The stability of fully deuterated amorphous silicon a-Si: D thin-film transistors is compared with their hydrogenated equivalent a-Si: H in terms of gate bias stress. The amorphous silicon channel and silicon nitride gate insulator layers were deposited by radio-frequency plasma-enhanced chemical-vapor deposition. The use of SiD 4 rather than SiH 4 for the deposition of a-Si: D changes the physical properties of the plasma given the same conditions of rf power, pressure, and gas flow rates. Consequently, a higher gas pressure is required to produce a-Si: D at the same growth rate and with similar bulk properties as a-Si: H. It is shown that a-Si: H and a-Si: D deposited at the same growth rate have very similar structural properties. Therefore transistors deposited at the same growth rate may be more sensibly compared to determine the effect of replacing H with D in amorphous silicon without significantly changing the silicon continuous random network. Using this criterion for comparison, no detectable difference is observed between hydrogenated and deuterated transistors in terms of stability under the application of a gate bias. The experimental results rule out the possibility of a giant isotopic effect in amorphous silicon. Furthermore, this result supports the idea that the rate-limiting step for dangling-bond defect creation in amorphous silicon is the breaking of a weak Si–Si bond, rather than breaking of a Si–H bond. © 2005 American Institute of Physics.