# Band gap

The electrical behaviour of semiconductors, insulators and conductors can be explained on the basis of the band gap theory. In any material, the difference in energy between the top of the outermost ‘valence’ band and the bottom of the ‘conduction’ band is referred to as the band gap. In insulators and semiconductors, there is a distinct band gap while in conductors there may be a negligibly small band gap or the conduction and valence bands may overlap. The band gap is also referred to as the ‘forbidden energy gap’, since no electrons are allowed energy levels in that range.

## Why electrons jump the band gap

For conduction to take place, it is necessary for the valence band electrons to gain enough energy in order to jump to the conduction band.

An intrinsic or pure semiconductor requires charge carriers, namely electrons or holes, for conduction to take place. Since ‘holes’ are simply electron deficient areas in the semiconductor atoms, it is the electrons which are the actual carriers of electronic current. This makes it necessary for the electrons to gain sufficient energy to get excited and jump the band gap.

The Fermi-Dirac calculations are used to find out the probability that an electron will get excited. The probability is directly proportional to , where Eg is the energy of the band gap, k stands for Boltzmann’s constant and T is the temperature of the semiconductor material.

## Allotting band gaps

The amount of band gap energy needed, which is measured in electron-volts, depends upon the application of the semiconductor. Band gap in various materials is altered by changing the composition; this is possible in semiconductor alloys such as indium-gallium arsenide and indium aluminium arsenide. Band gap can be reduced by thermal expansion in a process that happens with increasing temperature. This process of manipulation of the band gap is termed as band gap engineering.

Semiconductor structures with different materials can be made using methods like molecular beam epitaxy (MBE) and metal-organic chemical vapour deposition (MOCVD). MBE involves the use of a stream of particles of the metal to be deposited being directed to the substrate. MOCVD uses a gas, usually the halide or hydride of the element to be deposited. The gas is reacted with the substrate to leave behind a coating.

The band gap energies of typical semiconductor materials are given below:

• Silicon: 1.1eV
• Germanium: 0.7eV
• Indium nitride: 0.7eV
• Indium phosphide: 1.34eV
• Aluminium arsenide: 2.2eV

As compared to semiconductors, insulators have band gaps which are greater than 3eV and so do not exhibit the same properties.