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 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.
In semiconductor physics, two types of charge carriers are recognized. One of them is electrons. In addition, it is convenient to treat the traveling vacancies in the valence-band electron population (holes) as the second type of charge carriers which carry the positive charge.
