Editing Cathode (section)

===Vacuum tubes===
[[Image:4-1000A linear RF deck build by K5LAD.jpg|thumb|Glow from the directly heated cathode of a 1&nbsp;kW power [[tetrode]] tube in a radio transmitter. The cathode filament is not directly visible.]]

In a vacuum tube or electronic vacuum system, the cathode is usually a metal surface with an oxide coating that much improves electron emission,<ref>{{cite web | title=Valve Construction and Development: The Cathode|publisher=the Valve Museum|url=http://www.r-type.org/articles/art-249.htm | access-date=2 February 2025}}</ref> heated by a filament, which emits free electrons into the evacuated space. In some cases the bare filament acts as the cathode. Since the electrons are attracted to the positive nuclei of the metal atoms, they normally stay inside the metal and require energy to leave it; this is called the ''[[work function]]'' of the metal.<ref name="Avadhanulu">{{cite book

 |last        = Avadhanulu
 |first       = M.N.
 |author2     = P.G. Kshirsagar
 |title       = A Textbook of Engineering Physics For B.E., B.Sc.
 |publisher   = S. Chand
 |date        = 1992
 |pages       = 345–348
 |url         = https://books.google.com/books?id=lTUNWOR_cDgC&pg=PA345
 |isbn        = 978-8121908177
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20140102024212/http://books.google.com/books?id=lTUNWOR_cDgC&pg=PA345
 |archive-date = 2 January 2014
}}</ref> Cathodes are induced to emit electrons by several mechanisms:<ref name="Avadhanulu" />
* ''[[Thermionic emission]]'': The cathode can be heated. The increased thermal motion of the metal atoms "knocks" electrons out of the surface, an effect called thermionic emission. This technique is used in most vacuum tubes.
* ''[[Field electron emission]]'': A strong [[electric field]] can be applied to the surface by placing an electrode with a high positive voltage near the cathode.   The positively charged electrode attracts the electrons, causing some electrons to leave the cathode's surface.<ref name="Avadhanulu" /> This process is used in [[cold cathode]]s in some [[electron microscope]]s,<ref>{{cite encyclopedia
 |title       = Field emission
 |encyclopedia= Encyclopædia Britannica 
 |date        = 2014
 |url         = https://www.britannica.com/EBchecked/topic/206253/field-emission
 |access-date  = 15 March 2014
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20131202111836/https://www.britannica.com/EBchecked/topic/206253/field-emission
 |archive-date = 2 December 2013
}}</ref><ref name="Poole">{{cite book
 |last        = Poole
 |first       = Charles P. Jr.
 |title       = Encyclopedic Dictionary of Condensed Matter Physics, Vol. 1
 |publisher   = Academic Press
 |date        = 2004
 |pages       = 468
 |url         = https://books.google.com/books?id=CXwrqM2hU0EC&q=field+electron+emission&pg=PA468
 |isbn        = 978-0080545233
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20171224222803/https://books.google.com/books?id=CXwrqM2hU0EC&pg=PA468&dq=field+electron+emission
 |archive-date = 24 December 2017
}}</ref><ref name="Flesch">{{cite book
 |last        = Flesch
 |first       = Peter G.
 |title       = Light and Light Sources: High-Intensity Discharge Lamps
 |publisher   = Springer
 |date        = 2007
 |pages       = 102–103
 |url         = https://books.google.com/books?id=fWHQbhgxpAkC&pg=PA102
 |isbn        = 978-3540326854
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20171224222803/https://books.google.com/books?id=fWHQbhgxpAkC&pg=PA102
 |archive-date = 24 December 2017
}}</ref> and in microelectronics fabrication,<ref name="Poole" />
* ''[[Secondary emission]]'': An electron, atom or molecule colliding with the surface of the cathode with enough energy can knock electrons out of the surface. These electrons are called ''secondary electrons''. This mechanism is used in [[gas-discharge lamp]]s such as [[neon lamp]]s.
* ''[[Photoelectric emission]]'': Electrons can also be emitted from the [[electrode]]s of certain metals when light of [[frequency]] greater than the threshold frequency falls on it. This effect is called photoelectric emission, and the electrons produced are called ''photoelectrons''.<ref name="Avadhanulu" /> This effect is used in [[phototube]]s and [[image intensifier]] tubes.

Cathodes can be divided into two types:

====Hot cathode====
{{Main|Hot cathode}}
{{multiple image
| align = right
| direction = horizontal
| header   =
| image1   = Dubulttriode darbiibaa.jpg
| caption1 = Two indirectly-heated cathodes (orange heater strip) in ECC83 dual triode tube
| width1   = 137
| image2   = Triode-english-text.svg
| caption2 = Cutaway view of a [[triode]] vacuum tube with an indirectly-heated cathode ''(orange tube)'', showing the heater element inside
| width2   = 170
| footer   =
}}
[[File:Triode schematic labeled.svg|thumb|[[Schematic symbol]] used in [[circuit diagram]]s for vacuum tube, showing cathode]]

A hot cathode is a cathode that is heated by a [[electrical filament|filament]] to produce electrons by [[thermionic emission]].<ref name="Avadhanulu" /><ref name="Whitaker">Ferris, Clifford "Electron tube fundamentals" in {{cite book
 |last        = Whitaker
 |first       = Jerry C.
 |title       = The Electronics Handbook, 2nd Ed.
 |publisher   = CRC Press
 |date        = 2013
 |pages       = 354–356
 |url         = https://books.google.com/books?id=FdSQSAC3_EwC&pg=PA355
 |isbn        = 978-1420036664
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20140102024350/http://books.google.com/books?id=FdSQSAC3_EwC&pg=PA355
 |archive-date = 2 January 2014
}}</ref> The filament is a thin wire of a [[refractory metal]] like [[tungsten]] heated red-hot by an electric current passing through it. Before the advent of transistors in the 1960s, virtually all electronic equipment used hot-cathode [[vacuum tube]]s. Today hot cathodes are used in vacuum tubes in radio transmitters and microwave ovens, to produce the electron beams in older [[cathode-ray tube]] (CRT) type televisions and computer monitors, in [[x-ray generator]]s, [[electron microscope]]s, and [[fluorescent tube]]s.

