In this paper, the results of studies on the interface properties in the hydrogenated amorphous silicon thin-film transistors with silicon oxide gate insulators are presented by means of the bias-annealing method. The transfer characteristics before and after bias-annealing are measured. Using a computer program, the field-effect density-of-states are calculated from the transfer characteristics. The results show that the density of states distribution is determined by the Fermi energy during thermal equilibration. These experimental results are explained by the defect-pool model.