Electrical Properties of Solids

The electrical properties of solids are essential for understanding how different materials behave when subjected to an electric field. These properties are the result of the nature of bonding, arrangement of atoms, and the availability of charge carriers within the solid. Mastery of this topic is crucial not only for JEE Chemistry but also for future studies in electronics and materials science.

1.0Classification of Solids Based on Electrical Conductivity

Solids are primarily classified into three categories based on their ability to conduct electricity:

• Conductors: These are substances that allow the easy flow of electric current. Metals such as copper, aluminum, and silver are excellent examples. Their high electrical conductivity is due to the presence of a large number of free electrons, which move freely through the lattice and carry electric charge.
• Insulators: Materials like glass, rubber, and plastic are considered insulators because they do not conduct electricity under ordinary conditions. In these substances, all electrons are tightly bound to atoms, and there are no free charge carriers available for conduction.
• Semiconductors: These are materials whose electrical conductivity lies between that of conductors and insulators. Silicon and germanium are classic examples. Their conductivity can be significantly increased by adding impurities, a process known as doping. Semiconductors are the foundation of modern electronic devices.

2.0Band Theory of Solids

The Band Theory of Solids explains how electrons behave within a solid using quantum principles. Proposed by Felix Bloch in 1928, it describes how closely packed atoms’ orbitals overlap to form continuous energy bands instead of discrete levels.

When a vast number of atoms combine, their energy levels merge to form bands that determine a material’s electrical properties.

The three key energy bands are:

• Valence Band: Contains electrons involved in bonding.
• Conduction Band: Where free electrons move to conduct electricity.
• Forbidden Band (Band Gap): The energy gap between valence and conduction bands that dictates whether a substance is a conductor, semiconductor, or insulator.

3.0Mechanism of Electrical Conduction

Electrical conduction in solids depends on the movement of electrons and, in the case of semiconductors, "holes" (the absence of electrons in the valence band).

In Metals

• Electrons are delocalized and can move throughout the entire structure.
• When an electric field is applied, these free electrons drift, producing an electric current.

In Semiconductors

• Intrinsic Semiconductors: Pure semiconductors where some electrons gain enough energy to cross the band gap and leave behind holes. Both electrons (in the conduction band) and holes (in the valence band) act as charge carriers.
• Extrinsic Semiconductors: Produced by adding impurities (doping).
• n-type: Doping with elements that provide extra electrons (e.g., phosphorus in silicon).
• p-type: Doping with elements that create holes (e.g., boron in silicon).

In Insulators

• All electrons are tightly bound, so no charge carriers are present, and electrical conductivity is extremely low.

4.0Effect of Temperature on Electrical Properties

The electrical behavior of solids is also influenced by temperature:

• Metals: Conductivity decreases with increasing temperature because atomic vibrations (phonons) scatter the free electrons.
• Semiconductors: Conductivity increases with temperature since more electrons get sufficient energy to jump the band gap, increasing the number of charge carriers.
• Insulators: Generally remain non-conductive, but at very high temperatures, a tiny increase in conductivity may occur.

5.0Applications Based on Electrical Properties

Understanding the electrical properties of solids allows for the selection of appropriate materials for various applications:

• Metals are used in electrical wiring due to their high conductivity.
• Insulators are crucial for preventing unwanted flow of electricity in circuits and appliances.
• Semiconductors are the backbone of electronic components like diodes, transistors, and integrated circuits.