In semiconductors, the behavior of electrons can be better understood through the concept of energy bands. These energy bands are regions where electrons are allowed to exist within the crystal lattice structure of the semiconductor material. The two main types of energy bands in semiconductors are the valence band and the conduction band.

When a semiconductor is in its pure form, with no impurities, the valence band is fully occupied by electrons, while the conduction band is empty. The energy gap between these two bands is known as the band gap. It is key in determining the electrical behavior of the semiconductor.

The band gap can be seen as the minimum amount of energy required for an electron to transition from the valence band to the conduction band. Semiconductors with a small band gap (e.g., 0.6-0.7 eV) are called 'narrow-gap semiconductors,' while those with a larger band gap (e.g., approximately 1.1 eV for silicon) are referred to as 'wide-gap semiconductors.'

The band gap plays a crucial role in the conductivity of semiconductors. In conductors, such as metals, the valence and conduction bands overlap, making it easier for electrons to move between the two bands and thus allowing efficient electric current flow. On the other hand, in insulators, the valence and conduction bands are widely separated by a large band gap, making it challenging for electrons to transition, resulting in minimal electric current flow.