We present a detailed examination of the circuit performance issues associated with optimizing epitaxial Si and SiGe-base bipolar technology for the liquid-nitrogen temperature environment. We conclusively demonstrate that the common notion that silicon-based bipolar circuits perform poorly at low temperatures is simply untrue. Transistor frequency response is examined both theoretically and experimentally, with particular attention given to the differences between SiGe and Si devices as a function of temperature. ECL and NTL ring oscillator circuits were fabricated for each of the four profiles described in our companion paper (this issue). A minimum ECL gate delay of 28.1 ps at 84 K was measured for a SiGe-base profile, and is essentially unchanged from its room-tempera-ture value of 28.8 ps at 310 K. ASTAP models were calibrated to data and used to explore circuit operation under typical wire loading. For 5.65-mW ECL circuits driving 10-mm wire inter-connects, reductions in wire resistance as well as reduced logic swing operation yield 84 K circuit delays as much as 2.7 x faster than at 310 K. We conclude that epitaxial-base bipolar technology offers significant leverage for future cryogenic applications.