45. For which of the following substances, the resistance decreases with increase in temperature?

(A) Pure Silicon
(B) Copper
(C) Nichrome
(D) Platinum

Ans. (A) Pure Silicon

Explanation 1: 

1) In semiconductors free electrons available for conduction are very less, hence when temp increases valence electrons jumps into upper energy layer & becomes free electrons, hence conductivity increases , hence resistance decreases.

2) In metals there are already many free electrons, which is called 'ocean' of electrons, but when temp increases atoms i.e. their crystal structures starts vibrating( this phenomenon is difficult to understand bcoz its quantum physics phenomena) which obstructs free electrons, hence resistance decreases. But still resistance increased in metals at any particular temp is far far less than that of decreased resistance in semiconductors. 

Explanation 2:

1) Conduction takes place through free electrons of high energy. Other electrons are in the valence band with lower energy. In a semiconductor this energy gap is low enough such that at higher temperatures more electrons leave the valence band and move into the conduction band and leave behind a hole in the valence band. Hence conductivity dramatically improves. (The hole in the valence band also conducts as a positive charge)

2) A metal has large number of free electrons which frequently collide with atoms which hinder its random motion. At high temperatures there are higher random velocities with larger number of collisions which hinder free movement. Hence its resistance increases 

 

Source: Yahoo! Answers - 1. mansudtein; 2. ramesh_1960