The objective of this research is to investigate the design challenges of high efficiency broadband power amplifiers (PAs) using silicon germanium (SiGe) heterojunction bipolar transistor (HBT) and establish design methodologies to address these challenges. Broadband amplification with high efficiency is becoming crucial for emerging wireless broadband applications such as 5G/6G wireless communications, secured military/satellite communications, phased array radars, and high-resolution imaging. To support the system level requirement for low cost and high integration capability, SiGe HBT PAs can be a viable solution. To overcome an inherent efficiency and output power limitations of conventional distributed amplifiers for wideband applications, one part of this research provides two design examples of SiGe HBT non-uniform power amplifier (NDPA) and develops novel and systematic design approaches to improve efficiency over wide BW. To meet the demand for highly efficient PAs with higher peak-to-average power ratio and wider channel bandwidth for modern 5G NR-U/WLAN applications, the other part of this research establishes a design guideline for four types of series-combined transformer (SCT)-based Doherty output matching networks and provides a design example of SiGe HBT SCT-based Doherty PA for higher power back-off (PBO) efficiency enhancement.