Achieving very high-power efficiency at mmWave frequencies is critical for the deployment of 5G and 6G radio systems. Although compound semiconductors
technologies promise to reduce the power consumption of high-power amplifiers, their low-cost fabrication on a CMOS compatible platform remains challenging. We introduce in this presentation a GaN-on-Si platform achieving 68% PAE at 28GHz. The reliability challenges will be highlighted, toghether with the transistor optimization using different materials for the front- and back-barriers.