Power amplifiers (PAs) are key components of mobile base stations. In the last decade, the power efficiency of PAs for 3G/4G mobile base stations has risen to over 50% as a result of employing efficiency enhancement techniques, such as Doherty, envelope tracking, and outphasing, in combination with GaN devices and digital predistortion. This trend has significantly contributed to reducing the power consumption of mobile base stations. Furthermore, digital transmitters using switch-mode PAs have the potential of breaking through the 70% efficiency level. Achieving this goal will require advances not only in circuitry but also in device technology. For active antenna systems of 5G mobile systems, ease of integration, as well as high efficiency, becomes important for PAs, and thus, Si-based devices will play a major role.