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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.^[1]

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. 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), typically in smaller numbers than a real metal. The electrical properties of semimetals are partway between those of metals and semiconductors. 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,^[2] 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. However BN is a white semiconductor and graphite a black semimetal, because in one case the bandgap is positive (like case B in the figure) in the other negative (see C).

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.

1. ^ Burns, Gerald (1985). Solid State Physics. Academic Press, Inc., 339-40. ISBN 0-12-146070-3. 
2. ^ Wang, Yang; N. Mansour, A. Salem, K.F. Brennan, and P.P. Ruden (1992). "Theoretical study of a potential low-noise semimetal-based avalanche photodetector". IEEE Journal of Quantum Electronics 28 (2): 507-513. doi:10.1109/3.123280. Retrieved on 2006-06-08. 

• Metal
• Metalloid
• Semiconductor
• Solid-state physics