The dispersion of metal-oxide semiconductor capacitance, C(omega), or of the normalized conductance, G(omega)/omega, is measured at a fixed inversion bias. A theory has been developed which relates the frequency f* at which the step of the capacitance or the maximum of G(omega)/omega occurs to the minority carrier generation lifetime. The technique can be applied to trap-rich and nonhomogeneously doped semiconductor material. We have used it for the special case of trap-rich n-/n+ amorphous silicon (a-Si). The resulting generation lifetime turns out to be 3.5 . 10(-13) s.