Millimeter-wave 5G and 6G networks are emerging to support low-latency and high data rate mobile (e.g., up to 10 Gbps with ~10x lower latency than 4G LTE) and backhaul communications (e.g., 20 - 100 Gbps). Realizing high-performance millimeter-wave communication systems with both spectral and power efficiency will require advancing millimeter-wave transistor technologies with higher linearity and efficiency at 5G and 6G bands than the current state-of-the-art transistor technologies. In this talk, we will overview the challenges and recent advancements of millimeter-wave GaN transistor technologies.