Explanation

(b) When an impurity atom with 3 valence electrons (as aluminium) is introduced in a pure silicon crystal, all the three of its valence electrons form covalent bonds with one each valence electrons of the nearest silicon atom while the valence electron of the fourth nearest silicon atom is not able to form the bond, leading to formation of hole or $p$-type semiconductor. While phosphorus being a pentavalent impurity leads to formation of $n$-type semiconductor.
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