hat are extrinsic semi-conductors ? Explain donor type semi-conductor.

Explanation

Extrinsic semiconductors : A semiconductor obtained after adding the impurities in an intrinsic semiconductor is known as an extrinsic semiconductor.
Donor type semiconductor or n-type semi-conductor: When pentavalent impurity atoms are added to pure germanium or silicon crystal, we get an extrinsic semiconductor known as n-type semiconductor. When pentavalent impurity atom, say, arsenic is added to the silicon crystal, it replaces one of the silicon atoms. This arsenic atom forms four covalent bonds by sharing its four electrons with the neighbouring silicon atoms. The fifth valence electron of arsenic remains free. This electron is loosely bounded to its parent nucleus and is detached even at room temperature. Now it becomes a free electron. In this way, a large number of free electrons are available in the crystal when small amount of arsenic is added to silicon crystal. This type of crystal has large number of electrons and hence it is called n -type semiconductor. The majority charge carriers are electrons and minority charge carriers are holes in n-type semiconductor.