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An LED is an electronic device that emits light when sufficient current flows throughout the object.led is very heavily doped.
We know that in each diode there is a recombination of electrons and holes.There are 2 types of recombinations. This recombination of electrons and holes releases some energy which is invisible in most of the diodes(in the form of heat) but in some diodes it is visible in the form of light. When light is emitted on forward biasing of a P N junction than it is known as electroluminance. In this way, the frequency of emitted light from the diode may be in visible spectrum or in invisible spectrum. Emitted light from the diode can be divided into two parts, (i) light which is visible to human and (ii) light which is invisible to human. The frequency range visible to a normal man is 400nm (nenometer) to 700nm (nenometer). Light is generally coloured in this range. The waves of light with wavelength below 400nm (nenometer) are known as ultraviolet and waves of light with length greater than 7(K)nm are known as infra red.For LED, the semiconductor  Light emitted from LED depends upon two factors,
- On quantity of doping of P and N-type semiconductor material
- On the nature of the added materials. Gallium Phosphide and Gallium Arsenide are such materials which are mostly used in making LEDS. Red and green coloured LEDS are made up of gallium phasphide.
Coloured LEDS are generally used as indicator and light source for photo detectors. Infra red and ultraviolet LEDS are used for transmitting invisible signals. These are also used for photo detector in the form of invisible light source. In the remote control system infra red and ultraviolet LEDS are used generally. LEDS which are used generally have the current capacity of 20MA and maximum forward voltage capacity of 1.6V. LED emits light only ' when sufficient current flows through, it. On low current either no light or dim light will be recieved from the LED. Red coloured LEDS are used for warning yellow, green and orange coloured LEDS are used for other different purposes. If these are used in parallel with the speakers than they glows according to the signal. Now a days, computerized display board which are used for advertisement have LEDS. LEDS are also used in the graphic equalizer of a stereo audio system whenever it is to be used as supply indicator than a proper value resistance must be connected in series with it so that forward voltage and current can be controlled. Otherwise the LEDS will become defective.