How would you account for the following:
Impurity doped silicon is a semiconductor.

Silicon forms four covalent bonds thus two types of impurities can be added to silicon.
for example : boron can be added or phosphorus can be added.
In case of boron : boron form three bonds with silicon, it will result in an electron deficient bond andwill create a hole. These holes can move through crystal like positive charge giving rise electrical conductivity.
In case of phosphorus: when silicon forms four bonds with phosphorus one electron of phosphorous atom will remain unbonded due to which it become delocalizedn and contributes to electrical conductivity.