Asemimetal is a material with a small energy overlap between the bottom of theconductionband and the top of thevalence band, but they do not overlap inmomentum space. According toelectronic band theory, solids can be classified asinsulators,semiconductors, semimetals, ormetals. In insulators and semiconductors the filled valence band is separated from an empty conduction band by aband gap. For insulators, the magnitude of the band gap is larger (e.g., > 4 eV) than that of a semiconductor (e.g., < 4 eV). Because of the slight overlap between the conduction and valence bands, semimetals have no band gap and a smalldensity of states at theFermi level. A metal, by contrast, has an appreciable density of states at the Fermi level because the conduction band is partially filled.[1]
The insulating/semiconducting states differ from the semimetallic/metallic states in thetemperature dependency of theirelectrical conductivity. With a metal, the conductivity decreases with increases in temperature (due to increasing interaction of electrons withphonons (lattice vibrations)). With an insulator or semiconductor (which have two types of charge carriers –holes and electrons), both the carrier mobilities and carrier concentrations will contribute to the conductivity and these have different temperature dependencies. Ultimately, it is observed that the conductivity of insulators and semiconductors increase with initial increases in temperature aboveabsolute zero (as more electrons are shifted to the conduction band), before decreasing with intermediate temperatures and then, once again, increasing with still higher temperatures. The semimetallic state is similar to the metallic state but in semimetals both holes and electrons contribute to electrical conduction. With some semimetals, likearsenic andantimony, there is a temperature-independent carrier density below room temperature (as in metals) while, inbismuth, this is true at very low temperatures but at higher temperatures the carrier density increases with temperature giving rise to a semimetal-semiconductor transition. A semimetal also differs from an insulator or semiconductor in that a semimetal's conductivity is always non-zero, whereas a semiconductor has zero conductivity at zero temperature and insulators have zero conductivity even at ambient temperatures (due to a wider band gap).
To classify semiconductors and semimetals, the energies of their filled and empty bands must be plotted against thecrystal momentum of conduction electrons. According to theBloch theorem the conduction of electrons depends on the periodicity of the crystal lattice in different directions.
In a semimetal, the bottom of the conduction band is typically situated in a different part of momentum space (at a differentk-vector) than the top of the valence band. One could say that a semimetal is asemiconductor with a negativeindirect bandgap, although they are seldom described in those terms.
Classification of a material either as a semiconductor or a semimetal can become tricky when it has extremely small or slightly negative band-gaps. The well-known compound Fe2VAl for example, was historically thought of as a semi-metal (with a negative gap ~ -0.1 eV) for over two decades before it was actually shown to be a small-gap (~ 0.03 eV) semiconductor[2] using self-consistent analysis of the transport properties, electrical resistivity andSeebeck coefficient. Commonly used experimental techniques to investigate band-gap can be sensitive to many things such as the size of the band-gap, electronic structure features (direct versus indirect gap) and also the number of free charge carriers (which can frequently depend on synthesis conditions). Band-gap obtained from transport property modeling is essentially independent of such factors. Theoretical techniques to calculate the electronic structure on the other hand can often underestimate band-gap.
Schematically, the figure shows
The figure is schematic, showing only the lowest-energy conduction band and the highest-energy valence band in one dimension ofmomentum space (or k-space). In typical solids, k-space is three-dimensional, and there are an infinite number of bands.
Unlike a regularmetal, 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 resembledegenerate semiconductors more closely. This explains why the electrical properties of semimetals are partway between those of metals andsemiconductors.
As semimetals have fewer charge carriers than metals, they typically have lowerelectrical andthermal 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 highdiamagnetic susceptibilities and high latticedielectric constants.
The classic semimetallic elements arearsenic,antimony,bismuth, α-tin (gray tin) andgraphite, anallotrope ofcarbon. The first two (As, Sb) are also consideredmetalloids but the terms semimetal and metalloid are not synonymous. Semimetals, in contrast to metalloids, can also bechemical compounds, such asmercury telluride (HgTe),[3] andtin,bismuth, andgraphite are typically not considered metalloids.[4]Transient semimetal states have been reported at extreme conditions.[5] It has been recently shown that someconductive polymers can behave as semimetals.[6]