Band Gap¶
Non-Scalar Electronic
The Band Gap measures the finite energy difference between the highest occupied and lowest unoccupied energy levels in the valence and conduction bands, respectively, of a semiconducting or insulating material 1. It is one of the most fundamental electronic properties, governing optical absorption, electrical conductivity, and thermoelectric performance.
Direct and Indirect Band Gaps¶
Two types of band gap are possible: direct and indirect 2. The platform computes both types whenever possible.
When the gap is direct, the minimum energy difference between occupied and unoccupied states occurs at the same k-point in reciprocal space. For indirect band gaps, this minimum energy difference involves states at different k-points, and the transition requires a change in crystal momentum (typically assisted by a phonon).
The indirect band gap can be smaller than the direct gap. Classic examples include silicon (indirect gap ~1.1 eV, direct gap ~3.4 eV) and germanium.
Methods for Computing the Band Gap¶
The band gap can be obtained from several levels of theory, each with different accuracy-cost tradeoffs:
| Method | Typical Accuracy | Tutorial |
|---|---|---|
| GGA-DFT (PBE) | Underestimates by 30–50% | Band gap tutorial |
| HSE hybrid functional | Within ~0.2 eV of experiment | HSE (VASP), HSE (QE) |
| GW approximation | Typically within ~0.1 eV | GW (VASP) |
| ML force fields | Depends on training data | MatterSim |
GGA band gap underestimation
Standard DFT with the Generalized Gradient Approximation systematically underestimates band gaps due to the self-interaction error and missing derivative discontinuity in the exchange-correlation functional. For quantitative band gap predictions, hybrid functionals (HSE) or the GW approximation are recommended.
Example¶
Both types of band gaps are returned under the Results Tab, immediately below the main band structure dispersion plot.
For indirect-gap materials, the pair of k-vectors linking the corresponding minimal energy difference is indicated. For direct-gap semiconductors, the two gap types are presented as equivalent and both located at the Gamma point.
Schema¶
The JSON schema and an example representation for this property can be found here.