Charge Carrier
From: https://en.wikipedia.org/wiki/Charge_carrier#
Majority_and_minority_carriers
Charge carrier
In physics, a charge carrier is a particle or quasiparticle that is free to move
carrying an electric charge, especially the particles that carry electric
charges in electrical conductors.[1] Examples are electrons, ions and holes.
The term is used most commonly in solid state physics.[2] In a conducting medium
, an electric field can exert force on these free particles, causing a net
motion of the particles through the medium; this is what constitutes an
electric current.[3] The electron and the proton are the elementary charge
carriers, each carrying one elementary charge (e), of the same magnitude and
opposite sign.
In conductors
In conducting media, particles serve to carry charge:
- In many metals, the charge carriers are electrons. One or two of the valence
electrons from each atom are able to move about freely within the crystal
structure of the metal.[4] The free electrons are referred to as conduction
electrons, and the cloud of free electrons is called a Fermi gas.[5][6] Many
metals have electron and hole bands. In some, the majority carriers are
holes.[citation needed]
- In electrolytes, such as salt water, the charge carriers are ions,[6] which
are atoms or molecules that have gained or lost electrons so they are
electrically charged. Atoms that have gained electrons so they are negatively
charged are called anions, atoms that have lost electrons so they are positively
charged are called cations.[7] Cations and anions of the dissociated liquid
also serve as charge carriers in melted ionic solids (see e.g. the Hall–Héroult
process for an example of electrolysis of a melted ionic solid). Proton
conductors are electrolytic conductors employing positive hydrogen ions as
carriers.[8]
- In a plasma, an electrically charged gas which is found in electric arcs
through air, neon signs, and the sun and stars, the electrons and cations of
ionized gas act as charge carriers.[9]
- In a vacuum, free electrons can act as charge carriers. In the electronic
component known as the vacuum tube (also called valve), the mobile electron
cloud is generated by a heated metal cathode, by a process called thermionic
emission.[10] When an electric field is applied strongly enough to draw the
electrons into a beam, this may be referred to as a cathode ray, and is the
basis of the cathode ray tube display widely used in televisions and computer
monitors until the 2000s.[11]
- In semiconductors, which are the materials used to make electronic
components like transistors and integrated circuits, two types of charge carrier
are possible. In p-type semiconductors, "effective particles" known as electron
holes with positive charge move through the crystal lattice, producing an
electric current. The "holes" are, in effect, electron vacancies in the
valence-band electron population of the semiconductor and are treated as charge
carriers because they are mobile, moving from atom site to atom site. In n-type
semiconductors, electrons in the conduction band move through the crystal,
resulting in an electric current.
In some conductors, such as ionic solutions and plasmas, positive and negative
charge carriers coexist, so in these cases an electric current consists of the
two types of carrier moving in opposite directions. In other conductors, such as
metals, there are only charge carriers of one polarity, so an electric current
in them simply consists of charge carriers moving in one direction.
In semiconductors
There are two recognized types of charge carriers in semiconductors. One is
electrons, which carry a negative electric charge. In addition, it is convenient
to treat the traveling vacancies in the valence band electron population (holes)
as a second type of charge carrier, which carry a positive charge equal in
magnitude to that of an electron.[12]
Carrier generation and recombination
When an electron meets with a hole, they recombine and these free carriers
effectively vanish.[13] 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).[14] 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 transport in a piece of semiconductor. 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.[15]
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 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.[16]
Minority carriers play an important role in bipolar transistors and solar
cells.[17] Their role in field-effect transistors (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 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 an inversion layer), so conventionally the
source and drain designation for the carriers is adopted, and FETs are called
"majority carrier" devices.[18]
Free carrier concentration
Free carrier concentration is the concentration of free carriers in a 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 (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.[19]
See also