How are conductors, insulators, and semiconductors different in their ability to conduct electricity?

Materials are classified as conductors, insulators, or semiconductors based on their ability to conduct electricity, a property determined by their band structure and the presence of free charge carriers.123

Conductors

Metals are excellent conductors because they lack a band gap between their valence and conduction bands. This allows electrons to move freely through the material, forming an electron "cloud" that facilitates high electrical conductivity.1245

Insulators

Insulators are poor conductors because they possess wide forbidden energy gaps. These gaps are too large for thermal energy to excite electrons into the conduction band, resulting in very few or no free electrons available to carry current.2143

Semiconductors

Semiconductors have intermediate conductivity because they feature relatively narrow band gaps. While they act as insulators at absolute zero, thermal excitation or the introduction of impurities—a process known as doping—can provide enough energy to move electrons into the conduction band, creating free electrons and holes that allow for electrical conduction.12647