We compared threshold voltage shifts in amorphous Si, microcrystalline Si and polycrystalline Si thin-film transistors (TFTs) in terms of a recently developed thermalization energy concept for a dangling-bond defect state creation in amorphous Si TFTs. The rate of the threshold voltage shift in microcrystalline Si TFTs was much lower than in amorphous Si TFTs, but the characteristic energy for the process, which we identified as the mean energy to break a Si-Si bond, was virtually the same. This suggests that the same basic Si-Si bond breaking process was responsible for the threshold voltage shift in both cases. The lower magnitude in microcrystalline Si TFTs was due to a much lower attempt frequency for the process. We interpreted the attempt frequency in amorphous and microcrystalline silicon in terms of the localization length of the electron wavefunction and the effect of stabilizing H atoms being located only at grain boundaries.