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Phosphorus has one excess valence electron (compared with Si) after forming the four covalent bonds normally with silicon. This excess electron gives rise to electronic conduction. That is why silicon becomes semi-conductor on doping with phosphorus. It is called n-type semi-conductor.

Gallium has only three valence electrons. It creates an electron deficient bond or a hole when it is doped with silicon. Such holes can move in the crystal giving rise to electrical conductivity. Thus silicon doped with gallium is also semi-conductor due to movement of holes. It is called-p-type semiconductor.

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(i) These type of vacancy defects are called Schottky defects.
(ii) This defect decreases the density of the crystal.
(iii) NaCl shows this type of defect in the crystalline state.
(iv) This is the point defect which does not disturb stoichiometry of the solid.

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The frenkel defect is favoured by a large diffrance in size between the positive and negtive ion the metal cation are generally smaller than the anion which occupy the vacant lattice site or hole.

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Schottky defect also exists in pair to maintain electroneutrality. Therefore, it generates equal number of cation and anion vacancies. In case of doping of NaCl by CdCl₂, CI^– ions enter Cl^– ions site in NaCl while Cd²⁺ displace Na⁺. In doping so, one positive charge becomes extra. Therefore one Na⁺ is displaced so as to maintain electro-neutrality. That is doing only cation vacancies exist.

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 It is basically a vacancy defect in ionic solids. In order to maintain electrical neutrality, the number of missing cations and anions are equal.

Thus there is equal number of positive and negtive ion leave correct lattice point and go outside the lattice creating  a pair of vacancy. So there are less number of ions thannbefore whichresult in the decrease in its density.