SOLUTION

When a pure semiconductor is heated, its resistance goes down.

Detailed Explanation

In semiconductors at 0 K, all valence electrons are strongly bound to their atoms and are actively participating in the covalent bond formation. As a result, no free electrons are available for conduction and it acts as an insulator.

At room temperature (T> 0 K), the valence electron acquires a sufficient amount of thermal energy. As a result, breakage of covalent bonds takes place releasing free electrons.

Because there are so few electrons that are mobile at room temperature, only a small amount of current will flow through the semiconductor at room temperature. If the material is heated up, however, many of the valence electrons obtain enough energy to escape from their places and, therefore, the resistance of the material decreases rapidly at high temperatures.