The temperature dependence of the saturated light‐induced defect density (Nsat) of a‐Si:H was measured while precisely controlling sample temperature during illumination. Nsatdecreases if the sample temperature is held above 75 °C to 100 °C, and increases below 50 °C. At intermediate temperature Nsatlevels off. The two thermally activated regions of Nsatsuggest that the activation energy of the annealing process has a distribution with two peaks. In experiments geared to raising the stability of a‐Si:H, we found that Nsatis reduced to the lowest value measured so far by thermal annealing at 190 °C for 3 to 19 hours prior to light‐soaking. This result suggests that the microscopic structure of a‐Si:H plays on important role in stability.