Recent advances in communication, digital signal processing and computational systems demand very high performance electronic circuits. Heterojunction Bipolar Transistors (HBTs) have the potential of providing a more efficient solution to many key system requirements through intrinsic device advantages. This book reviews the present status of GaAs, InP and silicon-based HBT technologies and their applications to digital, analog, microwave and mixed-signal circuits and systems. It represents the first major effort to cover the complete scope of the HBT technology development in the past decade, starting from the fundamental device physics, material growth, device reliability, scaling, processing, modeling to advanced HBT integrated circuit design for various system applications.