Tunnel diode

Heavily doped semiconductors are called degenerate semiconductors. In a p-type degenerate semiconductor, Fermi level lie inside the conduction band. When a p-n junction of heavily doped semiconductors is formed, thickness of the layer becomes very thin (less than 10 nm).  Now electrons penetrate through the barrier. This property is called tunneling effect and diode is called tunnel diode.  On forward biasing, large tunneling current flows across the junction. Any further increase in forward bias voltage decreases the tunneling current.  Thus the tunneling current goes through a maximum. On reverse biasing, again large current flows but now in the reverse direction. This current is due to tunneling of electrons from the valence band of p-region to the conduction band of n-region. The tunneling current is usually insensitive to temperature variation. Due to the very high switching speeds, tunnel diodes can be used in oscillators, multivibrators, high speed logic circuits and low-noise microwave amplifiers.