Despite a high lateral breakdown voltage above 10 kV for large contact distances and a breakdown field of 5 MV cm −1 for short contact distances, an Al 0.9 Ga 0.1 N/GaN heterostructure with an 8 nm strained GaN channel grown on an AlN/sapphire template suffers from a low and anisotropic mobility. This work deals with a material study to elucidate this issue. Threading dislocations were observed along the growth direction in transmission electron microscopy pictures and are more in number in the (11−20) plane. Steps were also detected in this plane at the GaN channel interfaces. With the help of device simulations and static characterizations, the deep level transient spectroscopy technique allowed five traps located in the GaN channel to be identified. Most of them are associated with nitrogen-or galliumvacancy-related defects and are expected to be localized at the interfaces of GaN with the buffer and the barrier. It is likely that these electrically active defects contribute to reduce the mobility in the two-dimensional electron gas. In addition, a link was established between the mobility and the dependence of the quality of the channel interfaces on the crystallographic orientation.