- Energy Bands: In an isolated single atom,
there are single energy levels. When two atoms are brought near each
other, each single energy level splits. In a solid, the closeness of
atoms results in multiple splitting of energy levels. Now each energy
level splits into a very large number of closely spaced energy levels.
These energy levels form a band. Thus bands of energy levels, instead of
single energy levels are formed in solids.
- Valence Band: The band which is occupied
by the valence electrons or a band having highest occupied band energy
is known as valence band.
- Conduction Band: The next higher permitted
band, after the valence band, is known as conduction band. It is the
lowest unfilled energy band.
- Forbidden Band Gap: The region between the
conduction band and the valence band is known as forbidden band gap.
- In Insulators: In insulators the forbidden
band gap is wide and electrons from valence band can not easily jump
across the gap and go into conduction band.
- In Conductors: In conductors the conduction
band and the valence band overlap. Hence plenty of free electrons are
available for conduction.
- In Semiconductors: In semiconductors the
forbidden band gap is small. At 0°K the conduction band is empty and
valence band is completely filled. But at higher temperatures, electrons
acquire sufficient energy to jump from valence band to conduction band
and leave a hole behind in the valence band. The electrons move in the
conduction band and the holes move in the valence band when an electric
field is applied across a semiconductor.