The effective capture rates of electrons and holes by the defects of undopeda‐Si:H are measured using a technique based on the optical bias dependence of the modulated photocurrent. The evolution of the phase shift and modulated photocurrent spectra with the optical bias intensity is used to study the capture coefficient of the defects at different trap depths in the energy gap. The modulated photocurrent spectra are recorded using uniformly, or strongly absorbed light in order to obtain information about the trapping and recombination processes in the surface, the interface, and the bulk of the films. The calculated capture rates of carriers are studied in a series ofa‐Si:H films and information about the defect structure is obtained. It is found that the effective capture rates of the carriers by the defects are not constant in the studied films and may vary by up to two orders of magnitude from sample to sample. Finally, the importance of the atomic environment and the local strains of the defects ofa‐Si:H for the capture process of the carriers is discussed. ©1996 American Institute of Physics.