Answer
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Why does the resitivity of metals increase with rising temperature? As the metal gets hotter, the ions in the lattice experience more thermal motion. The conduction electrons moving through the metal collide more often with the jiggly ions, which increases the metal’s resistivity.
Why does the resitivity of some semiconductors decrease with rising temperature? See Figure 29-25. As the semiconductor becomes hotter, more electrons move from the valence band into the conduction band. Thus, there are more charge carriers available to carry current, and this decreases the semiconductor’s resistivity.
Note that the increased thermal motion of atoms does tend to increase the overall resistivity in semiconductors, as it does in metals, but this effect is generally overwhelmed by the larger number of available charge carriers in the conduction band.
