Written by Arda Kizilkaya
Blue light-emitting diodes (LEDs) stand as today’s most common electronic devices and serve as light sources in homes worldwide. Although red and green LEDs were invented decades ago, the color blue was nearly impossible if it wasn’t for Shunji Nakamura making radical discoveries to invent the blue LED. It all started in 1988, in Nichia, at the time a small company declining due to the competition against larger and more established companies. Shunji, Nichia’s researcher, desired to make more things but Nakamura’s supervisors were so disillusioned with his work that they told him to quit it, so in a desperate attempt to save his job, he brought a radical proposal to the company’s founder and president Nobuo Ogawa. He told Nobuo that he invented the blue LED which all of the established companies like Sony Toshiba and Panasonic had failed at. After suffering massive losses in the semiconductor market, Ogawa devoted 500 million yen, which then was worth 3 million dollars, to Nakamura’s moonshot Project. However, to understand how Nakamura created the blue LED, we need to understand how LEDs work.
When you have an isolated atom, you see that each electron of it has its discreet energy level and the atoms of the same element have near-identical energy levels when they are far apart from each other. However, when they approach each other to form a solid, the outermost electrons feel the pole of one another’s nuclei as well as the nucleus itself, so as a result, the energy levels shift, forming an energy band. The highest energy band with the most electrons in it is called the valence band, the next highest energy band is called the conduction band, and the gap between these bands is called the band gap. The ones used on LEDs are semi-conductors which have a valence band full of electrons; the band gap is small so the electrons can jump onto the conduction band. However, pure semiconductors were pretty much useless, so they had to do something called doping. This method took some of the material in the semiconductor like silicon, and replaced it with another atom to make 2 types of semiconductors: n-type or p-type. To make n-type semiconductors, some of the silicon is replaced with phosphorus giving an extra negatively charged layer. To make p-type semiconductors, some of the silicon is replaced with boron providing an extra positively charged layer. LEDs work by combining an n-type semiconductor, and a p-type semiconductor, and that creates an active layer where the semiconductors meet, to which the negative electrons and the positive holes are driven when electric voltage is applied to the semiconductor. When electrons and holes meet, they emit photons which create light. The light’s wavelength which decides its color relies entirely on the semiconductor; blue light appears at the short-wave, at the end of the rainbow, and can only be produced in some materials like Gallium Nitrate (GaN).
The Making of The Blue LED
Akasaki and Amano made a breakthrough in creating the first high-quality crystal and making a p-type layer. By coincidence, they discovered that their material was glowing more intensely when they were studying it in a scanning electron microscope. This suggested that an electronic beam was making the p-type layer more efficient.
Nakamura succeeded in creating high-quality gallium nitride crystals. He found his clever way of creating the crystal by first growing a thin layer of gallium nitride at a low temperature, and growing subsequent layers at a higher temperature. Nakamura now can also explain why Akasaki and Amano had succeeded with their p-type layer: the electron beam removed the hydrogen that connected to other atoms which was preventing the p-type layer from forming. For his part, Nakamura replaced their method with a simpler and cheaper method: by heating the material he managed to create, a functional p-type layer, in 1992. Now that Nakamura had both types of semiconductors; he only needed to increase its efficiency by lowering the band gap in the active layer by using indium, but he was well worked- too well that the electrons would not emit photons so he created a well-using aluminum and that's when the first functioning blue LED was made.
References:
How Shuji Nakamura’s invention lights up the world. (n.d.). National Inventors Hall of Fame®. https://www.invent.org/blog/inventors/shuji-nakamura-blue-led-lights Ned, A. (2021, March 28).
Why did blue LED earn a Nobel prize if reds and Greens already existed for decades? Medium. https://medium.com/predict/why-blue-led-earned-nobel-prize-if-reds-and-greens-already-existed-for-decades-d03e31a6fd8f (2023).
The official website of the Nobel Prize - NobelPrize.org. https://www.nobelprize.org/uploads/2018/06/popular-physicsprize2014-1.pdf
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