Answer
See explanation.
Work Step by Step
The conductivity of metals is due to the motion of free electrons. The electrons travel through a network of fixed atoms. The electrons are more inclined to bump into atoms and encounter resistance when the material heats up because the ions wiggle more, narrowing the channels between them and scattering electrons.
In a semiconducting material, the energy gap is of moderate size (see Figure 42.19), and the conduction band is mostly empty at low T. The material acts as an insulator.
With a rise in temperature, electrons can gain enough energy to hop into and partially fill the conduction band, which gives them a metallic band structure, making them better conductors. The number of mobile carriers increases with temperature.
