Si 3N 4 films were simultaneously deposited on GaAs and In 0.53Ga 0.47As in a PECVD reactor to explore NH 3 plasma exposure of both substrates without any subsequent ambient exposure prior to dielectric formation. For both GaAs and In 0.53Ga 0.47As MOS devices the interface defect density near midgap was significantly reduced due to NH 3 plasma and post-metallisation forming gas annealing (FGA). A 10 second NH 3 plasma combined with FGA resulted in a 50% reduction in the peak D it for the near-midgap defect on GaAs, and a 66% reduction in D it for the midgap defect on In 0.53Ga 0.47As. This comparable defect-response behaviour suggests that the defect observed at midgap for In 0.53Ga 0.47As is very similar in nature to the near-midgap defect in GaAs, and is potentially the same defect. This would indicate that the influence of Ga or As-related defect states are predominant compared to In-related defect states for the midgap defect response typically observed for In 0.53Ga 0.47As devices.