The passivation of InGaAs by thin AlN layers allows a significant reduction of the interface state density compared to that of the widely used Al2O3/InGaAs structure. The influence of the AlN layer thickness on the interface electrical properties, as well as the role of the post-deposition annealing, was carefully examined. Ultrathin AlN layers (∼1 nm) provide high quality interfacial electrical properties after a mild anneal (400 °C). Thick AlN passivation layers require annealing at higher temperature (500 °C) to achieve low interface states density. Possible explanations of the observed trend are suggested.