The localized electronic density of states gL(E) at the hydrogenated amorphous silicon/silicon nitride (a-Si:H/Si3N4) interfaces is evaluated by transient voltage spectroscopy (TVS). The gL(E) at the a-Si:H/Si3N4 interface is found to have a broad peak and a deep minimum, respectively, around 0.9 and 0.47 eV below the conduction-band edge. The positive fixed charge at the interface makes the flatband voltage of the a-Si:H/Si3N4 system negative and, hence, there is a finite band bending at the interface even at the gate bias of 0 V. The gL(E) of different samples fabricated under various preparation conditions varies from sample to sample, whereas threshold voltages of a-Si:H thin-film transistors are almost independent of their preparation, suggesting the presence of an equilibrium mechanism between the positive fixed charge and the defect density. A modification of gL(E) at a-Si:H/Si3N4 interfaces by bias annealing is definitely observed using TVS. The overall results obtained by TVS seem to support the defect pool model.