In
vacuum tubes, a
hot cathode or
thermionic cathode is a
cathode electrode which is heated to make it emit
electrons due to
thermionic emission. The heating element is usually an
electrical filament, heated by a separate
electric current passing through it. Hot cathodes typically achieve much higher power density than cold cathodes, emitting significantly more electrons from the same surface area.
Cold cathodes rely on
field electron emission or
secondary electron emission from positive ion bombardment and do not require heating. There are two types of hot cathode. In a
directly-heated cathode, the filament is the cathode and emits the electrons. In an
indirectly-heated cathode, the filament or
heater heats a separate metal cathode electrode which emits the electrons.