Doping of an Intrinsic Semiconductor: The mixing of a small amount of pentavalent (e.g. phosphorus) or trivalent (e.g., aluminium) substance as impurity in a pure semiconductor (e.g., $\mathrm{Ge}, \mathrm{Si}$ ) is called doping. Doping increases the conductivity of a semiconductor. For doping pure $\mathrm{Si}, \mathrm{Ge}$, pentavalent phosphorus is used to convert it to $n$-type semiconductor. Trivalent aluminium is used to convert pure $\mathrm{Si}, \mathrm{Ge}$ to $p$-type semiconductor.