Oscillation and amplification

Under certain conditions, diodes can be made to produce microwave radio signals. There are three types of diodes that do this: Gunn diodes, IMPATT diodes, and tunnel diodes.

Gunn 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 observed it in the sixties. A Gunn diode doesn’t work anything 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.

IMPATT diodes

The acronym IMPATT comes from the words impact avalanche transit time. This, like negative resistance, is a phenomenon the details of which are rather esoteric. 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 the Gunn diode.

Tunnel diodes

Another type of diode that will oscillate at microwave frequencies is the tunnel diode, also known as the Esaki diode. It produces only a very small amount of power, but it can be used as a local oscillator in a microwave radio receiver. Tunnel diodes work well as amplifiers in microwave receivers, because they generate very little unwanted noise. This is especially true of gallium arsenide devices.
The behavior of Gunn, IMPATT, and tunnel diodes is a sophisticated topic and is beyond the scope of this book. College-level electrical-engineering texts are good sources of information on this subject. You will want to know about how these devices work if you plan to become a microwave engineer.