LED is an acronym for Light Emitting Diode. Russian Oleg Vladimirovich Losev, working in a Soviet radio laboratory reported creation of the first LED in 1927. In the introduction to his 1927 patent application he stated,
'The proposed invention uses the known phenomenon of luminescence of a carborundum [a compound of silicon and carbon] detector and consists of the use of such a detector in an optical relay for the purpose of fast telegraphic and telephone communication, transmission of images and other applications when a light luminescence contact point is used as the light source connected directly to a circuit of modulated current.' (Ref #1)
Even earlier than this in 1907 the Englishman, Henry J. Round discovered the basis of electroluminescence, after noting 'yellowish light' after applying 10Volts across a carborundum crystal, (Ref #2). Low powered (often red) LEDs were first used commercially back in the 60s, but they have more recently been used for residential lighting (and backlighting in flat screen televisions)
A LED directly converts electricity into light. LEDs are made from joining together two semiconducting materials. On one side is a 'p-type' semiconducting material on the other side is a 'n-type' semiconducting material. P and N stand for positive and negative. These different materials are made by taking a pure semiconductor such as silicon, then mixing in small amounts of impurities such as Aluminium and Phosphorus, in a process known as doping. As electrons from the electrical current cross the boundary between the P and N materials the electrons move to a lower energy state and emit light of a certain colour. Different materials such as silicon carbide or gallium-nitride and different doping methods result in different coloured LEDs.
Because LEDs typically only emit monochromatic light, or light of a single colour, there are a couple of different methods to get white light (which is a mix of colours) from a LED. One method is to carefully mix up several different colored LEDs in a single light fixture to simulate white light. Another method uses a single blue or ultraviolet LED in combination with a phosphor, which absorbs the blue light and emits a broader spectrum of white light (similar to fluorescent lamps).
#1 - Losev, O. V., Soviet patent number 12191 (approved in 1929).
#2 - Round, H. J., Electrical World publication, 49, 308 (1907).
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