The phenomenon of electroluminescence was first observed in a piece of Silicon Carbide (SIC), in 1907 by Henry Joseph Round. The yellow light emitted by it was too dim to be of practical use and difficulties in working with Silicon Carbide meant that research was abandoned. Further experiments were carried out in Germany in the late 1920s by Bernhard Gudden and Robert Wichard Pohl, using phosphor materials made from Zinc Sulphide doped with Copper (ZnS:Cu), although once again, the low level of light produced meant that no in depth research was carried out. In 1936 George Destriau published a report on the emission of light by Zinc Sulphide (ZnS) powders, following the application of an electric current and is widely credited with having invented the term "electroluminescence".
British experiments into electroluminescence, using the semiconductor Gallium Arsenide (GaAs) in the 1950s led to the first "modern" Light Emitting Diode (LED), which appeared in the early 1960s. It is said that early experimental laboratory LEDs needed to sit in liquid nitrogen while operating and considerable effort was required to make the breakthroughs needed to create devices that would function efficiently at room temperature. The first commercial LEDs were only able to produce invisible, infra red light, but still quickly found their way into sensing and photo-electric applications.
The first visible (red) light LEDs were produced in the late 1960s, using Gallium Arsenide Phosphide (GaAsP) on a GaAs substrate. Changing to a Gallium Phosphide (GaP) substrate led to an increase in efficiency, making for brighter red LEDs and allowing the color orange to be produced.
By the mid 1970's Gallium Phosphide (GaP) was itself being used as the light emitter and was soon producing a pale green light. LEDs using dual GaP chips (one in red and one in green) were able to emit yellow light. Yellow LEDs were also made in Russia using Silicon Carbide at around this time, although they were very inefficient compared to their Western counterparts, which were producing purer green light by the end of the decade.
The use of Gallium Aluminium Arsenide Phosphide (GaAlAsP) LEDs in the early to mid 1980s brought the first generation of superbright LEDs, first in red, then yellow and finally green. By the early 1990's ultrabright LEDs using Indium Gallium Aluminium Phosphide (InGaAlP) to produce orange-red, orange, yellow and green light had become available.
The first significant blue LEDs also appeared at the start of the 1990's, once again using Silicon Carbide - a throwback to the earliest semiconductor light sources, although like their yellow ancestors the light output was very dim by today's standards. Ultra bright blue Gallium Nitride (GaN) LEDs arrived in the mid 1990s, with Indium Gallium Nitride (InGaN) LEDs producing high-intensity green and blue shortly thereafter.
The ultra bright blue chips became the basis of white LEDs, in which the light emitting chip is coated with fluorescent phosphors. These phosphors absorb the blue light from the chip and then re-emit it as white light. This same technique has been used to produce virtually any color of visible light and today there are LEDs on the market which can produce previously "exotic" colors, such as aqua and pink.
However, the story of LEDs has not just been about color, but brightness too. Like computers, LEDs are following their own kind of "Moore's Law", becoming roughly twice as powerful (bright) around every eighteen months. Early LEDs were only bright enough to be used as indicators, or in the displays of early calculators and digital watches. More recently they have been starting to appear in higher brightness applications and will continue to do so for some time to come.
The once humble Light Emitting Diode has truly come of age and is now making the jump from mere indicator to true...