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B TECH
COMPUTER SCIENCE ENGINEERING
CHEMISTRY(BTCH101-23)
TOPIC- BAND GAP TO EXPLAIN
CONDUCTORS,SEMICONDUCTORS AND INSULATORS
IMPORTANT TERMS:-
● VALENCE BAND: The energy level occupied by the valence electrons is known as the valence band.
● CONDUCTION BAND: The energy level above the valence band is known as the conduction band. It is the band in which electrons go after excitation.
● FORBIDDEN GAP: The gap between the valence band and conduction band is known as the forbidden band gap.
Depending upon the band gap, solids are classified into three categories:
● Conductors
● Semiconductors
● Insulators
Conductors:
● The energy gap between the conduction band and valence band is almost negligible.
● With an increase in temperature, the conductivity of metal decreases.
● Examples: copper and silver.
Insulators:
● The band gap between the valence band and conduction band is almost 5 to 10 eV.
● Here, the conductivity increases with an increase in temperature.
● Examples: Diamond and glass.
Semiconductors:
● The energy gap is almost 1 eV, which is an intermediate gap between metals and insulators.
● Here, conductivity increases with an increase in temperature.
● Examples: Germanium and Silicon.
The following table illustrates the band structures:
● Conductors: CB (Conduction Band) and VB (Valence Band) are overlapping.
● Insulators: CB and VB are separated by a large band gap.
● Semi-conductors: CB and VB are separated by a small band gap.
Semiconductors are of two types:
● Intrinsic semiconductors
● In the Si lattice, the 5th electron of the pentavalent impurity is loosely bound and can jump from the valence band to the conduction band, increasing the conductivity of the semiconductor.
P-type Semiconductors:
● Trivalent impurities such as Boron (B) and Aluminium (Al) are added.
● The three valence electrons of the trivalent impurity form three bonds with silicon, and one bond is left empty, which results in the formation of a hole.
● This hole acquires an electron from an adjacent atom, resulting in the formation of a bond and a new hole.
● In the p-type, the conductivity is due to the movement of holes.
Organic Semiconductors:
● These are usually conjugated and have high conductivity.
