In this paper, we report on a vertically scaled AlN/GaN HEMT technology. The comparison between a 3 nm and 4 nm barrier thickness shows both superior performances and robustness for the thinner barrier layer, which is attributed to the reduced mechanical strain into the heterostructure. The impact of the gateto-drain distance (LGD) on the RF performances has been studied at 40 GHz. Pulsed mode large signal characteristics reveal an outstanding combination of power added efficiency (PAE) of 72% with a saturated output power density (POUT) > 3.5 W/mm at VDS = 20V with LGD=0.5 µm. Furthermore, short term RF reliability assessment demonstrates a promising robustness.