Charge Storage Effect in the Microwave Detected Photoconductive Decay Method*

Excess photocarriers excited with a photon beam in a silicon (Si) wafer are annihilated through recombination processes in the surface and bulk regions in the wafer. When a p-type Si wafer has a positive oxide charge on its surface, for example, the charge induces surface potential, forming a depletion layer at the wafer surface. Excess minority carriers are then pulled into the depletion layer and survive there for a relatively long time when the wafer surface is strongly inverted. Survival of the carriers lasts even after the annihilation of the excess carriers in the bulk region. This 'charge storage effect', however, disappears when the injection level of the excess carriers is sufficiently high to lower the surface potential, making the surface recombination velocity high as a result.