Electrical Properties of Solids | Energy Band Theory

The fundamental electrical property of solids is their ability to conduct electric current. The electrical behavior of different materials is different.

Electrical properties of solids

Classification of solids is based on the bonds that hold the atoms or molecules together. The electrical properties of solids are their ability to conduct electricity.

Solids are classified into three types on the basis of their electrical conductivity.

• Conductor
• Insulator
• Semiconductor

Conductors

Those materials which conduct electricity easily are called conductors e.g. copper, silver, etc.

Their conductivities are of the order of 10⁷ (Ωm)^-1.

Insulators

 Those materials which do not conduct electricity are called insulators.

For example wood, diamond, etc with conductivities ranging between 10^-10 to 10^-2 (Ωm)^-1.

Semiconductors

These materials are not good as conductors and not bad as insulators are called semiconductors. For example silicon etc. Their conductivities range from 10^-6 to 10^-4 (Ωm)^-1.

The conventional free electrons theory based on the Bohr model of electron distribution failed to explain completely the electrical behavior of these three solids.

Band theory of solids is used for this purpose which is based on the wave mechanical model.

Energy band theory

Electrons in a single isolated atom have discrete energies and are bound to their nuclei. In solids, when a large number of atoms, say N, comes close enough to each other to form a solid, each energy level of an atom splits up into N sub-levels, called states.

This action is due to the action force exerted due to other atoms in the solid. These energy states are discrete but so close to each other that they look like continuous energy bands.

Forbidden Energy Band

There is a range of energy states which cannot be occupied by electrons. These are called forbidden energy states and their range is called the forbidden energy gap.

Valence Band

The electrons in the outermost shell are called valence electrons and the energy band formed by these electrons is called the valence band. This band may be either completely filled or partially filled but cannot be empty.

Conduction Band

The band above the valence band is called the conduction band. In this band, the electrons move freely and conduct electric current. The electrons in this band are called conduction electrons or free electrons. This band may be partially filled or empty.

Other Band

The bands below the conduction and valence bands are normally completely filled and they do not have any role in the electrical conduction process.

On the basis of the band theory of solids the conductors, insulators, and semiconductors are classified as under.

Insulators:

In insulators, the electrons are tightly bound in their valence shells and they are not free to move.

In these materials, the valence band is completely filled and the conduction band is empty. There is a large energy gap between the conduction and the valence band.

And due to this difference, they cannot move from the valence to the conduction band.

Conductors:

Conductors are those materials that have a large number of free electrons for electrical conduction.

In terms of energy band, the valence and the conduction bands are overlapped i.e. there is no distinction between these two bands.

Semiconductors:

These are those materials that have a partially filled valence band and a partially filled conduction band. A very narrow energy gap between the two bands exists and is normally of the order of 1eV.

At OK, there are no electrons in the conduction band and the valence band is completely filled. This means that at 0K, the Ge and Si behave like insulators.

With the increase in temperature, few of the electrons gain sufficient energy to move from the valence band to the conduction band.

When electrons move from the valence band to the conduction band, they leave behind a vacancy in the valence band. This vacancy is called a hole. The hole has a positive charge.

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