Vacuum electronics

Vacuum electronics studies and develops devices that control electron flow through a vacuum, including vacuum tubes and microwave amplifiers.

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Thermionic emission: The release of electrons from a heated material when they gain enough energy to escape its surface. Heated cathodes supply electrons in many vacuum tubes.
Diode: A two-terminal electronic device that conducts current more readily in one direction than the other. A vacuum diode uses an anode and cathode to rectify current.
Radar: A system that detects objects by transmitting radio waves and analyzing their reflected signals. Klystrons, traveling-wave tubes, and magnetrons provide radar transmitters with microwave power.
Solid-state electronics: Electronics built from semiconductor materials in which charge moves through solid devices. Semiconductor devices replaced vacuum tubes in most low-power and compact applications.
Thermionic valve: A vacuum tube, especially one used to rectify or amplify electrical signals. The valve was the early practical form from which vacuum-electronics engineering developed.
Electron gun: A device that emits and shapes a beam of electrons using an electron source and focusing electrodes. Electron guns provide controlled beams in tubes such as klystrons and traveling-wave tubes.
Triode: A vacuum tube with a cathode, anode, and control grid that regulates electron flow. The control grid lets a small voltage govern a larger current.
Satellite communications: The transmission of information between locations using artificial satellites as communication links. Traveling-wave tube amplifiers remain common in satellite payloads.
Bipolar junction transistor: A semiconductor device that uses two junctions to control current through a solid crystal. It supplanted many small-signal tube functions with lower voltage and power consumption.
John Ambrose Fleming: An English electrical engineer who invented the thermionic diode in 1904. His diode established one-way electron conduction as a practical circuit function.