Millimeter-wave fifth-generation (5G) systems will extensively leverage massive multiple-input multiple-output (MIMO) architectures to improve their link performance. These array systems will employ many power amplifiers (PAs) operating at moderate output power (P _out ), e.g., 16 PAs each with +7dBm P _out [1]. The PA energy efficiency is of paramount importance in MIMO systems for improved battery life and thermal management. Due to spectrum-efficient modulations with high peak-to-average power ratios, both PA peak efficiency and power back-off (PBO) efficiency are critical. To achieve 5G Gb/s data-rates with complex modulations, envelope tracking PAs require high-speed/high-precision supply modulators, and outphasing PAs need high-speed baseband computation, both of which pose substantial challenges in practice. Although Doherty PAs support high data-rates, existing silicon mm-wave Doherty PAs exhibit very limited PBO efficiency enhancement, mainly due to inefficient Doherty power combiners and imperfect main/auxiliary PA cooperation [2,3].