If a trivalent atom replaces a few silicon atoms in three dimensional network of silicon dioxide, what would be the type of charge on overall structure?
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.