Specific resistance is resistivity which is given by
ρ= where
n=no. of free electrons per unit volume and
τ= average relaxation time
For a conductor with rise in temperature
n increases and
τ decreases. But decrease in
τ is more dominant than increase in
n resulting an increase in the value of
ρ For a semiconductor with rise in temperature,
n increases and
τ decreases. But the increase in
n is more dominant than decrease in
τ resulting in decrease in the value of
ρ.