The difference between a conductor, a semiconductor, and an insulator

The ability of a material to conduct electrical current depends on its internal structure and the behavior of electrons. Based on this ability, we classify materials as conductors, semiconductors, and insulators. The main difference between them lies in the number of free charge carriers and the size of the so-called band gap.

Energy bands in materials

Electrons in a solid material can only exist in certain energy bands:

• Valence band – electrons are bound to atoms
• Conduction band – electrons can move freely and conduct current

In between them lies the band gap E_gEg​, which represents an energy barrier:

where:

• is the energy of the conduction band
• is the energy of the valence band

The size of ​ determines the electrical properties of the material.

Conductor

Conductors have a very large number of free electrons. The valence and conduction bands overlap or have a negligible gap between them:

This means that electrons can move freely without the need for additional energy.

The number of charge carriers is high:

Examples of conductors:

• copper (Cu)
• aluminum (Al)
• silver (Ag)

Conductors are used for transmitting electric energy, for example in electrical cables.

Insulator

Insulators have a large band gap:

Electrons do not have enough energy to move to the conduction band, so they cannot move freely.

The number of free charge carriers is very small:

Examples of insulators:

• plastic
• glass
• rubber
• ceramics

Insulators are used for protection against electric current.

Semiconductor

Semiconductors have a small band gap:

When energy is supplied (e.g. heat or light), an electron can move to the conduction band. This process creates:

• free electron
• hole (positive charge carrier)

The concentration of charge carriers depends on temperature:

where:

• is the Boltzmann constant
• is the temperature

Examples of semiconductors:

• silicon (Si)
• germanium (Ge)

Semiconductors are the foundation of modern electronics, such as transistors and integrated circuits.

Summary

The main difference between a conductor, semiconductor, and insulator lies in how easily electrons can move.

• Conductor: electrons move freely
• Semiconductor: electrons can move after energy supply
• Insulator: electrons practically do not move

The size of the band gap determines whether a material will be a good conductor, semiconductor, or insulator.