CBSE NOTES CLASS 12 PHYSICS
CHAPTER 14 SEMICONDUCTORS
Energy bands in solids
The collection of closely spaced energy levels is called an energy band. In a crystal due to inter-atomic interaction valence electrons of one atom are shared by more than one atom in the crystal and splitting of energy levels takes place.
- Valence band: This energy band contains valence electrons. This band may be partially or completely filled with electrons but never be empty. The electrons in this band are not capable of gaining energy from external electric field to take part in conduction of current.
- Conduction band: This band contains conduction electrons. This band is either empty or partially filled with electrons. Electrons present in this band take part in the conduction of current.
- Forbidden band: This band is completely empty. The minimum energy required to shift an electron from valence band to conduction band is called band gap (Eg).
TYPES OF SUBSTANCES ON THE BASIS OF ENERGY BANDS
Metals on the basis of energy bands
A substance is a metal either when the conduction band is partially filled and the balanced band is partially empty or when the conduction and valance bands overlap. When there is overlap electrons from valence band can easily move into the conduction band.
This situation makes a large number of electrons available for electrical conduction. When the valence band is partially empty, electrons from its lower level can move to higher level making conduction possible. Therefore, the resistance of such materials is low or the conductivity is high.
Semiconductors on the basis of energy bands
In this case a finite but small band gap (Eg < 3 eV) exists. Because of the small band gap, at room temperature some electrons from valence band can acquire enough energy to cross the energy gap and enter the conduction band. These electrons (though small in numbers) can move in the conduction band. Hence, the resistance of semiconductors is not as high as that of the insulators.
Insulators on the basis of energy bands
In this case, a large band gap Eg exists (Eg > 3 eV). There are no electrons in the conduction band, and therefore no electrical conduction is possible. Note that the energy gap is so large that electrons cannot be excited from the valence band to the conduction band by thermal excitation. This is the case of insulators.