SOLUTION

At low temperature, the valence band of a semiconductor is completely filled and the conduction band is completely empty. Therefore, a semiconductor virtually behaves like an insulator at low temperatures. However, at room temperature, some electrons cross over to the conduction band giving little conductivity to the semiconductor.

As the temperature is increased more valence electrons cross over to the conduction band and conductivity increases. This shows that the electrical conductivity of a semiconductor increases with the increase of temperature, i.e., a semiconductor has a negative temperature coefficient of resistance.

Whereas in the case of conductors, when the temperature increases its resistivity increases as electrons collide more frequently with vibrating atoms. This reduces the drift speed of electrons (and thus current reduces). Thus conductors have a positive temperature coefficient of resistance.