There are two types of hot cathodes:<ref name="Avadhanulu" />
* '''Directly heated cathode''': In this type, the filament itself is the cathode and emits the electrons directly. Directly heated cathodes were used in the first vacuum tubes, but today they are only used in [[fluorescent tube]]s, some large transmitting vacuum tubes, and all X-ray tubes.
* '''Indirectly heated cathode''': In this type, the filament is not the cathode but rather heats the cathode which then emits electrons. Indirectly heated cathodes are used in most devices today. For example, in most vacuum tubes the cathode is a nickel tube with the filament inside it, and the heat from the filament causes the outside surface of the tube to emit electrons.<ref name="Whitaker" /> The filament of an indirectly heated cathode is usually called the ''heater''. The main reason for using an indirectly heated cathode is to isolate the rest of the vacuum tube from the electric potential across the filament. Many vacuum tubes use [[alternating current]] to heat the filament. In a tube in which the filament itself was the cathode, the alternating [[electric field]] from the filament surface would affect the movement of the electrons and introduce hum into the tube output. It also allows the filaments in all the tubes in an electronic device to be tied together and supplied from the same current source, even though the cathodes they heat may be at different potentials.

In order to improve electron emission, cathodes are treated with chemicals, usually compounds of metals with a low [[work function]]. Treated cathodes require less surface area, lower temperatures and less power to supply the same cathode current. The untreated tungsten filaments used in early tubes (called "bright emitters") had to be heated to {{convert|1400|°C|°F|abbr=on}}, white-hot, to produce sufficient thermionic emission for use, while modern coated cathodes produce far more electrons at a given temperature so they only have to be heated to {{convert|425|–|600|C|F}}<ref name="Avadhanulu" /><ref name="IanPoole">{{cite web
 |last        = Poole
 |first       = Ian
 |title       = Vacuum tube electrodes
 |work        = Vacuum Tube Theory Basics Tutorial
 |publisher   = Radio-Electronics.com, Adrio Communications
 |date        = 2012
 |url         = http://www.radio-electronics.com/info/data/thermionic-valves/vacuum-tube-theory/tube-electrodes.php
 |access-date  = 3 October 2013
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20131104104630/http://www.radio-electronics.com/info/data/thermionic-valves/vacuum-tube-theory/tube-electrodes.php
 |archive-date = 4 November 2013
}}</ref><ref name="Jones">{{cite book
 |last        = Jones
 |first       = Martin Hartley
 |title       = A Practical Introduction to Electronic Circuits
 |publisher   = Cambridge Univ. Press
 |date        = 1995
 |location    = UK
 |pages       = 49
 |url         = https://books.google.com/books?id=EEcemABAU44C&pg=PA49
 |isbn        = 978-0521478793
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20140102024515/http://books.google.com/books?id=EEcemABAU44C&pg=PA49
 |archive-date = 2 January 2014
}}</ref> There are two main types of treated cathodes:<ref name="Avadhanulu" /><ref name="Whitaker" />

[[Image:Neon lamp on DC.JPG|thumb|upright=0.5|Cold cathode ''(lefthand electrode)'' in [[neon lamp]] ]]
* Coated cathode – In these the cathode is covered with a coating of [[alkali metal]] oxides, often [[barium]] and [[strontium]] oxide. These are used in low-power tubes.
* Thoriated tungsten – In high-power tubes, [[ion]] bombardment can destroy the coating on a coated cathode.   In these tubes a directly heated cathode consisting of a filament made of tungsten incorporating a small amount of [[thorium]] is used. The layer of thorium on the surface which reduces the work function of the cathode is continually replenished as it is lost by diffusion of thorium from the interior of the metal.<ref name="Sisodia">{{cite book
 |last        = Sisodia
 |first       = M. L.
 |title       = Microwave Active Devices Vacuum and Solid State
 |publisher   = New Age International
 |date        = 2006
 |pages       = 2.5
 |url         = https://books.google.com/books?id=mKs53pET-bkC&pg=SA2-PA4
 |isbn        = 978-8122414479
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20140102024156/http://books.google.com/books?id=mKs53pET-bkC&pg=SA2-PA4
 |archive-date = 2 January 2014
}}</ref>

====Cold cathode====
{{Main|Cold cathode}}
This is a cathode that is not heated by a filament. They may emit electrons by [[field electron emission]], and in gas-filled tubes by [[secondary emission]]. Some examples are electrodes in [[neon light]]s, [[cold-cathode fluorescent lamp]]s (CCFLs) used as backlights in laptops, [[thyratron]] tubes, and [[Crookes tube]]s. They do not necessarily operate at room temperature; in some devices the cathode is heated by the electron current flowing through it to a temperature at which [[thermionic emission]] occurs. For example, in some fluorescent tubes a momentary high voltage is applied to the electrodes to start the current through the tube; after starting the electrodes are heated enough by the current to keep emitting electrons to sustain the discharge.{{Citation needed|date=February 2024}}

Cold cathodes may also emit electrons by [[photoelectric emission]]. These are often called ''photocathodes'' and are used in [[phototube]]s used in scientific instruments and [[image intensifier]] tubes used in night vision goggles.{{Citation needed|date=February 2024}}