Thermionic emission is the emission of electrons from a heated filament or substance. When a filament like tungsten is heated to a high temperature, some electrons acquire sufficient energy and are able to break away from the surface of the material and go into space. This action is accelerated when the filament is heated in a vacuum by connecting it across a 6-V supply. Thermionic emission is used in electronic vacuum tubes, the simplest being the diode. The electrons emitted are also called thermions.
The rate of emission of electrons depends on:-
1. The material used for the filament.
2. The temperature of the filament.
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3. The surface area of the filament.