What is gunn diode?

The Gunn diode does not contain a PN diode junction. A Gunn diode is also called as a TED (transferred electron device). The Gunn diode or transferred electron device (TED) can be termed a diode because it does have two electrodes. It depends upon the bulk material properties rather than that of a PN junction. The Gunn diode operation depends on the fact that it has a voltage controlled negative resistance.

Gunn diode

Its largest use is in electronic oscillators to generate microwaves, in applications such as radar speed guns, microwave relay data link transmitters, and automatic door openers. In the Gunn diode, three regions exist: two of those are heavily N-doped on each terminal, with a thin layer of lightly n-doped material between.
Its internal construction is different from other diodes.
It consists only of N doped semiconductor material, whereas most diodes consist of both P and N-doped regions.
Therefore, it does not conduct only in one direction and cannot rectify alternating current like other diodes.