Leo Esaki
Leo Esaki, born in 1925 in Japan, is a physicist who discovered electron tunnelling in a semiconductor junction, invented the tunnel diode, and later proposed the semiconductor superlattice. He did the work that made his name while employed by Tokyo Tsushin Kogyo, the company later renamed Sony, and then built the second half of his career at IBM's research laboratory in the United States. In 1973 he shared the Nobel Prize in Physics, receiving half of the prize with Ivar Giaever, while Brian Josephson received the other half.
His early career placed him in industry, not a university, at a moment when the transistor was still new. Esaki studied physics at the University of Tokyo, graduating in the late 1940s, and in the 1950s joined Tokyo Tsushin Kogyo, a young electronics firm that was among the first in Japan to license transistor technology after the Bell Labs symposia and was working toward transistor radios. There he worked on germanium diodes and transistors, and his team was trying to improve the high-frequency behaviour of the base region of bipolar devices. This was solid-state physics done in a company laboratory by a researcher curious about why the devices behaved as they did, rather than a theory sent down from academia.
The tunnel diode came from careful measurement and a willingness to explain an anomaly. While examining heavily doped germanium pn junctions, he found that they showed a forward current that fell as voltage rose, a result classical diode theory could not explain. He recognized it as quantum tunnelling of electrons through the thin junction barrier and published the result in 1958, which gave semiconductor physics one of its first clear experimental demonstrations of a quantum effect. The device itself is covered in its own entry; what matters biographically is that the discovery was made at an industrial lab and made his reputation internationally, and that it came from doping a junction far beyond what engineers considered useful.
At IBM he moved from one narrow junction to engineered crystals. After joining the IBM Thomas J. Watson Research Center in the United States around 1960, he continued to study tunnelling and later became an IBM Fellow. In 1969 and 1970 he and Raphael Tsu proposed a semiconductor superlattice: a crystal built from alternating ultrathin layers of different semiconductors, so that the artificial periodicity creates new energy bands and a negative differential conductance. The idea, realized with molecular beam epitaxy in the early 1970s by Esaki's group, opened the field of band-gap engineering and led to quantum-well lasers, resonant tunnelling devices and the high electron mobility transistors found in modern communications equipment.
The 1973 prize recognized tunnelling in solids and set Esaki alongside two very different contemporaries. Giaever was honoured for tunnelling in superconductors and Josephson for the theoretical prediction of the Josephson effect, so the award celebrated electron tunnelling across three kinds of junction. A common misconception is that Esaki won for the transistor, or that he worked at Bell Labs; he did neither, and his own field was tunnelling and the physics of artificial semiconductor structures. Another is to credit the tunnel diode to the company rather than the researcher, when the discovery was his and the employer simply gave him a laboratory.
His later career went into education and administration and showed how an industrial scientist could become an institutional leader. After returning to Japan he became president of the University of Tsukuba in the 1990s, later led other educational institutions, and became a prominent voice on science education and creativity in Japan. His work remains relevant because the superlattices and quantum wells he imagined are in laser diodes and fast transistors in phones and fiber-optic networks today, and because tunnelling, first seen as a curiosity in a germanium diode, now governs flash memory and sets limits on how thin transistor gate insulators can become.
Read Leo Esaki as the physicist who carried quantum tunnelling out of textbooks and into semiconductor devices: first as a germanium diode at Sony's predecessor, then as the engineered superlattices at IBM that founded modern band-gap engineering.