Why is it required to add impurity to a pure semiconductor? Mention its condition and explain what impure semiconductors are.

(A) The conductivity of an intrinsic (pure) semiconductor depends on its temperature,but at room temperature,its conductivity is very low.
To increase its conductivity,it is necessary to add impurity to it.
When a small amount,say a few parts per million $(ppm)$,of a suitable impurity is added to the pure semiconductor,the conductivity of the semiconductor increases manifold. Such materials are known as extrinsic semiconductors.
The deliberate addition of a desirable impurity is called doping,and the impurity atoms are called dopants. Such a material is also called a doped semiconductor.
The dopant must be such that it does not distort the original pure semiconductor lattice.
It occupies only a very few of the original semiconductor atom sites in the crystal.
$A$ necessary condition to attain this is that the sizes of the dopant and the semiconductor atoms should be nearly the same.
$Si$ and $Ge$ are tetravalent and belong to the fourth group in the periodic table. Therefore,we choose the dopant element from the nearby fifth or third group so that the size of the dopant atom is nearly the same as that of $Si$ or $Ge$.
There are two types of dopants used for doping in $Si$ or $Ge$ with valency $3$ or $5$:
$(i)$ Pentavalent (valency $5$): Such as arsenic $(As)$,antimony $(Sb)$,and phosphorus $(P)$. This type of impurity is also called donor impurity.
$(ii)$ Trivalent (valency $3$): Such as indium $(In)$,aluminium $(Al)$,and boron $(B)$. This type of impurity is also called acceptor impurity.
The doping process results in two types of impure (extrinsic) semiconductors.