Over the period of the past five years, major advances in techniques for the low-temperature preparation of epitaxial films in the silicon!germanium materials system have led to remarkable progress in silicon-based device technology. The fundamental chemical principles underlying one of these growth methods, ultrahigh vacuum/chemical vapor deposition (UHV/CVD), are described in this overview. A variety of unique devices and structures, for example high-speed graded bandgap heterojunction bipolar transistors and n-type resonant tunneling diodes, will be discussed. The role of fundamental interface chemistry in making such structures possible will also be considered. © 1992 IEEE.