The rate‐limiting step for the plasma hydrogenation process is isolated and used to explain the effects of the hydrogenation process on the performance of thin‐film transistors (TFTs). In well‐hydrogenated TFTs device characteristics scale with device dimensions, but the performance of partially hydrogenated TFTs depends on device size and thickness, due to the mechanism of hydrogen diffusion through the quartz substrates into the channel polysilicon during hydrogenation. The mechanism was confirmed from results of experiments using quartz substrates coated with silicon nitride. The diffusion coefficient of hydrogen through the quartz and its activation energy are estimated.