Define the term "band gap" in solid-state physics.

The term "band gap" in solid-state physics refers specifically to the energy difference between the conduction band and the valence band within a material. In semiconductors and insulators, electrons reside in the valence band at lower energy levels, while the conduction band is where electrons can move freely, contributing to electrical conductivity. The size of the band gap determines how easily electrons can transition from the valence band to the conduction band when energy is supplied, such as through heat or light.

A smaller band gap indicates that less energy is required for this transition, which can enhance the material's electrical conductivity. Conversely, a larger band gap typically signifies a greater energy requirement for electrons to move into the conduction band, resulting in lower electrical conductivity. Understanding the band gap is crucial for applications in electronics and optoelectronics, as it influences the optical and electrical properties of materials.