The effects of positive and negative bias stress on hydrogenated amorphous silicon nitride/hydrogenated amorphous silicon (a-SiNx:H/a-Si:H) structures are investigated as a function of stress time, and stress temperature. It is shown that bias stress induces a parallel shift of the capacitance voltage (C-V) characteristics. The direction of the C-V shift depends on the sign of the applied stress voltage, while the magnitude of the C-V shift depends on stress time and temperature in a manner which is identical to that observed in a-Si:H thin-film transistors. In addition, it is shown that positive bias stress increases the number of localized states in the a-Si:H mobility gap, but negative bias stress does not. However, the observed increase cannot account for the corresponding C-V shift. These results lead us to conclude that the C-V shift is not induced by dangling bond defects in a-Si:H but rather by carrier trapping in the insulator.