Editing Rectifier (section)

=== Solid state ===

==== Crystal detector ====
{{Main|Crystal detector}}
[[File:CatWhisker.jpg|thumb|Galena cat's whisker [[crystal detector]]]]

The [[crystal detector]], the earliest type of [[semiconductor diode]], was used as a [[detector]] in some of the earliest [[radio receiver]]s, called [[crystal radio]]s, to rectify the radio [[carrier wave]] and extract the [[modulation]] which produced the sound in the earphones.   Invented by [[Jagadish Chandra Bose]] and [[Greenleaf Whittier Pickard|G. W. Pickard]] around 1902, it was a significant improvement over earlier detectors such as the [[coherer]].  One popular type of crystal detector, often called a ''cat's whisker detector'', consists of a crystal of some [[semiconducting]] [[mineral]], usually [[galena]] (lead sulfide), with a light springy wire touching its surface. Its fragility and limited current capability made it unsuitable for power supply applications. It was used widely in radios until the 1920s when [[vacuum tube]]s replaced it.  In the 1930s, researchers [[Diode#Solid-state diodes|miniaturized and improved]] the crystal detector for use at microwave frequencies, developing the first semiconductor diodes.
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==== Selenium and copper oxide rectifiers ====
{{Main|Metal rectifier}}
[[File:Selenium Rectifier.jpg|thumb|upright=0.7|[[Selenium rectifier]]]]
Once common until replaced by more compact and less costly silicon solid-state rectifiers in the 1970s, these units used stacks of oxide-coated metal plates and took advantage of the [[semiconductor]] properties of [[selenium]] or copper oxide.<ref>H. P. Westman et al., (ed), ''[http://lccn.loc.gov/43014665 Reference Data for Radio Engineers, Fifth Edition]'', 1968, Howard W. Sams and Co., no ISBN, Library of Congress Card No. 43-14665 chapter 13</ref> While [[selenium rectifier]]s were lighter in weight and used less power than comparable vacuum tube rectifiers, they had the disadvantage of finite life expectancy, increasing resistance with age, and were only suitable to use at low frequencies. Both selenium and copper oxide rectifiers have somewhat better tolerance of momentary voltage transients than silicon rectifiers.

Typically these rectifiers were made up of stacks of metal plates or washers, held together by a central bolt, with the number of stacks determined by voltage; each cell was rated for about 20 V. An automotive battery charger rectifier might have only one cell: the high-voltage power supply for a [[vacuum tube]] might have dozens of stacked plates. Current density in an air-cooled selenium stack was about 600 mA per square inch of active area (about 90 mA per square centimeter).
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==== Silicon and germanium diodes ====
{{Main|Diode}}
[[File:Dioden.JPG|thumb|A variety of silicon diodes of different current ratings. At left is a [[bridge rectifier]]. On the 3 center diodes, a painted band identifies the cathode terminal]] 
[[Silicon]] diodes are the most widely used rectifiers for lower voltages and powers, and have largely replaced other rectifiers.   Due to their substantially lower forward voltage (0.3V versus 0.7V for silicon diodes) germanium diodes have an inherent advantage over silicon diodes in low voltage circuits.  
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==== High power: thyristors (SCRs) and newer silicon-based voltage sourced converters ====
{{main|Silicon controlled rectifier}}
[[File:Manitoba Hydro-BipoleII Valve.jpg|thumb|200px|Two of three high-power [[thyristor]] valve stacks used for long-distance transmission of power from [[Manitoba Hydro]] dams. Compare with mercury-arc system from the same dam-site, above.]]

In high-power applications, from 1975 to 2000, most mercury valve arc-rectifiers were replaced by stacks of very high power [[thyristor]]s, silicon devices with two extra layers of semiconductor, in comparison to a simple diode.

In medium-power transmission applications, even more complex and sophisticated [[voltage sourced converter]] (VSC) silicon semiconductor rectifier systems, such as [[IGBT transistor|insulated gate bipolar transistors (IGBT)]] and [[Gate turn-off thyristor|gate turn-off thyristors (GTO)]], have made smaller high voltage DC power transmission systems economical. All of these devices function as rectifiers.

{{As of|2009}} it was expected that these high-power silicon "self-commutating switches", in particular IGBTs and a variant thyristor (related to the GTO) called the [[integrated gate-commutated thyristor]] (IGCT), would be scaled-up in power rating to the point that they would eventually replace simple thyristor-based AC rectification systems for the highest power-transmission DC applications.<ref>{{cite book|first1=Jos|last1=Arrillaga|first2=Yonghe H|last2=Liu|first3=Neville R|last3=Watson|first4=Nicholas J|last4=Murray|title=Self-Commutating Converters for High Power Applications|publisher=John Wiley & Sons|isbn=978-0-470-68212-8|date=12 January 2010}}</ref>

====Active rectifier====
{{main|Active rectification}}
[[Image:Diode mosfet.svg|thumb|250px|Voltage drop across a diode and a [[power MOSFET|MOSFET]].  The low on-resistance property of a MOSFET reduces ohmic losses compared to the diode rectifier (below 32&nbsp;A in this case), which exhibits a significant voltage drop even at very low current levels.  Paralleling two MOSFETs (pink curve) reduces the losses further, whereas paralleling several diodes won't significantly reduce the forward-voltage drop.]]

Active rectification is a technique for improving the efficiency of rectification by replacing [[diode]]s with actively controlled switches such as [[transistor]]s, usually [[power MOSFET]]s or [[power BJT]]s.<ref name="emadi"/> Whereas normal semiconductor diodes have a roughly fixed voltage drop of around 0.5 to 1 volts, active rectifiers behave as resistances, and can have arbitrarily low voltage drop.

Historically, [[vibrator (electronic)|vibrator]]-driven switches or motor-driven [[commutator (electric)|commutator]]s have also been used for [[mechanical rectifier]]s and synchronous rectification.<ref>
{{cite book
 | title = Standard polyphase apparatus and systems
 | edition = 5th
 | author = Maurice Agnus Oudin
 | publisher = Van Nostrand
 | date = 1907
 | page = [https://archive.org/details/standardpolypha00oudigoog/page/n248 236]
 | url = https://archive.org/details/standardpolypha00oudigoog
 | quote = synchronous rectifier commutator.
 }}</ref>

Active rectification has many applications. It is frequently used for arrays of photovoltaic panels to avoid reverse current flow that can cause overheating with partial shading while giving minimum power loss.