A cathode is the electrode from which a conventional current leaves a
polarized electrical device. (This definition can be recalled by using
the mnemonic CCD for cathode current departs.) A conventional current
describes the direction in which positive electronic charges move.
Electrons have a negative electrical charge, so the movement of
electrons is opposite to that of the conventional current flow
(consequently, the mnemonic cathode current departs also means that
electrons flow into the device's cathode).
1 Etymology 2 Flow of electrons 3 In chemistry
3.1 Electrolytic cell 3.2 Galvanic cell 3.3 Electroplating metal cathode (electrolysis)
4 In electronics
4.1 Vacuum tubes
4.1.1 Hot cathode 4.1.2 Cold cathode
5 See also 6 References 7 External links
The word was coined in 1834 from the Greek κάθοδος (kathodos),
'descent' or 'way down', by William Whewell, who had been consulted
Glow from the directly heated cathode of a 1 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 a metal surface which emits free electrons into the evacuated space. 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. Cathodes are induced to emit electrons by several mechanisms:
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. This process is used in cold cathodes in some electron microscopes, and in microelectronics fabrication, 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 lamps such as neon lamps. Photoelectric emission: Electrons can also be emitted from the electrodes 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. This effect is used in phototubes and image intensifier tubes.
Cathodes can be divided into two types: Hot cathode Main article: Hot cathode
Two indirectly-heated cathodes (orange heater strip) in ECC83 dual triode tube
Cutaway view of a triode vacuum tube with an indirectly-heated cathode (orange tube), showing the heater element inside
A hot cathode is a cathode that is heated by a filament to produce electrons by thermionic emission. 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 tubes. 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 generators, electron microscopes, and fluorescent tubes. There are two types of hot cathodes:
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 tubes, 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. 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 1400 °C (~2500 °F), 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 425–600 °C (~800–1100 °F) () There are two main types of treated cathodes:
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.
Cold cathode Main article: 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 lights, cold-cathode fluorescent lamps (CCFLs) used as backlights in laptops, thyratron tubes, and Crookes tubes. 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. Cold cathodes may also emit electrons by photoelectric emission. These are often called photocathodes and are used in phototubes used in scientific instruments and image intensifier tubes used in night vision goggles. Diodes
In a semiconductor diode, the cathode is the N–doped layer of the PN junction with a high density of free electrons due to doping, and an equal density of fixed positive charges, which are the dopants that have been thermally ionized. In the anode, the converse applies: It features a high density of free "holes" and consequently fixed negative dopants which have captured an electron (hence the origin of the holes). When P and N-doped layers are created adjacent to each other, diffusion ensures that electrons flow from high to low density areas: That is, from the N to the P side. They leave behind the fixed positively charged dopants near the junction. Similarly, holes diffuse from P to N leaving behind fixed negative ionised dopants near the junction. These layers of fixed positive and negative charges are collectively known as the depletion layer because they are depleted of free electrons and holes. The depletion layer at the junction is at the origin of the diode's rectifying properties. This is due to the resulting internal field and corresponding potential barrier which inhibit current flow in reverse applied bias which increases the internal depletion layer field. Conversely, they allow it in forwards applied bias where the applied bias reduces the built in potential barrier. Electrons which diffuse from the cathode into the P-doped layer, or anode, become what are termed "minority carriers" and tend to recombine there with the majority carriers, which are holes, on a timescale characteristic of the material which is the p-type minority carrier lifetime. Similarly, holes diffusing into the N-doped layer become minority carriers and tend to recombine with electrons. In equilibrium, with no applied bias, thermally assisted diffusion of electrons and holes in opposite directions across the depletion layer ensure a zero net current with electrons flowing from cathode to anode and recombining, and holes flowing from anode to cathode across the junction or depletion layer and recombining. Like a typical diode, there is a fixed anode and cathode in a Zener diode, but it will conduct current in the reverse direction (electrons flow from anode to cathode) if its breakdown voltage or "Zener voltage" is exceeded. See also
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