How do conductors, semiconductors, and insulators differ in electrical conductivity?

In materials science, the electrical conductivity of a substance is determined by its band structure, specifically the presence or absence of a band gap—a range of energy levels that electrons cannot occupy.

Classification by Band Gap

• Conductors (Metals): These materials lack a band gap at the Fermi level. The valence band (occupied states) and conduction band (unoccupied states) overlap or are continuous, allowing a fraction of electrons to move freely at all times. This results in high electrical conductivity.12
• Insulators: These materials possess a wide band gap between the valence and conduction bands. Because this gap is too wide for electrons to cross via thermal excitation, there are no free electrons available to carry a current, making them poor conductors.123
• Semiconductors: These materials have a relatively narrow band gap. While they act as insulators at absolute zero, the narrow gap allows electrons to be excited into the conduction band through thermal energy or impurities at room temperature, resulting in intermediate conductivity.124