ABSTRACT
Bandgap engineering generally has a positive influence on the low-temperature characteristics of
bipolar transistors [1]. SiGe HBTs operate very well, in fact, in the cryogenic environment (e.g., liquid
nitrogen temperature ¼ 77.3 K ¼ 3208F ¼ 1968C), an operational regime traditionally forbidden to Si BJTs. At present, cryogenic electronics represents a small but important niche market, with
applications such as high-sensitivity cooled sensors and detectors, semiconductor-superconductor
hybrid systems, space electronics, and eventually cryogenically cooled computers systems. While the
large power dissipation associated with conventional bipolar digital circuit families such as emitter-
coupled-logic (ECL) would likely preclude their widespread use in cooling-constrained cryogenic
systems, the combination of cooled, low-power, scaled Si CMOS with SiGe HBTs offering excellent
frequency response, low-noise performance, radiation hardness, and excellent analog properties repre-
sents a unique opportunity for the use of SiGe HBT BiCMOS technology in cryogenic systems.
Furthermore, independent of the potential cryogenic applications that may exist for SiGe HBT BiCMOS
technology, all electronic systems must successfully operate over an extended temperature range (e.g.,
55 to 1258C to satisfy military specifications and 0 to 858C for most commercial applications), and thus, understanding how Ge-induced bandgap engineering affects SiGe HBT device and circuit oper-
ation is important.
