Metals, Insulators and Semiconductors

A material is able to conduct electricity, if it contains free electrons in the conduction band when external electric field if applied to it. The free electrons thus work as charge carriers.

Metals

Metals have a large number of free electrons at room temperature. There is no gap between the valance and conduction band. The conduction band and valence band overlap eachother so valence band energies and conduction band energies are same. It is very easy for valence electron to transit to conduction band and become free electron even without supply of external energy. Metals have very low resistivity and high conductivity.

ρ (Resistivity)     ≈ 10^-2 – 10^-8 Ω m

σ (Conductivity) ≈ 10² – 10⁹ Sm^-1

Insulators

Insulators have completely filled valence band and conduction band is empty. There is a large energy gap (Eg > 5 eV) between valence and conduction bands due to which it is practically impossible for an electron in the valence band to jump to conduction band. Only at very high temperature or very high voltage, an electron can jump the gap which is known as the breakdown of insulator. At room temperature, no electron is present in conduction band. Insulators have very high resistivity and low conductivity.

ρ (Resistivity)     ≈ 10¹¹ – 10¹⁹ Ω m

σ (Conductivity) ≈ 10^-11 – 10^-19 Sm^-1

Semiconductors

Semiconductors have properties intermediate to metals and insulators. There is a finite small energy gap (Eg< 3 eV) between valence and conduction band. Because of the small gap, at room temperature some electrons from valence band can acquire enough energy to cross the energy gap and enter the conduction band. They have resistivity and conductivity intermediate to metals and insulators.

ρ (Resistivity)     ≈ 10^-5 – 10⁶ Ω m

σ (Conductivity) ≈ 10⁵ – 10^-6 Sm^-1