There is variation in the electrical conductivity of materials - metals, semiconductors, and insulators that are showcased with the help of the energy band diagrams.

Metals such as copper (Cu), zinc (Zn), or lead (Pb) have low resistivity and feature conduction bands that are either not fully occupied or overlap with the valence band, making a bandgap non-existent. This allows electrons in the highest energy levels of the valence band to easily transition to the conduction band upon gaining minimal kinetic energy from an applied electric field. As a result, with many available states close to filled ones, metals facilitate the easy flow of electrical current.

Semiconductors have a filled valence band and an empty conduction band separated by a small band gap, on the order of 1 eV, allowing for some valence electrons to be thermally excited to the conduction band at room temperature. This results in a moderate number of charge carriers, making semiconductors more conductive than insulators but less than metals. The band gap energy varies for semiconductors, like 1.12 eV for silicon (Si) and 1.42 eV for gallium arsenide (GaAs).

In insulators like SiO₂, the valence band is filled with electrons, and the conduction band is empty. Insulators have a large band gap, making it difficult for valence electrons to be excited by the conduction band at room temperature. This is because the valence electrons in insulators are involved in strong covalent bonds and require significant energy to break. A large energy gap separates the filled valence and empty conduction bands, and thermal energy at room temperature is insufficient to excite electrons across this gap. As a result, very few electrons are available for conduction, and the material cannot conduct electrical current effectively.