White LED lighting had to wait decades for someone to crack blue
Red LEDs glowed on gadgets for decades, but bright white lighting needed a blue one, and blue proved stubbornly hard. When Shuji Nakamura, Hiroshi Amano and Isamu Akasaki finally made it work in the early 1990s, they opened the door to LED bulbs and full-colour screens, and later shared a Nobel Prize.
An LED turns electricity straight into light. Inside the semiconductor, electrons fall into holes, gaps where electrons are missing, and each reunion releases a photon, a process called electroluminescence. The colour depends on the band gap, the energy an electron must shed. Shorter wavelengths need bigger gaps, so blue LEDs need higher voltages than red ones. White comes from combining several chips or coating a blue one with glowing phosphor. Because semiconductors bend light strongly, shaped dies and special coatings help it escape. Unlike laser light, LED output is neither coherent nor truly single-coloured, though its band is narrow enough to look pure to the eye.
The effect was spotted early. In 1906 Henry Joseph Round of Marconi Labs saw carborundum crystals give off yellow, green, orange or blue light under voltage, reporting it the next February. Soviet inventor Oleg Losev built a silicon carbide LED in 1927. Practical devices arrived only in 1962, when Texas Instruments engineers patented efficient infrared emission from gallium arsenide. They were costly until Monsanto and Hewlett-Packard drove the price below five cents apiece in the 1970s.
The first visible LEDs were dim and only red, used as indicator lamps and in seven-segment number displays; infrared ones went into remote controls. Blue arrived via layers of indium gallium nitride sandwiched in gallium nitride, and adjusting the indium-to-gallium ratio can in theory shift the colour anywhere from violet to amber. The 2014 physics Nobel recognised that breakthrough.
Compared with incandescent bulbs, LEDs use less power, waste less heat, last longer, survive knocks, fit smaller spaces and switch faster, which even makes them useful for data links. They do have limits: they prefer low-voltage direct current, flicker on pulsing or alternating supplies, and tolerate less heat. Today they light aircraft, cars, stages, streets, plant-growing rooms and medical devices.
Source: Light-emitting diode
