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A Gunn diode, also known as a transferred electron device (TED) is a form of diode used in high-frequency electronics. It is somewhat unusual in that it consists only of N-doped semiconductor material, whereas most 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 differential resistance.

The negative resistance, combined with the timing properties of the intermediate layer, allows construction of an RF relaxation oscillator simply by applying a suitable direct 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 therefore used to build oscillators in the 10 GHz and higher (THz) frequency range, where a resonant cavity is usually added to control frequency. The resonator can be based on a waveguide, coaxial cavity, YIG resonator, etc. Tuning is done mechanically, by adjusting the parameters of the resonator, or in case of YIG resonators by electric current.

Gallium arsenide Gunn diodes are made for frequencies up to 200 GHz, gallium nitride materials can reach up to 3 terahertz.