By accurately tuning the SiO2 interlayer thickness the density and polarity of charges in Si/SiO2/Al2O3 stacks can be controlled. We report on the number density, polarity, and physical location of charges present in the stacks as studied by optical second-harmonic generation (SHG). Depending on the SiO2 interlayer thickness (1–150¿nm) the effective charge density in the Si/SiO2/Al2O3 stacks ranges from 1013 to 1011¿cm-2 for both n- and p-type silicon. The polarity of the charges switches from negative to positive around a SiO2 interlayer thickness of 5–10¿nm at which point the effective charge density in the stacks is negligible. This switch in polarity is apparent from spectroscopic, time-dependent, and azimuthal SHG measurements. The observed trends in charge density and polarity can be explained by tunneling of electrons into defect states at the SiO2/Al2O3 interface as well as the presence of fixed and bulk charges at the Si/SiO2 interface and in the SiO2, respectively. This charge mechanism appears to hold generally for Si/SiO2/Al2O3 stacks as similar results were observed for SiO2 films prepared by various techniques.