Dangling bond defects are created during positive bias stress of amorphous silicon thin film transistors and there is an energy barrier between 0.9 and 1 eV for this process. We have studied how this energy barrier depends on the material parameters of the amorphous silicon, namely hydrogen content, hydrogen bonding, Urbach energy and intrinsic, deposition induced stress. We observe no dependence on the hydrogen content or hydrogen bonding type, but we do observe a clear dependence on the Urbach energy and the intrinsic stress. These measurements support a localized model for defect creation involving Si-Si bond breaking and the switching of a neighboring H atom to stabilize the broken bond. These results suggest that stable amorphous silicon TFTs can be obtained at low deposition temperatures by control of the deposition-induced, intrinsic stress.