Band Theory

The valence band refers to the energy level in an atom that contains the outermost electrons. It is the highest energy band filled with electrons at absolute zero temperature. Due to their proximity to the nucleus and other nearby atoms, electrons in the valence band are involved in the bonding interactions, such as covalent, ionic, or metallic bonds.

Electrons in the valence band have lower energy than the conduction band and typically do not contribute significantly to electrical conductivity. They are firmly bound to the atom, and no space is available for them to move freely. However, when external energy is applied to the material, such as heat or light, some electrons in the valence band can be excited to higher energy levels, contributing to the material’s conductivity or optical behavior.

On the other hand, the conduction band is situated above the valence band. It contains energy levels that are vacant under normal conditions, like room temperature. When an external energy source, such as heat or an electric field, is applied to the material, electrons can break free from their individual atoms and are excited from the valence band to the conduction band. Consequently, these free electrons can conduct electricity.

The mobility of electrons in the conduction band is essential for electrical conductivity in materials. When an electric field is applied, these electrons can move in response to the field, creating an electric current. The ability of a material to conduct electricity depends on factors such as the width of the energy gap between the conduction band and the valence band, as well as the number of available electrons in the conduction band.

The energy band gap is the energy difference between a material’s valence and conduction bands.

Conductors have overlapping valence and conduction bands, allowing electrons to move quickly through the material and conduct electricity. Examples of conductors include copper (Cu), aluminum (Al), and silver (Ag).

Insulators have a large band gap between their valence and conduction bands, making it difficult for electrons to move across them. Examples of insulating materials include glass, rubber, plastic, ceramics, and wood.

Semiconductors fall somewhere in between, with a small band gap that can be overcome by external factors like temperature or doping. Common examples of semiconductors include silicon (Si), germanium (Ge), gallium arsenide (GaAs), and indium phosphide (InP).

Understanding the energy band diagram is crucial in unraveling the electrical characteristics of materials. An energy band diagram illustrates the distribution of energy levels, especially the valence and the conduction bands, for electrons within a material. It is characterized by a vertical axis representing the allowed energy levels and a horizontal axis depicting the spatial arrangement of electronic states within the crystal lattice. The band gap is portrayed by the difference between the two energy levels.

The energy band diagrams for metals, insulators, and semiconductors provide a concise visual representation of their distinctive electrical properties. The valence and conduction bands overlap in a metal, allowing for a continuous distribution of energy states and high conductivity. Insulators showcase a clear band gap, with a significant energy difference between the valence and conduction bands, inhibiting electron movement.

Semiconductors, with their moderate band gap, exhibit a unique versatility. The band diagram for a semiconductor can be modified by introducing dopants, influencing its conductivity. N-type semiconductors, doped with elements like phosphorus, feature additional energy levels that facilitate electron mobility, while p-type semiconductors, doped with elements like boron, create holes in the valence band, promoting hole conduction.