Energy Bands in Solids

 Energy Bands in Solids

Energy bands in solid :

In a solid, the orbit of an electron is influenced not only by the charges of its own atom, but by nuclei and electrons of every atom in the solid.

 

In solid, each electron occupies a different position, no two electrons can see exactly the same pattern of surrounding charges. As a result, the orbit of the electrons are different.

 

The above figure shows the simple energy diagram, There are lakhs of electrons, in the first orbits of atoms in the solid and each of them has different energy, due to this the closely spaced energy levels differing very slightly in energy, form a cluster or band.

Semiconductor Silicon is commonly used for making transistors, IC’s etc. Since, its atomic number is 14 and each of its atom has only four electrons at the outermost orbit. Due to this, the third band of its atom is also known as valence band

The following figure shows the silicon  energy band diagram.

An addition band, called conduction band, is also shown above the valence band

 All the three lower band including valence band are shown completely filled. Although the third shell of an isolated atom of silicon is not completed filled (it has only four electrons whereas it could accommodate a maximum of eight electrons), the third energy band of solid silicon is completely filled.

 

It is so because in solid silicon each atom positions itself between four other silicon atoms and each of these neighbor share an electron with the central atom.

In this way, each atom has eight electrons, filling the valence band completely.

When we say the band is filled, it means that all its permissible energy levels in the band is occupied by electrons.

 

No electron in a filled band can move, because there is no place to move. Thus, an electron in a completely filled band cannot contribute to electric current.

 

The conduction band represents the next larger group of permissible energy levels. There is an energy gap E_G , between the valence band and conduction band.

An electron can be lifted from the valence band to the conduction band by adding some energy to the silicon. This energy must be more than the energy band gap E_G . The gap between the valence band and the silicon band is called the forbidden energy gap.

For silicon, it is 1.12eV and for Germanium it is 0.72eV.

 

The orbits in the conduction band are very large. An electron in the conduction band experiences almost negligible nuclear attraction. The electron in conduction band are move freely from one atom to another throughout the solid. This is why the electrons in the conduction band are called free electrons.