Apparatus for measuring low level oxide leakage in MOS capacitors is described. This measurement is useful in verifying generation lifetime depth profiles that are used to determine the depth of the defect-free zone (DFZ) in Czochralski-grown wafers. This method that is aimed at characterizing the silicon substrate underneath the oxide presupposes leakage-free oxide, since a leaky oxide may lower the lifetime and distort the lifetime profile in the substrate.