A Gunn diode is also called as a TED (transferred electron device). It is a two terminal passive semiconductor electronic component with negative resistance used in high frequency electronics.
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.