(from physicsworld.com) The Japanese semiconductor pioneer Isamu Akasaki has died at the age of 92. His work in the late 1980s and early 1990s led to the development of blue light-emitting diodes (LEDs), which soon found a wide range of applications from low-energy light bulbs and mobile-phone displays to televisions. For the work Akasaki shared the 2014 Nobel Prize for Physics together with fellow Japanese-born researchers Hiroshi Amano and Shuji Nakamura.
Akasaki was born in Chiran, Japan, on 30 January 1929 and graduated from Kyoto University in 1952. After receiving a PhD in electronics in 1964 from Nagoya University, he moved to Matsushita Research Institute Tokyo before returning to Nagoya in 1981 where he remained for the rest of his career. From 1992, Akasaki held a joint position with Meijo University, which is also in Nagoya.
It was at Nagoya and Meijo where Akasaki conduced much of his Nobel-prize-winning research. The first red LED was created in the early 1960s and researchers then managed to create devices that emitted light at ever-shorter wavelengths, reaching green by the end of that decade. However, creating devices that could deliver enough blue light was a struggle. But doing so was essential for a source of white light – needing, as it would, red, green and blue LEDs.
Crystal maze
At Nagoya in the 1980s, Akasaki and Amano focussed on making blue LEDs from the compound semiconductor gallium nitride (GaN) given that it has a large band-gap energy corresponding to ultraviolet light. Yet they needed to overcome several challenges, including the ability to create high-quality crystals of GaN with good optical properties. To do so they used metalorganic vapour phase epitaxy techniques to deposit thin films of high-quality GaN crystals onto substrates.
Another issue was to learn how to dope the GaN so it is a “p-type” semiconductor, which is crucial for creating an LED. Akasaki and Amano noticed, however, that when GaN doped with zinc is placed in an electron microscope, it gives off more light than if undoped, which suggested that electron irradiation improved the p-doping.
This effect was later explained by Nakamura, who was based at the Nichia Corporation and was working independently on GaN blue LEDs. In the early 1990s, both groups then used their high-quality, p-doped GaN to make high-brightness blue LEDs, achieved by combining them with other GaN-based semi-conductors in multilayer “hetero-junction” structures. Today, GaN-based LEDs are used in back-illuminated liquid-crystal displays in devices ranging from mobile phones to TV screens.
In 2014, Akasaki, along with Amano and Nakamura, were awarded the Nobel Prize for Physics for “the invention of efficient blue light-emitting diodes [LEDs] which has enabled bright and energy-saving white light sources”. Akasaki was awarded many other prizes during his career including the Japanese Order of Culture in 2011 and the Queen Elizabeth Prize for Engineering in 2021. He died on 1 April from pneumonia.
