The threshold voltage stability of fully deuterated (a- Si : D ) and hydrogenated amorphous silicon (a- Si : H ) thin-film transistors (TFTs) is compared. The difference in the kinetic energy of D ⁺ and H ⁺ ions upon impact with the growing surface during radio-frequency plasma-enhanced chemical vapor deposition leads to material having different physical properties for the same nominal deposition conditions. However, a- Si : D and a- Si : H grown at the same growth rate by adjusting the gas pressure have almost identical properties. By using the growth rate as a normalizing parameter for comparing a- Si : H and a- Si : D TFTs, it is shown that there is no difference in the stability of a- Si : D compared with a- Si : H TFTs. This study rules out the possibility of a giant isotopic effect in amorphous silicon TFTs, and supports the model for Si dangling bond defect creation in a- Si : H where the breaking of weak Si–Si bonds is the rate-limiting step.