We report on a comparison of the ultrathin (sub-10 nm barrier thickness) AlN/GaN heterostructure using two types of buffer layers: 1) carbon doped GaN high electron mobility transistors (HEMTs) and 2) double heterostructure field effect transistor (DHFET). It is observed that the carbon doped HEMT structure shows better electrical characteristics, with a maximum drain current density Id of 1.3 A/mm, a transconductance Gm of 500 mS/mm and a maximum oscillation frequency fmax of 234 GHz while using a gate length of 220 nm. The low trapping effects together with high frequency performance and excellent electron confinement under high bias enabled to achieve a state-of-the-art combination at 18 GHz of output power density (Pout > 6 W/mm) and power added efficiency (PAE) close to 40% at Vds as high as 30V. At 40 GHz, a PAE above 35% is still observed in spite of the rather large gate length. A key feature is the low gate leakage current of only few tenths of μA/mm that remains stable after many load-pull sweeps at various frequency in the case of carbon doped HEMT, which is attributed to a significant reduction of the self-heating as compared to the DHFET.