Semimetal

This diagram illustrates a direct semiconductor (A), an indirect semiconductor (B), and a semimetal (C).

A semimetal is a material with a small overlap in the energy of the conduction band and valence bands.

However, the bottom of the conduction band is typically situated in a different part of momentum space (at a different k-vector) than the top of the valence band. One could say that a semimetal is a semiconductor 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 (or k-space). In typical solids, k-space is three dimensional, and there are an infinite number of bands.

Unlike a regular metal, 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 semiconductors 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 and thermal conductivities. 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, antimony, and bismuth. 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
• Metalloid
• Semiconductor
• Solid-state physics