can see part of the bond wire, which establishes the contact to the anode. On the picture to the left the bond wire is shown as a horizontal line on the right side above the carrier.
The cathode (-) is marked by a bevel at the left of the housing socket. The cathode connection of brand-new LEDs is shorter (standard rule: cathode = short). In most of the LEDs the reflector is the cathode, there is also a standard rule: the technical current direction is “indicated” by the arrow built by the electrode’s form (see picture to the right). Attention: in rare cases the configuration is just the other way round !
High-power LEDs are operated with higher power. This requires a higher performance in the heat elimination, expressed in specific construction types. The heat can either be carried off via the feeders, or the power is supplied via two bond wires and the heat is eliminated through a separated reflecting tub.
The industrial processing of THD (through-hole-device) LEDs is complex and expensive. LEDs are therefore also produced in SMD (surface-mounted-device) housings. An additional possibility is the direct “bonding” of the LED chip on a circular board (chip on board – COB).
Multicolor light diodes consist of several (2 or 3) diodes in one housing.
The version with 2 connections has 2 LEDs in opposite directions connected in parallel. Depending on the polarity the one or the other diode is emitting light. An alternating voltage excites both diodes and produces an overlapping color.

Technology

Through specific selection of the semiconductor materials and the endowment, the characteristics of the produced light can be varied. Especially the spectral segment (in the visible segment this corresponds to the color) and the efficiency can be controlled this way:

• aluminum-gallium-arsenide (AlGaAs) – red and infrared, max. wavelength 1,000nm
• gallium-aluminum-arsenide (GaAlAs) – e.g. 665 nm: red, max. wavelength 1,000nm
• gallium-arsenide-phosphide (GaAsP) and aluminum-indium-gallium-phosphide (AlInGaP) – red, orange and yellow
• gallium-phosphide (GaP) – green
• silicone-carbide (SiC) – the first commercial blue LED; low efficiency
• indium-gallium-nitride (InGaN)/gallium-nitride (GaN) – UV, blue and green
• white LEDs are in most of the cases blue LEDs with a phosphoric layer operating as luminescence converter (see: White LED)

Several epitaxie procedures are employed for the production of LED-semiconductors.

Characteristics

Incandescent lamps emit heat, whereas light diodes do not. They emit light in a limited spectral segment, the light is almost monochromatic. They are therefore – compared to other light sources, where color filters have to filter the major part of the spectrum – very effective for signal installations.
The lifetime of an LED is defined as the period during which the light of the LED has fallen to half of its original intensity. Light diodes are getting weaker and weaker in their light intensity, they usually do not fall out. Light diodes are shock resistant. They do not have a hollow body, which could implode. Their lifetime depends on the respective semiconductor material and the operating conditions (heat, electric current). The indicated lifetime ranges from several 1,000 hours in the case of 5 W LEDs to more than 100,000 hours in the case of LEDs operating with low powers. High temperatures (e.g. through high voltages) shorten the lifetime of LEDs drastically.
The short switch periods of LEDs are very important, e.g. in optoelectronics. The modulation frequency can amount up to 100 MHz.
Light diodes have an exponentially increasing current-voltage-curve. During their operation the electric current has to be limited by an additional construction element, in the simplest case a resistor or a constant power source. In this case a more or less intensive and in most of the cases unintended alteration of the nominal current (and therefore also alterations in the light intensity) and even the destruction of the light source can be avoided effectively. The power input varies from model to model between 2 mA (e.g.: miniaturized SMD LED), 20mA (e.g.: 5-mm-LED) and abt. 700 or more mA in the case of LEDs for illumination purposes. The forward voltage ranges in this case from 1.3 V (infrared LED) to abt. 4 V (InGaN-LED: green, blue, UV).

White LED

Different procedures are used to produce white light with light diodes:

• Three light diodes in the colors red, green and blue (RGB) are connected with each other and produce white light (used for: displays, effective illumination). This can be obtained either by separate LEDs or by three LED-chips in one housing. White light can also be obtained by connecting only two LEDs in the colors blue and yellow.
• The LED-chip is covered with fluorescence coloring. Similar to fluorescent lamps, high-energy short-wave light (blue/UV light) is transformed into low-energy long-wave light. Through the correct selection of components the additive color mix brings White as result. The selection of the colorings varies in this case with the employed materials. Either you excite different colorings (e.g.: RGB) with one UV-LED, whose combination produces white light, or you take a blue LED as base and use only one coloring (yellow, in most of the cases Cer-endowed YAG).

The use of several colorings is more expensive and results in a lower light output, but it allows to obtain good color reproduction characteristics (color reproduction index Ra 90).

Commercial illuminations – due to cost reasons – do always use the version with one light diode in combination with colorings:
Customary white LEDs usually consist of one blue LED coated with a yellow-fluorescent layer of Cer-endowed yttrium-aluminum-garnet powder. This method is the most economic way of producing white light with LEDs, due to the fact that blue LEDs are the most effective (UV-LEDs, e.g., do not bring half of the light output), and the high UV percentage, that each blue LED reflects in addition, by which this fluorescent layer is transformed into mainly yellow light.
The kind of coloring used for coating is in this case decisive for the quality. The graphic shows clearly, that the fluorescent coating with yellow light in the medium ranges produces a broadband light leading therefore to a balanced spectrum. The overlapping with most of the red colorings, on the other hand, is very bad, because it encroaches the color reproduction and leads to a weak red-reproduction on colored displays. RGB-LEDs, however, (not shown on this graphic) produce three, quite “peaked”, i.e.: narrowband color portions. Although narrowband colorings might cause problems, their reproduction is usually more effective, achieving brilliant colors, especially on colored displays. Variations in the fluorescent layer result (especially on the rims) in a inhomogeneous light color depending on the beam angle. White LEDs are manufactured for different kinds of white light (well known are: the cold light as from neon lamps and the warm light as from incandescent lamps). The differences in the light color can be indicated with the color temperature (measured in Kelvin). A color temperature of 3,000 K corresponds to a warm, reddish light color, whereas 6,000 K describe a cold color similar to daylight.