Explain $n$-type and $p$-type semiconductors.

(N/A) $(i)$ $n$-type semiconductors: These semiconductors are obtained by adding electron-rich impurities to silicon or germanium. Silicon and germanium belong to group-$14$ and have four valence electrons each. In their crystals,each atom forms four covalent bonds with its neighbors. When doped with group-$15$ elements like $P$ or $As$,which contain five valence electrons,they occupy some lattice sites in the silicon or germanium crystal. Four out of five electrons are used to form four covalent bonds with four neighboring silicon atoms. The fifth electron is extra and is delocalized. These delocalized electrons increase the conductivity of the doped silicon or germanium,and this increase is due to the negatively charged electron. Hence,silicon doped with an electron-rich impurity is called an $n$-type semiconductor.
$(ii)$ $p$-type semiconductors: These semiconductors are obtained by adding electron-deficient impurities to silicon or germanium. When silicon or germanium is doped with a group-$13$ element such as $B$,$Al$,or $Ga$,which contains only three valence electrons,an electron hole is formed where the fourth electron is missing. This is also known as an electron vacancy or hole. Under an applied electric field,electrons from neighboring atoms can move to fill the hole,effectively moving the hole in the opposite direction. Since the hole behaves as a positive charge,this is called a $p$-type semiconductor.