The missing color
By the 1970s, LEDs could produce red, yellow and green light. Blue was missing, and that gap mattered. Red, green and blue together can make a wide range of colors on a screen, the same principle used in color television, and blue light can be used to produce white light by other means. Researchers knew this, and many tried to make a bright blue LED through the 1970s and 1980s. For years the results were poor, and many major laboratories moved on to other materials.
Why blue was hard
Blue light has more energy per photon than red, so the semiconductor must have a wide energy gap. Two main candidates were zinc selenide and gallium nitride. Zinc selenide was easier to grow in crystals but wore out quickly under operation. Gallium nitride was more durable but very hard to make: good crystals were difficult to grow, and making the material conduct in the right way for an LED was even harder. An LED needs both an n-type layer, rich in free electrons, and a p-type layer, with positive holes. Gallium nitride could be made n-type, but producing a useful p-type layer was a stubborn problem.
The Nagoya and Nichia work
Isamu Akasaki, working in Japan with his student Hiroshi Amano at Nagoya University, persisted with gallium nitride in the 1980s. They developed a way to grow better crystals on a sapphire base using a thin buffer layer of aluminum nitride, reported in the mid-1980s. They then obtained p-type gallium nitride by doping with magnesium and treating it with an electron beam in 1989, and demonstrated a blue-emitting junction device soon after.
Shuji Nakamura, a researcher at the Japanese company Nichia Chemical Industries, took a different route to similar goals in the early 1990s. He developed a different growth method for gallium nitride and found that heating the magnesium-doped material in a particular way produced p-type conduction. His team made high-brightness blue LEDs, announced in 1993, and later a blue-green LED and blue laser diodes.
The three men shared the 2014 Nobel Prize in Physics for the invention of efficient blue LEDs, which the Nobel committee said had enabled bright and energy-saving white light sources. Others also contributed to the field, and the work of each group built on gallium nitride research done by many earlier researchers.
From blue to white
A blue LED does not by itself give white light. The usual solution is to coat or surround a blue LED with a phosphor, a material that absorbs some of the blue and re-emits it as yellow. The mixture of the remaining blue and the added yellow appears white to the eye. Nichia announced white LEDs of this kind in the mid-1990s. The exact mix of phosphors can be adjusted to give warmer or cooler white, which is one reason modern LED bulbs are sold in different color temperatures.
Another approach combines separate red, green and blue LEDs to mix white. This is useful for color-changing lamps and some display lights, though phosphor-converted white is more common in general lighting.
Where blue LEDs ended up
The practical effects spread widely.
- General lighting. White LED bulbs and tubes became possible, and as efficiency improved they began to compete with fluorescent and filament lamps in the 2000s and 2010s.
- Screens. Liquid crystal screens need a backlight, and white LEDs replaced earlier fluorescent backlights in televisions, monitors and smartphones. Flat-screen displays became thinner and more efficient than the cathode ray tubes they displaced. Direct-emitting LED displays, including large outdoor video walls, use red, green and blue LEDs as pixels.
- Indicators and signs. Full-color LED signs, vehicle lights and decorative lighting became common.
- Optical discs. Blue-violet laser diodes, based on the same gallium nitride materials, allowed higher-capacity optical discs, as used in Blu-ray players.
A note on the story
Popular tellings of the blue LED sometimes make it a lone-genius story, but the record shows that progress depended on crystal growth methods, new doping techniques and many failed experiments. Nakamura’s work at a smaller company, with limited resources and in a field most large labs had left, is often highlighted, and he later became involved in a well-known legal dispute with his former employer over inventor compensation. Such disputes are part of the history, but details are best checked in dedicated accounts.
Why it was called a breakthrough
The blue LED matters mostly because it completed a set. With blue available, LEDs could cover the full visible spectrum and also provide white light, which turned a technology for indicators into one for lighting. Before blue LEDs, an LED bulb was not a realistic replacement for a filament bulb. Afterward, with falling costs, it became possible.