The Light emitting diode is a special type of diode, where the PN junction emits light. This is due to recombination of charge carriers (electrons). The most widely-used semiconductor in LEDs is gallium.
By using gallium arsenide(GaAs) we get a diode with a infrared to red emission. The GaAs LED has a forward voltage drop of about 1.4V.
Gallium arsenide phosphide (GaAsP) is used in diodes with frequencies between red and yellow. they have a voltage drop of ~2V.
Gallium phosphide diodes have a green to blue-green colour, and a voltage drop of about 3V.
By using different amounts of arsenide and phosphide in the manufacturing proccess it is possible to make many different colours.
Blue LEDs have proven to be harder to make, and are therefore expensive. They are made from silicon carbide or gallium nitride but are less efficient than other LEDs. But development of alternative production proccesses is underway.
LEDs are available in several shapes and sizes. You can also buy multicolor LEDs, that have both a red and green chip so you can get any colour between red and green. RGB diodes are also available but are very expensive and not very widely-used. White LEDs have also been developed and are available.
Visible light LEDs are used for indicator lights.
High intensity LEDs can be used as lighting, but only at short ranges. Infrared LEDs are used for data transmission.
A lesser known fact about LEDs is that they are also light sensitive, and have a high sensitivity to light of the same frequency that they emit. Therefore a LED can be used as a light sensor.
LEDs must always be used in series with a resistor to limit the current. The LED must be connected the right way to work, usualy anode(+) is longer than the cathode(-). Modern LEDs usually have built in protection against connection of wrong polarity, but older ones might burn out. On used LEDs it might be hard to identify the electrodes. but most LEDs have the following configuration:(The cathode is marked by a flat edge on the package).
_--_
/ \
|. __ |
||\ \ ||
||| |||
======== - Flat edge.
|| ||
A || || C
||--- Longer leg