Doping is the process of addition of impurities to the crystal lattice of the semi-conductor in order to increase its conductivity. Semi-conductors have the conductivity between insulators and conductors. Impurity added is called dopant.
Silicon contains 4 valence electrons in it's outermost shell. Adding Phosphorus to the crystal lattice of silicon which contains 5 valence electrons (electron rich impurity) causes bonding between 4 valence electrons of phosphorus and 4 valence electrons of silicon. 1 electron of phosphorus is free to move to the conduction band which is responsible for conduction of electricity. This type of conductor is called n-type conductor.
While Gallium contains 3 valence electrons (electron deficient impurity) causes bonding to 3 free electrons of silicon and 1 electron of silicon is free thereby causing a hole. This free electron is available for conduction. This type of conductor is called p-type conductor.
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