A semiconductor diode, discovered in 1957 by L. Esaki (1925– ), based on the tunnel effect. It consists of a highly doped p−n semiconductor junction, which short circuits with negative bias and has negative resistance over part of its range when forward biased. Its fast speed of operation makes it a useful device in many electronic fields. In a tunnel diode the shape of the characteristic for current and voltage is affected sharply by the tunnel effect, with a large tunnelling current associated with tunnelling from the valence band to the conduction band (see energy bands). As the forward bias on the junction is increased the tunnel effect becomes less and less pronounced.