bandgap energy E g

https://dx.doi.org/10.1351/goldbook.B00593
The energy difference between the bottom of the conduction band and the top of the valence band in a semiconductor or an insulator.
Source:
PAC, 1996, 68, 2223 (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2229