Editing Charge carrier (section)

==In semiconductors==
There are two recognized types of charge carriers in [[semiconductor]]s. One is [[electron]]s, which carry a negative [[electric charge]]. In addition, it is convenient to treat the traveling vacancies in the [[valence band]] electron population ([[electron hole|holes]]) as a second type of charge carrier, which carry a positive charge equal in magnitude to that of an electron.<ref>{{cite web
 |url=http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/intrin.html
 |title=Intrinsic Semiconductors
 |access-date=May 1, 2021
 |first=R.
 |last=Nave}}</ref>

===Carrier generation and recombination===
{{main|Carrier generation and recombination}}
When an electron meets with a hole, they [[carrier generation and recombination|recombine]] and these free carriers effectively vanish.<ref>{{cite web
 |url=https://ecee.colorado.edu/~bart/book/book/chapter2/ch2_8.htm
 |title=Carrier recombination and generation
 |date=2011
 |access-date=May 1, 2021
 |first=B.
 |last=Van Zeghbroeck
 |archive-date=May 1, 2021
 |archive-url=https://web.archive.org/web/20210501181954/https://ecee.colorado.edu/~bart/book/book/chapter2/ch2_8.htm
 |url-status=dead
 }}</ref> The energy released can be either thermal, heating up the semiconductor (''thermal recombination'', one of the sources of [[waste heat]] in semiconductors), or released as [[photon]]s (''optical recombination'', used in [[light-emitting diode|LEDs]] and [[laser diode|semiconductor laser]]s).<ref>{{cite web
 |url=https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-720j-integrated-microelectronic-devices-spring-2007/lecture-notes/lecture4.pdf
 |title=Lecture 4 - Carrier generation and recombination
 |date=February 12, 2007
 |access-date=May 2, 2021
 |first=Jesús
 |last=del Alamo
 |page=3
 |publisher=MIT Open CourseWare, Massachusetts Institute of Technology}}</ref> The recombination means an electron which has been excited from the valence band to the conduction band falls back to the empty state in the valence band, known as the holes. The holes are the empty states created in the valence band when an electron gets excited after getting some energy to pass the energy gap.

=== Majority and minority carriers ===
The more abundant charge carriers are called '''majority carriers''', which are primarily responsible for [[current (electricity)|current]] transport in a piece of semiconductor. In [[n-type semiconductor]]s they are electrons, while in [[p-type semiconductor]]s they are holes. The less abundant charge carriers are called '''minority carriers'''; in n-type semiconductors they are holes, while in p-type semiconductors they are electrons.<ref>{{cite web
 |url=https://www.physics-and-radio-electronics.com/electronic-devices-and-circuits/semiconductor/majority-and-minority-carriers.html
 |title=Majority and minority charge carriers
 |access-date=May 2, 2021}}</ref> The concentration of holes and electrons in a doped semiconductor is governed by the [[Mass action law (electronics)|mass action law]].

In an [[intrinsic semiconductor]], which does not contain any impurity, the concentrations of both types of carriers are ideally equal. If an intrinsic semiconductor is [[doping (semiconductor)|doped]] with a donor impurity then the majority carriers are electrons. If the semiconductor is doped with an acceptor impurity then the majority carriers are holes.<ref>{{cite web
 |url=http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/dope.html
 |title=Doped Semiconductors
 |access-date=May 1, 2021
 |first=R.
 |last=Nave}}</ref>

Minority carriers play an important role in [[Bipolar junction transistor|bipolar transistors]] and [[solar cell]]s.<ref>{{cite web|url=https://inst.eecs.berkeley.edu/~ee105/sp04/handouts/lectures/Lecture21.pdf|title=Lecture 21: BJTs|access-date=May 2, 2021|first=J. S.|last=Smith}}</ref> Their role in [[field-effect transistor]]s (FETs) is a bit more complex: for example, a [[MOSFET]] has p-type and n-type regions. The transistor action involves the majority carriers of the [[field-effect transistor|source]] and [[Field-effect transistor|drain]] regions, but these carriers traverse the [[field-effect transistor|body]] of the opposite type, where they are minority carriers. However, the traversing carriers hugely outnumber their opposite type in the transfer region (in fact, the opposite type carriers are removed by an applied electric field that creates an [[Inversion layer (semiconductors)|inversion layer]]), so conventionally the source and drain designation for the carriers is adopted, and FETs are called "majority carrier" devices.<ref>{{cite web
 |url=https://www.eetimes.com/back-to-the-basics-of-power-mosfets/
 |title=Back to the basics of power MOSFETs
 |date=February 22, 2007
 |access-date=May 2, 2021
 |first=Dan|last=Tulbure
 |publisher=EE Times}}</ref>

===Free carrier concentration===
{{Main|Charge carrier density}}
''Free carrier concentration'' is the [[concentration]] of free carriers in a [[doping (semiconductor)|doped semiconductor]]. It is similar to the carrier concentration in a metal and for the purposes of calculating currents or drift velocities can be used in the same way. Free carriers are electrons ([[electron hole|holes]]) that have been introduced into the [[conduction band]] ([[valence band]]) by doping. Therefore, they will not act as double carriers by leaving behind holes (electrons) in the other band. In other words, charge carriers are particles that are free to move, carrying the charge. The free carrier concentration of doped semiconductors shows a characteristic temperature dependence.<ref>{{cite web
 |url=http://truenano.com/PSD20/chapter2/ch2_6.htm#2_6_4_4
 |title=Carrier densities
 |date=2011
 |access-date=July 28, 2022
 |first=B.
 |last=Van Zeghbroeck}}</ref>