We report on high power-added-efficiency using AlN/GaN heterostructure with a carbon doped buffer layer for millimeter wave applications (C-doped HEMTs). The carbon doped HEMTs show high electrical characteristics with a maximum drain current density Id of 1.5 A/mm, an extrinsic transconductance Gm of 500 mS/mm and a maximum oscillation frequency fmax above 200 GHz while using a gate length of 120 nm. The high RF performance obtained on the carbon doped HEMT combined to an excellent electron confinement under high bias enabled to achieve a state-of-theart combination at 40 GHz of output power density (POUT = 7 W/mm) and power added efficiency (PAE) above 50% up to VDS = 25V in pulsed mode.