Two kinds of insulators, PECVD silicon nitride (SiN) and DECR silicon dioxide (SiO2), are compared using top gate metal/insulator/semiconductor structures based on amorphous silicon (a-Si:H). The comparison is based on quasistatic capacitance measurements following several thermal bias annealing steps at different bias-anneal voltages V-ba. For positive bias annealing, both Al/SiN/a-Si:H and Al/SiO2/a-Si:H structures show similar behaviour with increasing V-ba, indicating changes in the a-Si:H bulk defect density. For negative bias annealing, the two kinds of structures behave differently owing to their different interfaces. Experimental results indicate the presence of a bump in the interface trap level density at the SiN/a-Si:H interface, which is not observed at the SiO2/a-Si:H interface.