Why does the resistance decreases with increase in temp for semi- conductors?

[Abhishakes]

Why does the resistance decreases with increase in temp for semi- conductors?

 

[vanesch]

Essentially because there are more electron-hole pairs (which contribute to conduction) when temperature rises. This effect wins from the standard effect in metals which increases the probability of scattering and hence the slowing down of the carriers (very crude picture) with increasing temperature. In metals, the first effect is absent, as all or most of the carriers are already available.