A characterization method is presented, which determines fixed charge and interface defect densities in passivation layers. This method bases on a bias voltage applied to an electrode on top of the passivation layer. During a voltage sweep, the effective carrier lifetime is measured by means of microwave detected photoconductivity. When the external voltage compensates the electric field of the fixed charges, the lifetime drops to a minimum value. This minimum value correlates to the flat band voltage determined in reference impedance measurements. This correlation is measured on p-type silicon passivated by Al₂O₃ and Al₂O₃/HfO₂ stacks with different fixed charge densities and layer thicknesses. Negative fixed charges with densities of 3.8 × 10¹²cm^-2 and 0.7 × 10¹²cm^-2 are determined for Al₂O₃ layers without and with an ultra-thin HfO₂ interface, respectively. The voltage and illumination dependencies of the effective carrier lifetime are simulated with Shockley Read Hall surface recombination at continuous defects with parabolic capture cross section distributions for electrons and holes. The best match with the measured data is achieved with a very low interface defect density of 1 × 10¹⁰eV^-1 cm^-2 for the Al₂O₃ sample with HfO₂ interface.