What Is the Conduction Band in Physics?

The conduction band is a fundamental concept in solid-state physics and is essential for understanding the electrical properties of materials such as metals, semiconductors, and insulators. It is directly related to electronic structure, band theory, and the distinction between different classes of solids.

Formation of Energy Bands in Solids

In isolated atoms, electrons occupy discrete energy levels. When a large number of atoms come together to form a solid, these energy levels split due to atomic interactions, resulting in a continuous range of allowed energies called energy bands. The most relevant bands are the valence band and the conduction band. Detailed concepts about atom structure can be found in the Structure of Atom resource.

Definition of the Conduction Band

The conduction band is the range of energy levels in a solid where electrons are free to move within the material. Electrons in this band are not bound to individual atoms and are capable of contributing to electrical conduction. This characteristic distinguishes materials based on their ability to conduct electricity.

Distinction Between Conduction Band and Valence Band

The valence band is the highest range of electron energies in which electrons are normally present at zero temperature. The conduction band lies above the valence band and is typically empty at absolute zero. The gap between these two bands is called the forbidden energy gap or band gap. For an overview on bands, refer to the Conduction Band Overview.

Conduction Band Energy and Electron Excitation

Electrons must gain sufficient energy to move from the valence band to the conduction band. This energy is provided by thermal excitation or external means. The minimum energy required is called the band gap energy $(E_g)$. In semiconductors, $E_g$ is small, enabling easier promotion of electrons.

Band Structure in Metals, Semiconductors, and Insulators

In metals, the conduction band and valence band overlap, leading to a significant density of free electrons and high electrical conductivity. In semiconductors, the conduction and valence bands are separated by a small band gap, while in insulators, this gap is large, preventing electron flow.

Conduction Band in Semiconductors

In semiconductors such as silicon, the conduction band is usually empty at absolute zero. At higher temperatures, some electrons gain enough energy to move from the valence band to the conduction band, allowing limited conduction. The electrical conductivity increases with temperature. More details can be obtained from the Classification of Solids article.

Conduction Band Minimum and Energy Calculations

The lowest energy point in the conduction band is called the conduction band minimum (CBM). The excitation of electrons from the valence band maximum (VBM) to the CBM can be quantified by the band gap:
$E_g = E_{CBM} - E_{VBM}$
where $E_{CBM}$ is the conduction band minimum energy and $E_{VBM}$ is the valence band maximum energy.

Role of the Conduction Band in Electrical Conductivity

The presence of electrons in the conduction band creates mobile charge carriers, enabling the flow of electric current. Conductivity $(\sigma)$ depends on the number of electrons in the conduction band ($n$), their charge ($e$), and mobility ($\mu$):

$\sigma = n e \mu$

Band Theory and Fermi Level

Band theory explains physical properties of solids based on band structure. The Fermi level denotes the highest occupied energy level at absolute zero. The position of the Fermi level relative to the conduction band indicates electrical behavior. Semiconductors have the Fermi level close to the midpoint of the band gap. For additional context, see Electronic Devices.

Conduction Band and Electrical Conduction Mechanism

When electrons enter the conduction band, they can move freely within the crystal lattice. In an electric field, these free electrons carry current across the material. The number of electrons in the conduction band determines the magnitude of current flow. Insights into conduction processes are elaborated in Conduction Explained.

Effect of Doping on the Conduction Band in Semiconductors

Doping introduces impurity atoms to a pure semiconductor, altering its electrical properties. Donor impurities create extra energy levels near the conduction band, increasing the probability of electrons entering the conduction band, thus enhancing electrical conductivity.

Comparison of Conduction and Valence Bands

Key Points on Conduction Band

• Conduction band enables free movement of electrons
• Electrical properties depend on band occupancy
• Band gap influences conductivity
• Doping increases conduction band electron population

Related Physics Concepts

The study of conduction bands is related to electronic structure, band theory, binding energy, and solid-state physics. For further reading, the concept of Binding Energy is also fundamental in understanding atomic and solid-state phenomena.