Light Emitting Diodes
Light Emitting Diodes or LED´s, are one of the most widely used of all the different types of the semiconductor diodes available today. They are the most visible type of diode which emit a narrow bandwidth of either visible light at different colored wavelengths, invisible infra-red light for the remote controls or laser type light when the forward current is passed through them. A "Light Emitting Diode" or LED as it is more commonly known, is basically just a specialized type of PN junction diode, made from the very thin layer of heavily doped semiconductor material. When diode is forward biased, electrons from semiconductors conduction band recombine with the holes from valence band releasing sufficient energy to generate photons which emit a monochromatic (single color) of light. Cause of this thin layer a reasonable number of these photons can leave the junction and radiate producing a colored light output. So we can say that when operated in the forward biased direction Light Emitting Diodes are semiconductor devices which convert electrical energy into light energy.
The construction of the light emitting diode is quite different from the normal signal diode. The PN junction of an LED is surrounded by the transparent, hard plastic epoxy resign hemispherical shaped shell or body which protects the LED from vibration and shock both. Surprisingly, an LED junction does not emit that much light so the epoxy resin body is constructed in such a manner that the photons of light emitted by junction are reflected away from the surrounding substrate base to which the diode is attached and are focused upwards by the domed top of the LED, which itself behaves like a lens concentrating the amount of light. This is why the emitted light appears to be brightest at the top of LED.
However, not all the LEDs are made with a hemispherical shaped dome for the epoxy shell. Some LEDs also have a rectangular or cylindrical shaped construction which has a flat surface on top. Also, nearly all the LEDs have their cathode, (K) terminal identified by either a notch or flat spot on the body, or by one of the leads being shorter than other.
Not like normal incandescent lamps and bulbs which generate large amounts of heat when illuminated, light emitting diode produces a "cold" generation of light which leads to the high efficiencies than normal "light bulb" cause most of the generated energy radiates away within visible spectrum. Since the LEDs are solid-state devices, they can be exceptionally small and durable and offers much longer lamp life than the normal light sources.
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