The recombination lifetimes of silicon wafers were studied using microwave photo-conductivity decay(μ PCD) method with variable photo-injection function. For samples with surface inversion layers, the lifetimes increased as the injected carrier concentration decreased, and converged toward the value determined by the SiO²/Si interface trap density. It was found that the bulk recombination lifetimes could be measured in middle-high injection. Any clear dependence was not observed in the samples with surface accumulation layers. The recombination lifetimes of light-irradiated p-type silicon with iron contamination revealed strong dependence on injected carrier concentration due to breaking up iron-boron pairs. Molybdenum contamination of the epitaxial layers in p/p+ and p/p− wafers were also investigated using the differential μPCD technique.