Insulators: With increase in temperature, the conducting property increases. So we call the semi-conductor material have negative temperature coefficient i.e. with increase in temperature, resistance decreases.
Semiconductors: With increase in temperature, the conductivity of the semi-conductor material increases. As with increase in temperature, outermost electrons acquire energy and hence by acquiring energy, the outermost electrons leave the shell of the atom.
Hence with increase in temperature, number of carriers in the semiconductor material increases and which leads to increase in conductivity of the material. So we call the semi-conductor material have negative temperature coefficient i.e. with increase in temperature, resistance decreases.
Conductors: The outermost shell of conductors is mostly free at room temperature and hence due to the fact that conducting materials leave the outermost electrons, the nucleus of the atom of conducting material is more positive as it is a positive ion.
Cu Cu+ + e
Hence taking out more electrons from the penultimate shell of the atom is very difficult and when the temperature is increased, the energy supplied is not enough to take out more electrons but due to the energy because of increase in temperature, the nucleus of the atoms start vibrating and hence obstruct the flow of electrons already in the free space. So with increase in temperature, conductivity of the conductors decreases and resistance increases. Hence we say conductors have positive temperature coefficient.
Sensistor: When there is access doping present in the semiconductor, it is called sensistor. The sensistor also have positive temperature coefficient.