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Semimetal
 
 
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A semimetal is a material with a small overlap in the energy of the conduction
Electrical conduction

Electrical conduction is the movement of electric charge particles through a transmission medium . The movement of charge constitutes an Current ....
 band
Electronic band structure

In solid-state physics, the electronic band structure of a solid describes ranges of energy that an electron is "forbidden" or "allowed" to have....
 and valence band
Valence band

In solids, the valence band is the highest range of electron energy where electrons are normally present at absolute zero.In semiconductors and Electrical insulations, there is a band gap above the valence band, followed by a conduction band above that....
s.

However, the bottom of the conduction band is typically situated in a different part of momentum space (at a different k-vector
Wave vector

A wave vector is a vector representation of a wave. The wave vector has magnitude indicating wavenumber , and the direction of the vector indicates the direction of wave propagation....
) than the top of the valence band. One could say that a semimetal is a semiconductor
Semiconductor

A semiconductor is a material that has electrical conductivity between those of a Electrical conductor and an electrical insulation; it can vary over that wide range either permanently or dynamically....
 with a negative indirect bandgap, although they are seldom described in those terms.

Schematically, the figure shows
A) a semiconductor with a direct gap (like e.g.






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Semimetal
A semimetal is a material with a small overlap in the energy of the conduction
Electrical conduction

Electrical conduction is the movement of electric charge particles through a transmission medium . The movement of charge constitutes an Current ....
 band
Electronic band structure

In solid-state physics, the electronic band structure of a solid describes ranges of energy that an electron is "forbidden" or "allowed" to have....
 and valence band
Valence band

In solids, the valence band is the highest range of electron energy where electrons are normally present at absolute zero.In semiconductors and Electrical insulations, there is a band gap above the valence band, followed by a conduction band above that....
s.

However, the bottom of the conduction band is typically situated in a different part of momentum space (at a different k-vector
Wave vector

A wave vector is a vector representation of a wave. The wave vector has magnitude indicating wavenumber , and the direction of the vector indicates the direction of wave propagation....
) than the top of the valence band. One could say that a semimetal is a semiconductor
Semiconductor

A semiconductor is a material that has electrical conductivity between those of a Electrical conductor and an electrical insulation; it can vary over that wide range either permanently or dynamically....
 with a negative indirect bandgap, although they are seldom described in those terms.

Schematically, the figure shows
A) a semiconductor with a direct gap (like e.g. CuInSe2),
B) a semiconductor with an indirect gap (like Si) and
C) a semimetal (like Sn or graphite).


The figure is schematic, showing only the lowest-energy conduction band and the highest-energy valence band in one dimension of momentum space
Momentum space

The Momentum space associated with a particle is a vector space in which every point corresponds to a possible value of the momentum vector . Representing a problem in terms of the momenta of the particles involved, rather than in terms of their positions, can greatly simplify some problems in physics....
 (or k-space). In typical solids, k-space is three dimensional, and there are an infinite number of bands.

Unlike a regular metal
Metal

In chemistry, a metal is a chemical element whose atoms readily lose electrons to form positive ions , and form metallic bonds between other metal atoms and ionic bonds between nonmetal atoms....
, semimetals have charge carriers of both types (holes and electrons), so that one could also argue that they should be called 'double-metals' rather than semimetals. However, the charge carriers typically occur in much smaller numbers than in a real metal. In this respect they resemble degenerate semiconductor
Degenerate semiconductor

A degenerate semiconductor is a semiconductor with such a high Doping -level that the material starts to act more like a metal than as a semiconductor....
s more closely. This explains why the electrical properties of semimetals are partway between those of metals and semiconductors.

As semimetals have fewer charge carriers than metals, they typically have lower electrical
Electrical conductivity

Electrical conductivity or specific conductance is a measure of a material's ability to electrical conduction an electric current. When an electrical potential difference is placed across a conductor, its movable charges flow, giving rise to an electric current....
 and thermal conductivities
Thermal conductivity

In physics, thermal conductivity, , is the List of materials properties of a material that indicates its ability to conduct heat. It appears primarily in Heat conduction#Fourier's law for heat conduction....
. They also have small effective masses for both holes and electrons because the overlap in energy is usually the result of the fact that both energy bands are broad. In addition they typically show high diamagnetic susceptibilities and high lattice dielectric constants.

The classic semimetallic elements are arsenic
Arsenic

Arsenic is a well-known chemical element that has the symbol As and atomic number 33. Arsenic was first documented by Albertus Magnus in 1250....
, antimony
Antimony

Antimony is a chemical element with the symbol Sb and atomic number 51. A metalloid, antimony has four allotropy forms. The stable form of antimony is a blue-white metalloid....
, and bismuth
Bismuth

Bismuth is a chemical element that has the symbol Bi and atomic number 83. This heavy, brittle, white crystalline trivalent poor metal has a pink tinge and chemically resembles arsenic and antimony....
. These are also considered metalloids but the concepts are not synonymous. Semimetals, in contrast to metalloids, can also be compounds, such as HgTe, and tin and graphite are typically not considered metalloids.

Graphite and hexagonal boronnitride (BN) are an interesting comparison. The materials have essentially the same layered structure and are isoelectronic, which means that their band structure should be rather similar. However, BN is a white semiconductor and graphite a black semimetal, because the relative position of the bands in the energy direction is somewhat different. In one case the bandgap is positive (like case B in the figure), explaining why BN is a semiconductor. In the other case the conduction band lies sufficiently lower to overlap with the valence band in energy, rendering the value for the bandgap negative (see C).

See also

  • Metal
    Metal

    In chemistry, a metal is a chemical element whose atoms readily lose electrons to form positive ions , and form metallic bonds between other metal atoms and ionic bonds between nonmetal atoms....
  • Metalloid
    Metalloid

    is a term used in chemistry when classifying the chemical elements. On the basis of their general physical and chemical properties, nearly every element in the periodic table can be termed either a metal or a nonmetal....
  • Semiconductor
    Semiconductor

    A semiconductor is a material that has electrical conductivity between those of a Electrical conductor and an electrical insulation; it can vary over that wide range either permanently or dynamically....
  • Solid-state physics
    Solid-state physics

    Solid-state physics, the largest branch of condensed matter physics, is the study of rigid matter, or solids, through methods such as quantum mechanics, crystallography, electromagnetism and metallurgy....


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