Oscillation and amplification
Under the certain conditions, diodes can be made to generate microwave radio signals. There are 3 types of diodes which do this: Gunn diodes, IMPATT diodes, and tunnel diodes.
Gunn diodes
A Gunn diode can generate around 1 W of RF power output, but it works at levels of around 0.1 W. Gunn diodes are made from the gallium arsenide usually.
A Gunn diode oscillates due to the Gunn Effect, which is named after J. Gunn of International Business Machines (IBM) who observed it in the sixties. A Gunn diode does not work anything like a detector, rectifier, or mixer; instead, oscillation takes place as a result of a quirk called as negative resistance.
Gunn diode oscillators are tuned by using varactor diodes. A Gunn-diode oscillator, is connected directly to microwave horn antenna, is called as a Gunnplexer. These devices are popular with the amateur radio experimenters at frequencies of 10 GHz and above.
IMPATT diodes
The acronym IMPATT comes from words impact avalanche transit time. This, is a phenomenon the details of which are 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
The other type of diode which will oscillate at the microwave frequencies is tunnel diode, also called as Esaki diode. It generates a very small amount of power, but it can be taken in use as a local oscillator in a microwave radio receiver.
Tunnel diodes work well as amplifiers in microwave receivers, as they generate very little unwanted noise. This is true for gallium arsenide devices. The behavior of Gunn, IMPATT, and tunnel diodes is a sophisticated topic and is be- yond the scope of this book. College level electrical engineering texts are fine sources of information on this subject.