Why do the semiconductors have negative temperature coefficient whereas the metals have positive values?

Semiconductors: When temperature increases, more electrons jump to conduction band from valance bond. Hence resistance decreases.

Metals: Already plenty of electrons are there in conduction band. When temperature increases, the electrons in conduction band of metal vibrate and collide each other during their journey. Hence the the resistance of metal increases with increase of temperature.

S.Lakshminarayana