semi-conductors are materials which conduct poorly, but still conduct. As their temperature rises, they conduct better as it causes more electrons to rise into the valence range typically. With normal conductors, i.e. copper, silver, etc, the heat does the opposite.
semi-conductors are typically made impure when used in components. The impurities give the semi-conductor the potential to generate a conductive channel and this is where the transistor fits in.