ABSTRACT
Silicon-Germanium BiCMOS technology is making inroads in a wide variety of mixed-signal and high-performance circuit applications due to its attractive combination of excellent RF performance metrics at conservative lithographic feature size, together with the high integration levels and yield associated with standard silicon manufacturing [1]. In a SiGe HBT, the introduction of an epitaxial germanium layer in the base works to increase the dc current gain, thereby decoupling base doping levels from dc current gain. This allows the HBT designer to dope the base highly, leading to very small extrinsic base resistance, which favors broadband noise (noise figure) performance. Thus, a germanium profile working in tandem with increased base doping levels helps to improve dynamic switching and noise characteristics of the SiGe HBT [2]. This makes them immediately attractive for use in high-frequency, high-performance, low-noise circuits like low-noise amplifiers (LNA) [3–6], power amplifiers (PA) [7–11], and voltage-controlled oscillators (VCO) [12–16]. Introducing epitaxial germanium has an added advantage—it makes the SiGe HBT technology an ideal candidate for use in extreme environment applications (e.g., operation under cryogenic temperatures and ionizing radiation). In stark contrast with Si BJTs, the dc and ac performances of SiGe HBTs improve with reducing ambient temperature. This paves the way for usage of SiGe platforms in applications like terahertz communications [17], smart phased array systems-on-chip [18], radio astronomy [19], and airborne remote surveillance systems, where uninterrupted sensing, transmitting, and computing under varying temperatures is critical to mission success.
