If a trivalent atom replaces a few silicon atoms

If a few tetrahedral Si atoms in a three-dimensional network structure of SiO2 are replaced by an equal number of trivalent atoms, then one valence electron of each Si

atom will become free.

As a result, the substitution of each Si atom by a trivalent atom will introduce one unit negative charge into the three-dimensional network structure of the SiO2.

Thus, the SiO2 structure will become negatively charged.