The band gap refers to the energy difference between the valence band, which is the highest range of electron energies in a solid where electrons are normally present, and the conduction band, which is the range of electron energies where electrons can move freely and conduct electricity. In more detail: 1. **Valence Band**: This is the energy band that contains the electrons involved in bonding. Electrons in this band are generally not free to move, as they are bound to their respective atoms.
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A band gap is an interval of energy containing no bulk states in the specified band description. A global gap must remain empty for all crystal momenta; a separation between two bands at one momentum alone does not establish it.
In a simple semiconductor, the gap separates the filled valence band from the empty conduction band at zero temperature. Light can create an electron–hole excitation across it, with the transition also constrained by momentum and selection rules. A direct band gap aligns the relevant extrema at one momentum, while an indirect transition may require a phonon to balance momentum. Surface, defect or bound exciton states can add lower-energy features without making them ordinary bulk one-electron bands. Thermal carrier activation and optical thresholds consequently probe related but distinct quantities.