Question

5. a. Draw simple band pictures to distinguish a metallic conductor from a semi-conductor. b. Explain how and why the temperature dependence of the electrical conductivity can be used to distinguish a metallic conductor from a semiconductor. c. Pure niobium oxide (NiO) is an insulator. When it is heated in the presence of oxygen gas the resulting material becomes a semiconductor. Does the oxidation of Ni(II) to Ni(III) correspond to n-doping or p-doping? Explain using band pictures.

Answer 1

Firstly in order to conduct electricity or heat electron must available in conduction band.

a) In case of conductor there is no band gap between Valence band and Conduction band but in semi-conductors there is band gap ranging from 1eV to 1.7eV between Valence band and Conduction band.

Conductors Semi-conductors Conduction Band Conduction Band Band Gap Valence Band Valence Band Electrons Energy Energy

b) Electrical conductivity of conductors decreases with increase in temperature. When temperature of conductor rises the particles inside the conductor gains kinetic energy and starts vibrating along their mean positions so flow of electron will be hindered.

In Semi-conductors electrical conductivity increases with increase in temperature, becomes maximum and then decreases with further increase in temperature. This is because the heat we apply first will help electrons to jump from valence band to conduction band and the point comes maximum electron excites, after the point electrical conductivity starts to decrease for further increase in temperature, this is due to the increase in molecular kinetic energy flow of electron will be hindered.

c) When NiO is heated in presence of oxygen it oxidises from Ni(II) to Ni(III), this leads to increase in concentration of anion within the crystal. We can relate this as p-type semiconductor since we are increasing number of anion.

Conduction Band e holes Energy