We report on AlN/GaN HEMT technology for millimetre-wave applications with various buffer architectures. A high carbondoped buffer enables high electron confinement at the expense of trapping effects, which is attributed to the enhanced point defects and dangling bonds when using higher carbon concentration. On the other hand, reduced carbon-doped buffer combined with the insertion of an AlGaN back barrier underneath a non-intentionally doped GaN channel layer enables not only low trapping effects but also prevents punch through effects and related drain leakage current under high drain bias. This results in a unique combination of excellent electron confinement and low trapping effect while using gate lengths as low as 100 nm and high drain bias (≥ 20 V). As a result, the optimized structure using a thin AlGaN back barrier enabled outstanding RF performances at 40 GHz with a record PAE of 70% and 66% under high output power density at VDS = 20 V in pulsed and CW mode, respectively.