With SiGe, who needs GaAs?

Summary form only given. With SiGe-based bipolar transistors making rapid inroads into high-frequency small-signal and analog applications, there is the question of whether they will permanently displace GaAs or other compound-semiconductor heterojunction bipolar transistors (HBTs). Often the attempts to answer this question have raised more questions because they were based on apple-to-orange comparisons. In this talk, we attempt to make an apple-to-apple comparison of the SiGe and GaAs HBTs by examining and comparing the intrinsic physical properties of the materials and the electrical parameters of these transistors. The comparison is made for devices of comparably advanced structures and of the same design rules. The conclusions are that the commonly practiced SiGe HBT is not really a heterojunction bipolar transistor at all, and that GaAs HBTs are inherently faster than SiGe "HBTs". Therefore, GaAs and other compound-semiconductor HBTs will have a place in applications where the higher speed makes an important difference. The inherent advantage of SiGe bipolar transistor over GaAs HBT lies in its being compatible with CMOS VLSI processing.