The resistivity is given by:\\\\(\\rho =\\dfrac{m}{ne^{2}\ au }\\\\)where n = number density of electrons\\\\(\\rho\\\\) = relaxation timeWith the rise in temperature, number density of electrons and holes (in case of semiconductors) increases, where \\\\(\ au\\\\) remains constant. So, \\\\(\\rho\\\\) (resistivity) decreases.