The resistivity of a material is given by $\rho = \frac{m}{ne^2\tau}$. For insulators and semiconductors, the number density of free charge carriers ($n$) increases exponentially with an increase in temperature. This increase in $n$ more than compensates for any decrease in the relaxation time ($\tau$). As a result, their resistivity ($\rho$) and resistance decrease with an increase in temperature, which means they possess a negative temperature coefficient of resistance. In contrast, metals have a positive temperature coefficient of resistance because their charge carrier density is nearly independent of temperature, while the relaxation time decreases due to more frequent collisions at higher temperatures.