How does the bandgap separate the valence band from the conduction band?

In a semiconductor, the bandgap acts as a forbidden energy region that separates the valence band from the conduction band.12

When a large number of atoms form a crystal, their discrete atomic energy levels spread into continuous energy bands. The valence band represents the highest energy band that is completely filled with electrons at absolute zero, while the conduction band is the next higher band, which remains empty under those conditions.23

The bandgap is the range of energy levels between these two bands that electrons are physically unable to occupy. Because there are no available states within this gap, electrons must gain sufficient energy—typically through thermal excitation or the absorption of light—to "jump" across the gap from the valence band into the conduction band, where they become free to move and participate in electrical conduction. The size of this gap determines the material's electrical properties, distinguishing semiconductors from insulators, which have much wider gaps.123