Leo Esaki observed a distinctive tunnelling response in heavily doped germanium junctions. A narrow barrier allows carriers to tunnel between occupied and empty electronic states on opposite sides. Increasing the voltage can reduce the overlap of available states and hence reduce the current, producing negative differential resistance. The effect turns quantum tunnelling into a useful device characteristic: a falling current-versus-voltage slope is a local differential property, not a requirement that current and voltage have opposite signs.[ref]
New Phenomenon in Narrow Germanium Junctions by Leo Esaki. Physical Review 109, 603 (1958), narrow-junction tunnelling response
Portrait of Leo Esaki
. Unknown photographer (Asahi Shinbun / The Asahi Shimbun). Public domain under Japanese copyright law (as stated on Commons File page). Source usage terms, via Wikimedia Commons. Source. Articles by others on the same topic
Leo Esaki is a Japanese-American physicist known for his significant contributions to the field of semiconductor physics, particularly in the development of the tunnel diode. He was born on March 12, 1925, in Osaka, Japan. Esaki's work in the 1950s led to the discovery of the Esaki diode, which exploits quantum tunneling effects to allow current to flow in the reverse direction under certain conditions.