tunnel diode

A tunnel diode is an electronic component that puts quantum tunneling to work in a circuit. It is a semiconductor junction in which both sides are very heavily doped, so the boundary layer between them is exceptionally thin — only a few nanometres. Across so narrow a barrier, electrons do not need enough energy to climb over; they simply tunnel straight through it, appearing on the far side as if the barrier were partly transparent.

This tunneling gives the device a strange and prized property: a region of negative differential resistance. In an ordinary resistor, raising the voltage always raises the current. In a tunnel diode, over a certain range, raising the voltage actually makes the current fall, because the energy levels that allowed tunneling slide out of alignment. A component whose current drops as voltage rises can amplify signals and sustain oscillations, which is why tunnel diodes once served in very fast and high-frequency circuits.

Because tunneling is essentially instantaneous and does not rely on charge slowly drifting across a junction, tunnel diodes can switch extremely quickly, into the microwave range. Leo Esaki discovered the effect in 1957 and shared a Nobel Prize for it. Though largely superseded by other devices today, the tunnel diode was an early, vivid proof that a quantum effect once thought purely theoretical could be engineered into practical electronics.