In plasma‐deposited a‐Si:H films, the time dependence and saturation value of the metastable defect density produced by high‐intensity illumination are found to depend on the material properties. From a wide range of deposition techniques, we have observed a strong correlation between the steady‐state defect density Nss and the monohydride Si–H bond concentration observed in infrared transmission, with [SiH]2000 values between 2% and 16%. We propose a microscopic description of the light‐induced defect creation and light‐induced defect annealing phenomena in a‐Si:H, based on the trapping of mobile H at two specific sites: the Si–H bond and the Si dangling bond. This model predicts a steady‐state defect density proportional to the monohydride [SiH]. For the production of mobile H in three‐center (Si–H–Si) configurations, a low‐energy mechanism with no necessity for Si–H bond breaking is suggested from the observed changes in the infrared absorption of the various H‐bonding configurations during light soaking. With the assistance of an electron‐hole pair recombination, it consists of the excitation of two adjacent H atoms from a (Si–H H–Si) configuration along with a Si–Si bond reconstruction. © 1996 American Institute of Physics.