The variation of specific resistance with temperature is given by
$\rho=\rho_{0}\left[1+\alpha\left(T-T_{0}\right)\right]$
where $\rho = $ specific resistance at temperature $T$
$\rho_{ref} = $ specific resistance of reference temperature
$\alpha=$ temperature coefficient of resistance which is negative for semiconductor.
$\rho=\rho_{0}-\rho_{0} \alpha\left(T-T_{0}\right)$
Comaring with straight line equation, $y=m x+c$
$\Rightarrow m$, slope is negative.
