A tunnel diode, also known as an Esaki diode, is a special type of diode that can be operated at very high frequencies, i.e., well into the microwave frequency range.  It was invented by Leo Esaki of Tokyo Tsushin Kogyo in 1957.

A tunnel diode has this capability because it operates on the principle of electron tunneling effect.  Making use of extremely doped p-type and n-type materials, a tunnel diode achieves very high concentrations of carriers that can interact readily without having to pass over the junction's potential barrier.  Instead,  the carriers simple pass through the potential hill.  This is the electron tunneling effect mentioned earlier.

The very heavy doping of the p-n junction of a tunnel diode results in a broken bandgap, somewhat causing the conduction band electron states on the n-side of the junction to be aligned with the valence band hole states on the p-side.

Tunnel diodes are usually fabricated using germanium, although gallium arsenide and silicon tunnel diodes also exist.  They are used in applications such as oscillators, amplifiers, frequency converters, and detectors.

When the forward bias voltage is initially applied in a tunnel diode, the current increases as the filled electron states in the n-side conduction band aligns with the empty valence band hole states in the p-side.  As the voltage is further increased, these states begin to misalign, causing the current to decrease.  The current further drops as the voltage is increased, which means that the tunnel diode is exhibiting a negative resistance under these conditions.  Increasing the voltage further eventually prevents the occurrence of electron tunneling, causing the tunnel diode to just operate like a normal diode.

Figure 1.  Photo of a tunnel diode (left) and the two circuit symbols commonly used for the tunnel diode (right)

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