Solid materials are categorized by their electrical conductivity into insulators, semiconductors, and metals. Semimetals represent a distinctive class of crystalline solid that occupies a subtle boundary between these established groups, exhibiting characteristics of both metals and semiconductors. Their electrical properties are not defined by a clear energy gap or a continuous sea of free electrons. Understanding these materials requires examining their unique electronic band structure, which gives rise to their unusual physical behavior.
The Unique Electronic Structure of Semimetals
The definition of a semimetal is rooted in electronic band theory, which describes the allowed energy levels for electrons within a solid. In all solids, electrons reside in a filled valence band and can move into an empty conduction band. The defining feature of a semimetal’s electronic structure is that the top of its valence band slightly overlaps with the bottom of its conduction band in energy. This overlap is extremely small, typically less than a few hundredths of an electron volt.
This minute overlap means the material has no true energy gap separating the occupied and unoccupied states. Consequently, electrons can move into the conduction band without needing external excitation. In pure semimetals, the number of charge carriers (electrons and the “holes” left behind) is extremely low. The concentration of these carriers is far lower than in a typical metal, yet it remains non-zero even at absolute zero temperature. This configuration results in a low density of states at the Fermi level.
How Semimetals Differ from Conductors and Semiconductors
The difference between semimetals, metals, and semiconductors is determined by the specific arrangement of their valence and conduction bands. Metals have bands that overlap significantly or have a partially filled highest occupied band. This configuration ensures a vast number of mobile charge carriers, resulting in exceptionally high electrical conductivity that decreases as temperature rises due to atomic vibrations.
In contrast, semiconductors exhibit a small, measurable energy gap separating the filled valence band from the empty conduction band. At absolute zero, a semiconductor acts as an insulator. Increasing temperature or adding impurities provides enough energy to excite electrons across this gap, significantly raising conductivity. Insulators feature a much larger energy gap, making electron crossing practically impossible.
Semimetals sit precisely at the threshold between these two extremes, exhibiting a zero band gap or the subtle overlap between bands. This unique positioning results in electrical conductivity that is temperature-dependent, much like a semiconductor, because thermal energy increases the number of available carriers. Unlike a semiconductor, the semimetal’s bands are already touching or overlapping, meaning it is never a perfect insulator, even at the lowest temperatures.
Notable Examples and Their Characteristics
Several well-known elements and compounds exhibit semimetallic properties, with Bismuth and Graphite being two of the most commonly cited examples. Bismuth is the heaviest stable, naturally occurring element, and its semimetallic structure causes unusual characteristics. For example, it is the strongest natural diamagnetic material, meaning it generates an opposing magnetic field when placed in an external magnetic field.
Bismuth also possesses one of the lowest thermal conductivities among the elements, a feature that makes it a subject of interest in thermoelectric research. Graphite, an allotrope of carbon composed of stacked two-dimensional layers, is another significant semimetal. Graphite’s semimetallic nature is highly anisotropic, meaning its properties vary dramatically depending on the direction of measurement.
Within the carbon layers, electrons are highly mobile, granting high electrical conductivity similar to a metal. However, the weak bonding between these layers results in very poor conductivity perpendicular to the planes. This directional dependence contrasts sharply with the isotropic conductivity found in most traditional metals. Other elemental semimetals include Antimony and alpha-tin (gray tin).
Current Technological Applications
The unique properties arising from the semimetallic band structure are currently being explored for several advanced technological applications. One significant area is in thermoelectric devices, which convert temperature differences into electrical voltage and vice versa. Semimetals like Bismuth telluride compounds are highly valued for their thermoelectric performance due to the combination of low thermal conductivity and sufficient electrical conductivity.
Researchers are also focusing on topological semimetals, a newer sub-class of material where the electronic bands cross in a highly specific way, producing unique quantum behavior. These materials are investigated for use in spintronics, a field that seeks to utilize the electron’s spin to create more energy-efficient electronic devices. The low density of states in many semimetals also makes them desirable for creating ultra-low resistance contacts in microelectronic components.