Al2O3 synthesized by atomic layer deposition (ALD) on H-terminated Si(100) exhibits a very thin (∼1 nm) interfacial SiOx layer. At this interface, a high fixed negative charge density, Qf, is present after annealing which contributes to ultralow surface recombination velocities <2 cm/s. Here, we identify the thickness of the interfacial SiO2 layer as a key parameter determining Qf. The SiO2 thickness was controlled by intentionally growing ultrathin SiO2 interlayers (0.7-30 nm) by ALD. Optical second-harmonic generation spectroscopy revealed a marked decrease in Qf for increasing SiO2 thickness between 0 and 5 nm. This phenomenon is consistent with charge injection across the interfacial layer during annealing. For thicker SiO2 interlayers (>∼5 nm), the polarity of the effective charge density changed from negative to positive. The observed changes in Qf and the associated field-effect passivation had a significant influence on the injection-level-dependent minority carrier lifetime of Si.