GaN-based material enables higher power density, superior breakdown voltage, and higher thermal conductivity as compared to its counterparts. On the other hand, the poweradded-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 solidstate power amplifiers. GaN-based 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 GaN-based transistors high performances and robustness.