Driven by the large absolute bandwidths that are available at the sub-mm-wave frequency range around 300 GHz, wireless high-data-rate communication systems and high-resolution imaging applications are being extensively investigated in recent years. Due to their superior characteristics in terms of noise figure and cutoff frequencies, InGaAs-channel HEMT devices have proven to be a key technology to implement the required active front-end MMICs for these wireless THz systems, enabling ultra-high bandwidths and state-of-the-art noise performance. This work describes the modeling, design, and characterization of 300-GHz HEMT-based power amplifier cells and demonstrates the implementation of highly compact amplifier MMICs and broadband waveguide modules. These amplifiers are key components for the implementation of high-performance chipsets for wireless THz systems, providing high output power for the utilization of next-generation communication and imaging applications. A unique amplifier topology based on multi-finger cascode and common-source devices is developed and evaluated, demonstrating more than 20-mW measured output power at the sub-mm-wave frequency range around 300 GHz.