In gaseous substances, the arrangement of molecules is not closed. in liquid, the molecular arrangement is moderate but in solid the molecules are so closely arranged. that the electrons in the atoms of molecules tends to move into the orbitals of neighbouring atoms. Hance the electron orbital's overlap when the atom comes together.
Due to the intermixing of atoms in solids, instead of single energy levels, there will be bands of energy levels formed. these set of energy levels , which are closely packed are called as energy bands.
(1) valance band : valence band is the band of electron orbital electrons can jump out or moving into the conduction band when excited. The valence band is the band having the highest occupied energy.
(2) conduction band: conduction band is there range of energy cross ponding to the energy of free electrons. The conduction band is the band having the lowest occupied energy.
(3) forbidden energy gap : the energy gap between valence band and conduction band is called forbidden energy gap.
It is denoted by eg and its unit of electron volt (ev).
Note: forbidden energy gap is a region in which no electron can stay as, there is no allowable energy level or stay is present.
(a) forbidden energy gap (eg) of silicon(si) at room temperature (300k) = 1.12 ev.
(b) eg for germanium (Ge) at room temperature (300k) = 0.72 ev.
(c) eg for si at absolute zero temperature ( T=0k) = 1.21 ev
(d) eg for ge at absolute zero temperature ( T=0k) = 0.78
• silicon is more temperature sensitive than germanium.
While germanium is more conductivity then silicon.
•. Energy gap is inversely proportional to temperature.
i.e , Eg ∝ 1/ temperature.
• classification of material based on forbidden energy gap:-
Depending upon the size of the forbidden gap the insulator, the semiconductor and the conductor are formed.
(A) insulator : insulator are such material in which the conduction cannot take place, due to the large forbidden gap.
Ex : wood ,mica, glass ,plastic etc.
The structure of energy bands in insulator is as shown in the following figure.
Characteristics : the following are the characteristics of insulators.
• Atoms of insulators have more than 4 electrons in their valence shell.
• the forbidden energy gap is very large.
• valence band electrons are bonded tightly to atoms.
• the balance of forbidden energy gap of an insulator will be of 6 electron volt.
• for some insulators as the temperature increases the might show some conduction.
• no free electrons and holes are present.
• resistivity ( ρ ) ≈ 10^12 Ωcm.
(B). Semiconductor : semiconductors are such materials in which the forbidden energy gap is small and the conduction takes place its some external energy is applied.
Ex: silicon, germanium, selenium etc.
The following figure shows the structure of energy bands in semiconductors.
Characteristics : The following are the characteristics of semiconductor.
• The forbidden energy gap is very small.
• The forbidden gap for germanium is 0.7 electron volt whereas for silicon is 1.1 electron volt.
• a semiconductor actual is neither and insulator, Nor good conductor.
• as the temperature increases, the conductivity of a semiconductor increases.
•conductivity is due to free electrons and holes.
• resistivity ( ρ ) ≈ 10 ^-3 Ωm to 10^3 Ωm.
•some importance forbidden energy gap of semiconductor are -
Silicon ( si ) = 1.1 ev
Germanium ( Ge ) = 0.66 ev
Germanium arsenide (GaAs) = 1.41
Indium phosphide (in p) = 1.34
Zinc tellurite (Zn Te) = 2.26
Cadmium tellurite ( CdTe) = 1.43.
( c) conductors : conductors are such materials in which the forbidden energy gap disappears as the valence band and conduction bands become very close that they overlap.
Ex : copper , aluminium, silver , gold etc
The following figure shows the structure of energy bands in conductors.
Characteristics : the following are the characteristics of conductors.
•There exists no forbidden gap in a conductor. ( Eg = 0 ev )
• The valance band and the conduction band gets overlapped.
•no holes are present.
• conductivity is due to free electrons.
• A slight increase in voltage , increase the conduction.
• There is no concept of hole formation, as a continuous flow of electrons contribute the current.
• resistivity ( ρ ) ≈ 10 ^ -12 Ωm.
