We report on breakthrough power-added-efficiency (PAE) Q-band performances using a vertically scaled AlN/GaN HEMT technology. The comparison between a 3 nm and 4 nm barrier thickness shows both superior performance and robustness for the thinner barrier layer attributed to the reduced mechanical strain into the heterostructure. Large signal characteristics at 40 GHz revealed an outstanding PAE of 73% at VDS = 30V associated to an output power density > 5 W/mm in pulsed mode. Also, the load-pull measurements mapping across the 4-inch wafer demonstrates a high uniformity and reproducibility of the results. Consequently, significantly improved PAE can be expected for next generation of high power MMICs operating in the millimeter-wave range.