Charge carrier
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In physics, a charge carrier denotes a free (mobile, unbound) particle carrying an electric charge. Examples are electrons and ions. In semiconductor physics, the travelling vacancies in the valence-band electron population (holes) are treated as charge carriers.
In ionic solutions, the charge carriers are the dissolved cations and anions. Similarly, cations and anions of the dissociated liquid serve as charge carriers in liquids and melted ionic solids (see eg. the Hall-Heroult process for an example of electrolysis of a melt).
In plasma, such as an electric arc, the electrons and cations of ionized gas and vaporized material of electrodes act as charge carriers. (The electrode vaporization occurs in vacuum too, but then the arc is not technically occurring in vacuum, but in low-pressure electrode vapors.)
In vacuum, in an electric arc or in vacuum tubes free electrons act as charge carriers.
In metals, the charge carriers are the electrons forming the Fermi gas in the metal lattice.
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[edit] Majority and minority carriers in semiconductors
In semiconductors, electrons and holes act as charge carriers. The more abundant charge carriers are called majority carriers. In N-type semiconductors they are electrons, while in P-type semiconductors 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.
Minority carriers play an important role in bipolar transistors and solar cells. However, their role in field-effect transistors (FETs) is a bit more complex: for example, a MOSFET has both P-type and N-type regions. The transistor action involves the majority carriers of the source and drain regions, but these carriers traverse the 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 a depletion layer), so conventionally the source and drain designation for the carriers is adopted, and FETs are called "majority carrier" devices.
When an electron meets with a hole, they recombine and these free carriers effectively vanish. 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 photons (optical recombination, used in LEDs and semiconductor lasers).
