Why Blue LED Inventors Started a Revolution in LED Lighting

LEDs have become so commonplace nowadays that we can’t even remember a time when they weren’t around, yet they have a fairly recent history, and they wouldn’t have taken off the way they did were it not for blue LEDs.

Interestingly, not all LED light colours are equal. When the first LEDs were developed in 1962, only infrared diodes were possible, and they were only bright enough to be used as indicators. In addition, their unit cost of US$200 was prohibitive to mass usage. As a result, they were primarily used for laboratory and electronics equipment, then in household appliances as an on/off bulb on various devices.

Green LEDs made their apparition at around the same time, with limited use too, but it wasn’t for another 10 years that the next colour was developed, yellow. Used to improve the brightness of red and red-orange LEDs, it did so tenfold, making them suitable for a wider range of applications.

Blue LED light proved much more elusive and it took three decades for Japanese scientists Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura to invent the first efficient blue LED, in 1994, for which they received the Nobel Prize in Physics in 2014.

One of the obstacles to developing blue LED light was finding the right material, which is what determines the colour of the LED. Semiconductors using zinc selenide and silicon carbide didn’t produce efficient light emission. Gallium nitride (GaN) had been investigated in the 1950s as a substrate, but abandoned as it proved difficult to grow crystals of the required quality to give good optical properties.

Another issue was the difficulty in p-doping – the process by which impurities are deliberately introduced into a pure intrinsic semiconductor to modulate its electrical properties – but in the late 1980s, Amano and Akasaki found out that zinc doping gave better results, which paved the way to their eventual success in creating blue light less than a decade later.

This technical development was instrumental in creating white light LEDs. It was discovered that coating the emitter with phosphor resulted in the absorption of some of the blue light to produce yellow light which, combined with the remaining blue light, appears white to the human eye.

While this first version of white light was revolutionary, it wasn’t terribly efficient and research continued. By using different phosphors, it became possible to also create green and red light through the same process of fluorescence, which, once mixed with blue, is also perceived by humans as white, but with superior illumination powers.

Nowadays, we are spoilt for choice, even as far as white light is concerned, and you can even choose between warm or cool white LEDs. In addition, as the technology behind LEDs is the same as computers’, they will continue to benefit from the same advances to become ever more efficient and suitable for a wider range of applications. Definitely worth a Nobel Prize!

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