In this paper, we demonstrate Q-band power performance of carbon doped AlN/GaN high electron mobility transistors (HEMTs) using a deep sub-micrometer gate length (120 nm). With a maximum drain current density ID of 1.5 A/mm associated to a high electron confinement and an extrinsic transconductance gm of 500 mS/mm, this structure shows excellent electrical characteristics. A maximum oscillation frequency fmax of 242 GHz has been observed. As a result, a state-of-the-art combination at 40 GHz of output power density (POUT = 7 W/mm) and power added efficiency (PAE) of 52% up to VDS = 25V has been obtained. The achievement of such outstanding performance is attributed to the reduced thermal resistance (RTH) as compared to the commonly used double heterostructure by means of Raman thermography.