Why is it required to add impurity in pure semiconductor? Mention it's condition and explain what are impure semiconductors?

Explanation

The conductivity of an intrinsic semiconductor depends on its temperature , but at room temperature its conductivity is very low.
To increase their conductivity, it need to add impurity in 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 is increased manifold. Such materials are known as extrinsic semiconductor.
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 has to be such that it does nto distort the original pure semiconductor lattice. It occuupies 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 they belongs to the fourth grouup in the periodic table and therefore we choose the dopant element from 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 whose valency is 3 to 5.
(i) Pentavalence (valency - 5) : Such as arsenic (As), antimony (Sb) and phosphorous (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 provides two types of impure semiconductor.