What Do Valence and Conduction Bands Represent?

• Conduction band
• Valence band
• Band gap

What you will learn

• Explain why electron energies in a solid form bands.
• Tell the valence band from the conduction band and state what lifts an electron to the conduction band.
• Link conduction-band electrons and valence-band holes to electrical conduction.

Before you start

No prior knowledge is needed. The picture here is a simple one; the source material also says it captures only the main features.

Why do electron energies form bands?

Because when atoms bond, electron energies no longer sit at separate levels. According to the University of Cambridge lesson on energy bands, if N valence atomic orbitals of the same energy combine to form bonds, N energy levels result. The levels do not coincide; they spread out into energy bands.

Electrons cannot have energy values outside these bands. Within a band the levels lie very close together, so they are sometimes treated as continuous. In fact a band is made of a finite number of very closely spaced levels.

• Single levels
• Upper band
• Lower band
• Stretch with no levels

What are the valence band and the conduction band?

In the simple picture, the valence band is the bonding band and the conduction band is the anti-bonding band. When each atom contributes one electron to each orbital, the valence band is completely filled and the conduction band is empty.

An electron can move from the valence band to the conduction band only if it is given an energy at least as great as the band gap energy. That can happen, for example, when the electron absorbs a photon of sufficiently high energy. The band gap is the energy difference between the highest occupied state in the valence band and the lowest unoccupied state in the conduction band.

• Electron in the conduction band
• Hole in the valence band

How do the bands relate to conduction?

In silicon and germanium, an electron that gains enough thermal energy can break a covalent bond, becoming a free electron of negative charge and leaving a vacancy, or hole, of positive charge, according to the Harvey Mudd College lecture on semiconductor materials. The energy threshold is 1.1 eV for silicon and 0.7 eV for germanium.

In an electric field, both free electrons and holes contribute to electrical conduction. A free electron corresponds to an electron in the conduction band, and a hole is the vacancy left in the valence band.

• Crystal
• Free electron and hole

Real-world example

Think of a two-storey building with floors. Everyone stands on the lower floor and the upper floor is empty. To reach the upper floor each person must climb a set height, and nobody can hover between floors. The lower floor is the valence band, the upper floor the conduction band, and the height to climb is the band gap.

This is only a comparison to help you remember. An electron does not climb stairs; it gains energy.

Common misunderstandings

A band is not a place in space. It is a range of energy that electrons are allowed to have.

Not every band is full or empty as drawn. That is the simple picture when each atom contributes one electron to each orbital; in a simple metal the valence band is not full.

A hole is not a real particle flying through the crystal. It is the vacancy an electron leaves, and its motion can be described as that of a positively charged particle.

Frequently asked questions

What is the valence band?

The lower band of energy, matching bonding orbitals, filled with electrons in the simple picture.

What is the conduction band?

The higher band of energy, matching anti-bonding orbitals, empty in the simple picture; electrons that reach it contribute to conduction.

What is the band gap?

The energy difference between the highest occupied state of the valence band and the lowest unoccupied state of the conduction band.

Summary

• Electron energies in a solid spread into bands, and electrons cannot have energy outside them.
• In the simple picture the valence band is full and the conduction band empty; an electron reaches the conduction band when given at least the band gap energy.
• Conduction-band electrons and valence-band holes both contribute to conduction.