Leo Esaki (born 1925), Japanese physicist at IBM in Yorktown Heights, New York, USA. Esaki studied the quantum- mechanical tunnelling of electrons through potential energy barriers in semiconductor diode junctions. This tunnelling causes some electrons to flow in the opposite direction to the normal electron flow. In 1960 he invented the Esaki or tunnel diode. Esaki diodes consist of heavily-doped semi-conductors with narrow junctions. They exhibit negative resistance, where the current flowing is reduced by increasing the voltage applied. Esaki diodes are small, fast, and use little power; they are widely used in computers. Esaki shared the 1973 Nobel Physics Prize. Photographed between 1975 and 1980.