Using Heterojunction Bipolar Transistors (HBTs) as power amplifiers presents significant challenges due to their thermal behavior, particularly issues such as self-heating and current collapse in multi-finger configurations. Addressing these challenges requires careful design of the current biasing network. This thesis aims to investigate and address these challenges in HBT power amplifier design. The thesis begins by constructing an HBT model based on the Ebers-Moll bipolar transistor model. This model is utianalyzes and characterizesffects of self-heating and current collapse using the tool SDD( User-Defined Models) in ADS (Advanced Design System). Subsequently, the thesis explores various possibilities for biasing network design for HBT power amplifiers.