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Band gap

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Description

Molecular structure makes a big difference. On the molecular level, in different types of solid material molecules or compounds, the distance between atoms that make up the molecules creates certain electrical features of the material. For instance, the distance between atoms in silicon carbide, which is a compound of silicon and carbon, varies from the distance between atoms in graphene, which is an allotrope of carbon. Electrons orbiting the atoms in these molecules may overlap or be far apart based on the structure of that molecule or compound. The electrical effects of molecule’s structure produces an energy range. Specifically the electrons orbit in two bands, a valence band and a conduction band, and these electrons may jump to the other band within the molecule. Between these two bands of orbiting electrons is a gap. The band gap describes the space in all solids where no electrons go, between those two bands, valence and conduction. The gap, also known as an energy gap, varies by material from no band gap to wide band gap. The size the gap can affect the properties of the material and how it behaves in a transistor. The gap can also be manipulated and is the focus of continuing research.

Small band gap materials are conductors. They have no gap between the two bands of electrons. Narrow band gaps are used for insulators. Wide band gaps are used for semiconductors.

No band gap
Graphene, the first 2D semiconductor discovered, is a tough, highly conductive material that has no band gap. That hasn’t stopped researchers from trying to add band gap by stretching or stressing graphene. Research studies so far say they have reached 0.5 to 2.1 electronvolt band gaps.

Narrow band gap
A narrow band gap is at a range of 1.11 eV, narrower than silicon.

Wide band gap
Silicon carbide (SiC) and gallium nitride (GaN) are compound materials that have existed for over 20 years, starting in the military and defense sectors. They are very strong materials compared with silicon and require three times the energy to allow an electron to start to move freely in the material. This larger energy gap (or wider band gap) gives these materials superior qualities, such as faster switching, higher efficiency, and increased power density.*

Power semiconductors based on SiC and GaN are considered wide band gap technologies, which means they provide faster switching speeds and higher breakdown voltages than silicon-based solutions.

Tuning the gap
Band gap characteristics can shift depending on how the material is formed. In grown, rather than exfoliated materials, the substrate’s coefficient of thermal expansion can be used to control the amount of strain in the deposited film, shifting the band gap and emission characteristics.

Conventional III-V and II-VI semiconductors share many structural and electrical characteristics with silicon, germanium, and each other. So materials scientists can vary semiconductor composition as needed to tune the band gap, lattice parameters, and other properties without dramatic changes to the overall integration scheme.

Two-dimensional semiconductors, in contrast, come from different parts of the periodic table. They have different electrical properties, different lattice structures, and different chemical characteristics.

 

*Applied Materials provided some content.