a-SiNx:H thin films, prepared by the PECVD technique, were optically and electrically characterized. The frequency dependent trap admittance derived for the MIS structure in the accumulation gate voltage regime was revised. Based on these results an omitted multiplying factor was included into the derivation. An extension of the tunnelling model in the depletion bias regime was used in comparison with the widely accepted statistical model and finally, the more general feature of the tunnelling-based approaches was discussed. (C) 1999 Elsevier Science B.V. All rights reserved.