When a small amount of impurity atoms are added to a semiconductor then generally its resistivity

The conductivity of a semiconductor is given by \(\sigma=n_{e} e \mu_{e}+n_{h} e \mu_{h}\) where \(n_{e}\) and \(n_{h}\) = number of electrons and holes per unit volume and \(\mu_{\mathrm{e}}\) and \(\mu_{\mathrm{h}}=\) mobility of electrons and holes. When impurity atoms are added in a semiconductor, then the concentration of holes and electrons increases. So, the conductivity Increases and hence the resistivity \(\left(\rho=\frac{1}{\sigma}\right)\) of semiconductors decreases.