When two or more atoms come together to form a molecule, their atomic orbitals combine and molecular orbitals of distinct energies result. In a solid, there are a large number of atoms, and therefore a large number of atomic orbitals that may be combined into molecular orbitals. These groups of molecular orbitals are so closely placed together to form continuous regions of energies, known as the bands.
The energy difference between these bands is known as the band gap.
Conductor, Semiconductor, and Insulators
In order to conduct electricity, valence electrons must cross orbitals of different energies to move throughout the solid. This is determined by the band gap. The valence electrons in conductors occupy a band that has many empty orbitals. Thus, only a small amount of energy is required to move the electrons to these empty orbitals. This small energy difference is “easy” to overcome, so they are good conductors of electricity. Semiconductors and insulators observe two types of bands - a valence band, with few to no empty orbitals, and a conduction band, with empty orbitals. The energy difference or the band gap between the valence band and the conduction band decides the ease with which the electrons can move. In insulators, the band gap is so “large” that very few electrons can reach the empty orbitals of the conduction band; as a result, insulators are poor conductors of electricity. Semiconductors, on the other hand, have comparatively small band gaps. As a result, they can conduct electricity when “moderate” amounts of energy are provided to move electrons out of the filled orbitals of the valence band and into the empty orbitals of the conduction band. Thus, semiconductors are better than insulators but not as efficient as conductors in terms of electrical conductivity.
This text has been adapted from Openstax, Chemistry 2e, Section 8.4 Molecular Orbital Theory.