One valence electron of each Si atom will become free if a few tetrahedral Si atoms in a three-dimensional network structure of SiO2 are replaced with an equal number of trivalent atoms.

As a result, replacing each Si atom with a trivalent atom will result in one unit negative charge being introduced into the SiO2 three-dimensional network structure.

As a result, the SiO2 structure will take on a negative charge.