Heterojunction Bipolar Transistors Using Si-Ge Alloys

Advanced epitaxial growth techniques permit the use of pseudomorphic Si_1-x, Ge_x alloys in silicon technology. The smaller bandgap of these alloys allows for a variety of novel band-engineered structures that promise to significantly enhance silicon-based technology. In this paper, we discuss the growth and properties of pseudomorphic Si_1-x, Ge_xstructures and then focus on their applications, especially the Si_1-x, Ge_x base heterojunction bipolar transistor (HBT). We show that HBT's in the Si_1-x Ge, system allow for the decoupling of current gain and intrinsic base resistance. Such devices may be made by using a variety of techniques including molecular-beam epitaxy and chemical vapor deposition. We describe the evolution of fabrication schemes for such HBT's and describe the dc and ac results obtained. We show that optimally designed HBT's coupled with advanced bipolar structures can provide performance leverage.