Electrical Properties of Conductors, Semiconductors and Insulators

Energy Bands in Solid

 

In an atom, electrons of different orbit exhibits different energy level. When another atom comes close to this atom, the first orbit electrons experienced a force which will depend upon this neighboring atom due to which energy level of first orbit electrons are slightly changed. Similarly in solids there are millions of atoms and corresponding to each atom there are millions of I_st orbit electron. In solids atoms are closely packed due which force experienced by an electron will depend upon the neighboring atom also. Therefore force experienced by each first orbit electron is slightly different. Therefore each first orbit electron in solid will have slightly different energy than other. So corresponding to each electron there are millions of energy levels. So the energy of adjacent level is closed to each other and from a band of energy. Similarly, the energy level of higher orbit electron will leads to formation of energy band.

However inner band electrons do not contribute in materials properties. The formation of band mainly depends upon the outermost electrons of atoms therefore these electrons describe the materials properties like electrical conductivity.

Classification of Energy Band

• Valance Band

• Conduction Band

• Forbidden Energy Gap

 

 

Valance band

It is the energy band formed by the series of energy levels containing valance electrons i.e. outer most orbit electron.

It is the highest band in which electrons are still associated with their atoms.

Valance band can fully or partially filled by electrons.

Valance band Electrons are not capable of taking energy from applied field and hence they do not contribute in current flow.

Valance band electron need thermal energy to free from the grip of their parent atom.

 

Conduction Band

It is the first band above valance band and it is also known as empty band of minimum energy. Whenever external energy is applied electrons from valance band jumps and comes in the conduction band these electron are free to move anywhere in the material. That’s why they are also known as free electrons.

It can be partially filled by electrons. Its electrons are capable of taking energy from applied electric fields movement of these electrons results into current flow.

Forbidden Energy Gap

 

It is the energy band gap between Valance band and Conduction band. In this band no electron is allowed to stay. The width of forbidden energy gap depends upon atomic bonding and nature of the material. For example width of this gap is very high in case of insulators but it is negligible or zero in case of metals. The width of forbidden energy gap decreases with increases in temperature due to which this gap is the major factor for determining the electrical conductivity of the materials.