Under certain conditions, diodes can be made to produce microwave RF signals. Three types of diodes that can do this are Gunn diodes, IMPATT diodes, and tunnel diodes.
A Gunn diode can produce up to 1 W of RF power output, but more commonly it works at levels of about 0.1 W. Gunn diodes are usually made from gallium arsenide. A Gunn diode oscillates because of the Gunn effect, named after J. Gunn of International Business Machines (IBM), who first observed it in the 1960s. A Gunn diode doesn’t work like a rectifier, detector, or mixer. Instead, the oscillation takes place as a result of a quirk called negative resistance.
Gunn-diode oscillators are often tuned using varactor diodes. A Gunn-diode oscillator, connected directly to a microwave horn antenna, is known as a Gunnplexer. These devices are popular with amateur-radio experimenters at frequencies of 10 GHz and above.
The acronym IMPATT comes from the words impact avalanche transit time. This, like negative resistance, is a rather esoteric phenomenon. An IMPATT diode is a microwave oscillating device like a Gunn diode, except that it uses silicon rather than gallium arsenide. An IMPATT diode can be used as an amplifier for a microwave transmitter that employs a Gunn-diode oscillator. As an oscillator, an IMPATT diode produces about the same amount of output power, at comparable frequencies, as a Gunn diode.
Another type of diode that will oscillate at microwave frequencies is the tunnel diode, also known as the Esaki diode. It produces enough power so it can be used as a local oscillator in a microwave radio receiver, but not much more. Tunnel diodes work well as amplifiers in microwave receivers, because they generate very little unwanted noise. This is especially true of gallium arsenide devices.