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

When a small amount of Impurity atoms are added to a semiconductor, then generally its resistivity decreases.
Explanation:
The conductivity of a semiconductor is given by
\(\sigma=\mathrm{n}_{\mathrm{e}} \mathrm{e} \mu_{\mathrm{e}}+\mathrm{n}_{\mathrm{h}} \mathrm{e} \mu_{\mathrm{h}}\)
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.