The density of Si dangling bond defects in plasma‐deposited Si‐richa‐Si1−xCx:H anda‐Si1−xNx:H is argued to depend primarily on deposition temperature via the hydrogen diffusion rate. The optimum temperature equals about 0.62 of the bulk‐bonded hydrogen evolution temperature, or about 500 °C fora‐Si3N4:H. A model of hydrogen diffusion in alloys is proposed.