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.