An Gunn diode is a form of diode used in high-frequency electronics. It is somewhat unusual in that it consists only of N-doped silicon, whereas ordinary diodes consist of both P and N-doped regions. In the Gunn-diode, three regions exist: two of them are heavily N-doped on each terminal, with a thin layer of lightly doped material in between. When a voltage is applied to the device, the electrical gradient will be largest across the thin middle layer. Eventually, this layer starts to conduct, reducing the gradient across it, preventing further conduction. In practice, this means a gunn diode has a region of negative resistance.

The negative resistance, combined with the timing properties of the intermediate layer, allows one to construct an oscillator by just applying a suitable current through the device. The oscillation frequency is determined partly by the properties of the thin middle layer, but can be adjusted by external factors. Gunn diodes are therefor used to build oscillators in the 10 GHz and higher frequency range, where a resonant cavity is usually added to control frequency.