Resistivity() of a material is the resistance offered by the material to the the flow of charges. It is defined as the resistance(R) of the material of a unit area(A) per unit length(l).
The following diagram shows the resistivity versus temperature plots of the conductor and semiconductor.
In case of a conductor, the resistivity increases as the temperature increases. Here, the number of charge carriers is independent of the temperature. Increasing temperature decreases the relaxation time or the time between two collisions. Thus, more collisions occur which increases the resistance.
In a semiconductor, the density of charge carriers increases as temperature increases. This effect overcompensates for the increase in relaxation time. Hence, the resistivity decreases.
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