The Gunn diode type.

In the early sixties, J.B. Gunn working at IBM laboratories discovered that certain semiconductors materials like the Gallium Arsenide, electrons can exist in a high-mass low velocity state as well as their normal low-mass high-velocity state and they can be forced into the high-mass state by a steady electric field of sufficient strenght in such away they form domains which cross the field at a constant rate causing current to flow as a series of pulses.
This principle is known as the "GUNN effect" that is used to build a diode made of an epitaxial layer of N-type Gallium Arsenide grown on a Gallium Arsenide substrate.
By applying a small voltage between the Ohmic contacts to the N-Layer and substrats it produces the electrical field responsible for the domains. The frequency of the current pulses generated in this way, depends either on the transit time through the N-layer as well as its thickness.
If such a kind of diode is mounted in a suitable tuned cavity resonator, the current pulses produce oscillation due to schock exicitation and so generating radio frequency power up to 1 W in a frequency range from 10 to 30 GHz. Fig 258
Gunn diodes are used in low and medium power transmitters, motion detection systems, local and locking oscillators.