How Does the Band Gap Affect Conduction?

• Small band gap
• Medium band gap
• Large band gap

What you will learn

• State what the band gap decides about conduction.
• Explain why a semiconductor conducts at room temperature while an insulator hardly does.
• See why a metal conducts well without a band gap to cross.

Before you start

It helps to read the article on valence and conduction bands first. This article repeats the main idea where needed.

How does the band gap affect conduction?

An electron can reach the conduction band only if it gains energy at least as great as the band gap, according to the University of Cambridge lesson on energy bands. The larger the gap, the more energy is needed, so fewer electrons make it.

That energy can come from a photon of sufficient energy. In a material with a small band gap it can also come from thermal vibration.

• Conduction band
• Valence band
• Thermal vibration

Why does a semiconductor conduct moderately?

Because in a material with a small band gap, a small but appreciable number of electrons are excited from the filled valence band into the empty conduction band at room temperature simply by thermal vibration. That puts the conductivity of semiconductors between that of metals and insulators.

A material with a band gap of less than about 3 eV is generally regarded as a semiconductor, and one above about 3 eV is commonly regarded as an insulator. This is a rough distinction in the Cambridge lesson, rather than an absolute classification threshold. The same lesson also describes wide-band-gap semiconductors.

In silicon, an electron that gains enough thermal energy, 1.1 eV, can break a bond and become a free electron, according to the Harvey Mudd College lecture. That figure is the thermal energy threshold for silicon in that lecture.

• Metal
• Semiconductor

What about metals?

Metals conduct well because the valence band is not full. In a simple metal such as sodium, empty states lie just above the highest occupied states, so electrons can easily gain energy from an applied electric field and jump into them.

A metal therefore does not need electrons to cross a band gap to conduct.

• Conducts well
• Semiconductor
• Insulator

Real-world example

Think of a stream you need to cross. If it is one step wide, many people get across. If it is as wide as a river, almost nobody does. The width of the stream is the band gap, and the people crossing are electrons reaching the conduction band.

This is only a comparison to help you remember. Electrons gain energy rather than step across.

Common misunderstandings

The band gap is not a barrier object. It is a stretch of energy with no states for electrons.

The 3 eV figure is not an absolute boundary. The source says the distinction between a semiconductor and an insulator is less precise.

A semiconductor is not always an insulator. At room temperature a small number of electrons already reach the conduction band.

Frequently asked questions

Does a large or a small band gap conduct better?

Within this model, a small gap makes thermal excitation into the conduction band easier. Read the comparison within the conditions of that model.

How does a semiconductor differ from an insulator?

Mainly in the band gap: below about 3 eV is usually called a semiconductor, above it usually an insulator.

Does a metal have a band gap?

In this article's picture, a simple metal has a valence band that is not full, so electrons gain energy easily.

Summary

• An electron must gain at least the band gap energy to reach the conduction band.
• A small gap lets thermal vibration lift some electrons, so a semiconductor conducts moderately.
• A metal has a valence band that is not full, so it conducts well.