Power-added-efficiency (PAE) becomes a key parameter in order to support complex waveforms with high peak-to-average ratio and large instantaneous bandwidth required for 5G applications for instance. Overall, a high PAE is mandatory in order to reduce the power dissipation, which is a major issue for future compact solid-state power amplifiers. GaN-based High electron mobility transistors (HEMTs) on SiC substrate have already demonstrated attractive efficiencies in the mm-wave range. However, state-of-the-art GaN HEMT PAE performances are typically limited to 50% or below in the Ka-band and above, especially for high power densities > 3 W/mm. This is mainly due to the lack of power gain, the enhanced trapping effects and reduced electron confinement when downscaling the device size. In this talk, promising solutions based on highly scaled epitaxial heterostructures will be addressed, enabling mm-wave high performances and robustness.