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Materials fall into three categories based on electrical conductivity. Metals have overlapping valence and conduction bands with no energy band gap, enabling excellent electron flow. Insulators have a large band gap (>3 eV), preventing electron promotion at room temperature. Semiconductors like silicon have a narrow band gap (~1.1 eV), allowing moderate conductivity through thermal excitation of electrons from the valence band to the conduction band.

Semiconductors have a smaller energy band gap than insulators, making it easier for thermal energy at room temperature to excite electrons from the valence band to the conduction band. This creates charge carriers—free electrons and holes—enabling moderate current flow. Insulators require much greater energy to promote electrons across their large band gap, resulting in very few available carriers and poor conductivity.

When thermal energy excites electrons from the valence band to the conduction band in semiconductors, two types of charge carriers form. Free electrons move through the conduction band and carry negative charge. Holes are voids left behind in the valence band where electrons were removed, acting as positive charge carriers. Both contribute to the moderate conductivity of semiconductors.

Semiconductor conductivity increases with temperature because higher thermal energy promotes more electrons from the valence band to the conduction band, generating additional charge carriers. This temperature dependence distinguishes semiconductors from metals, whose conductivity decreases with temperature. Light illumination and doping impurities also influence semiconductor conductivity by creating or providing additional carriers.

Band gap energy is the energy difference between the valence band and conduction band. It determines how easily electrons can transition to the conduction band and thus controls a material's electrical conductivity. Silicon has a band gap of 1.12 eV and gallium arsenide has 1.42 eV. Smaller band gaps enable easier carrier generation and recombination, making semiconductors useful for electronic devices.

Metals have no energy band gap because their valence and conduction bands overlap. This allows electrons in the highest energy levels of the valence band to easily transition to the conduction band with minimal kinetic energy from an applied electric field. The abundance of available electron states near filled ones enables free electron movement and excellent electrical conductivity.

Insulators have a large energy band gap (typically greater than 3 eV) with a filled valence band and empty conduction band. Valence electrons are locked in strong covalent bonds and require significant energy to break free. Thermal energy at room temperature is insufficient to excite electrons across this large gap, resulting in very few charge carriers and preventing effective electrical current flow.