How Blue LEDs Work, and Why They Deserve the Physics Nobel

In the early hours of October 7, 2014, while most of America slumbered, members of the Swedish Academy of Sciences were meeting to decide to whom they should award the Nobel Prize in Physics. The Nobel Committee often recognizes grand discoveries, like the prediction of the Higgs boson and the observation of the accelerating expansion of the universe. Thus many were surprised to see the 2014 prize honor a very practical invention: blue light emitting diodes. Yet this year’s awardees, Shuju Nakamura, Isamu Akasaki and Hiroshi Amano, richly deserve the prize.

The technical achievement

LEDs are one of many useful spinoffs from the basic science of quantum mechanics. In the standard quantum cartoon, electrons orbit the center of atoms, kind of like planets around the Sun. (The planetary model is not completely accurate, but it has many good features that help visualize what is going on in the quantum realm.) Imagine a simple atom with a single electron. According to quantum mechanics, the electron can be in a series of discrete orbits, as if a single planet around our Sun could be in the orbit of Mercury or Venus or any of the planets, but nowhere in between. Electrons near the nucleus have lower energy, while ones farther away have more energy.

When an electron moves from a high-energy orbit to a lower-energy one, it emits energy in the form of light. The color of the emitted light depends on the energy difference between the old orbit and the new one. The bigger the energy difference, the bluer the light.

With a few tweaks, these basic ideas also apply to groups of atoms, like those which make up molecules and solid materials. Which brings us to LEDs. To make an LED, you must fuse together two different types of semiconductor, one with electrons to spare (“n-type”) and one with extra room to hold those electrons (“p-type”). When the proper voltage is placed across the diode, electrons can move and fall into the holes, releasing energy. Early diodes only released a little energy, so they emitted infrared light. To make a diode that could produce visible light, scientists had to widen the energy gap. By the early 1970s, scientists had discovered how to make bright red, yellow, and green light by choosing the proper semiconductor materials and judiciously spiking, or “doping,” them with impurities.

But scientists struggled to make blue light, which requires a very high energy gap. Finally, in 1994, Shuji Nakamura, then employed by the Nichia Corporation, developed high-brightness Blue SMD LED using indium gallium nitride (InGaN), a mix of gallium nitride and indium nitride. By adjusting the amount of indium in the semiconductor, he tuned the energy gap to produce blue light.

Nakamura wasn’t the first to attempt to use gallium nitride to create LEDs, however most solid state physicists of the era had moved on to different materials. First, nobody knew how to prepare a surface on which gallium nitride crystals could grow, and further, nobody knew how to make p-type layers of GaN. Isamu Asaki and Hiroshi Amano showed it could be done using sapphire as a substrate and were eventually able to create the required p-layer of the material. Quite by accident, they also discovered that a scanning beam microscope increased the brightness of the light emitted by LED.

Nakamura grew his own GaN crystals and developed a simpler method for making the p-type layers using careful heating. He was also the first to understand why the electron beam boosted the light output of LEDs: it was removing hydrogen, just as his own heating technique did.

Modern blue through-hole LED require a more sophisticated approach, including varying the amount of indium and gallium, although the basic technique is the same as Nakamura’s. Starting with a sapphire substrate, several alternating layers of gallium nitride are added, some doped with indium and others doped with aluminum. These extra elements are key to increasing the efficiency and brightness of the blue LEDs. Further, with the introduction of aluminum, it is possible to make even bluer LED—even ultraviolet ones.

The greatest benefit on mankind

To understand why this seemingly mundane development warrants such recognition, one must return to Alfred Nobel’s will, in which he provided the seed money to start the prizes that bear his name. When Nobel’s brother died, a French newspaper mistakenly published Alfred’s obituary. Alfred Nobel was horrified to see himself called a “merchant of death” and a man “who became rich by finding ways to kill more people faster than ever before.” He resolved to repair his legacy by bequeathing a prize to be awarded to “those who…have conferred the greatest benefit on mankind.” Nobel wanted to be remembered as a man who helped make the world a better place.

This year’s award clearly fits the bill. “I really think that Alfred Nobel would be very happy about this prize,” said Per Delsing, head of the Nobel Committee for Physics, in announcing the prize. ”It’s really an invention and it’s really something that will benefit most people.”

The invention of bright blue LEDs brought entirely new industries into existence. Now, blue, red and green LEDs could be combined to make white—or any other color—light. This development led to the power-efficient screens for cell phones, TVs, computers, iPads, and many other electronic miracles of the modern world.

However the real impact of blue LED goes well beyond our rainbow-colored gadgets. Today, LEDs are bright enough to use as light sources. Just as Thomas Edison’s original incandescent light bulb revolutionized the early years of the 20 ^th century, LEDs are poised to revolutionize the 21 ^st .

LEDs can now emit far more light using far less power than incandescent and fluorescent bulbs. For instance, an incandescent light bulb can emit about 16 lumens per watt of electrical power, and a fluorescent light manages about 70 lumens per watt. In comparison, a modern white LED can emit 300 lumens per watt, meaning that it consumes only about 5% of the power of an incandescent. Given that about a quarter of the world’s electricity is used to generate light, the invention of efficient lighting can have considerable economic and environmental impact, potentially reducing the greenhouse gas emissions that are driving anthropogenic global warming. In the developing world, bright, efficient LEDs could provide solar-powered, off-grid energy for homes, hospitals, and more.

This year’s Nobel Prize in physics is very well deserved and reflects Alfred Nobel’s desired legacy of recognizing discoveries and developments that have greatly benefited mankind.