In this work, we show that a careful architecture of buffer layers should be employed in order to perform high performance millimeter-wave GaN devices. The use of higher bias operation (VDS ≥ 20 V) will be possible while using short gate length only if the thermal resistance induced by the buffer layers is reduced. In particular, it is shown that a thick AlGaN back barrier (DHFET structure) results in a huge drop of the PAE at VDS = 25V despite the pulsed mode conditions. With an enhanced thermal dissipation as compared to the DHFET, the carbon-doped structure delivers much higher performances illustrated by a state-of-the-art combination of PAE (> 50%) and an output power density of 7 W/mm at 40 GHz and VDS = 25V.