By examining the purest material available to date, authors have shown that the Staebler-Wronski effect is an intrinsic property of a-Si:H. It appears that surface band bending can play an important role in the defect generation. Impurities can enhance the defect generation only when they exceed a crtical concentration of approximately 10**2**0 cm** minus **3 for oxygen and 10**1**9 cm** minus **3 for nitrogen, respectively. The observed enhancement probably originates from an 'alloying' effect, however, and a different mechanism related to impurities may be involved.