In a semi-conductor such as silicon, carbon or germanium, as temperature rises, electrons leave the crystalline arrangement and are set free to move around. The location they move out from is left with what is called a hole. The conductance in semiconductors is due to the holes and the free electrons. As the temperature rises it creates more free electron and holes. This gives semiconductors a negative temperature coefficient of resistance. In a conductor like a metal the resistance rises with temperature as the increased vibrations in the atoms impedes the free movement of electrons.