What is the Difference Between a Conductor and an Insulator, What Exercise Chapter 1 the Solid State Chemistry Class

Question 22: In terms of band theory, what is the difference
(i) Between a conductor and an insulator
(ii) Between a conductor and a semiconductor

Answer (i)
A conductor may conduct electricity due to  movement of electrons or ions. If valance band is partially filled or it overlaps with a higher energy unoccupied conduction band, then electrons can flow easily flow under an applied electric field and the metal shows conductivity.
If the gap between filled valence band and the next higher unoccupied band (conduction band) is large, electrons cannot jump to it and such a substance has very small conductivity and it behaves as an insulator .


(ii)
A conductor may conduct electricity due to  movement of electrons or ions. If valance band is partially filled or it overlaps with a higher energy unoccupied conduction band, then electrons can flow easily flow under an applied electric field and the metal shows conductivity.
In case of semiconductors, the gap between the valence band and conduction band is small. Therefore, some electrons may jump to conduction band and show some conductivity. Electrical conductivity of semi conductors increases with rise in temperature because with the rise of temperature more electrons can jump to the conduction band. Substances like silicon and germanium show this type of behaviour and are called semiconductors