A Precision SiGe Reference Circuit Utilizing Si and SiGe Bandgap Voltage Differences

Silicon-germanium (SiGe) BiCMOS technology platform provides designers with a unique opportunity to have access to both Si-Si and SiGe-Si p-n junctions. By taking advantage of the coexistence of these two p-n junctions, this paper presents a new temperature compensation technique for SiGe reference circuits. The source of the appearance of curvature in the thermal characteristics of reference circuits is due to the fact that the temperature-dependent nonlinearities in the base-emitter junction are not completely canceled across the temperature range of interest. Here, it is shown that under specific biasing conditions, the two Si-Si and SiGe-Si p-n junction voltages exhibit similar nonlinear temperature dependences. As such, a two-step temperature compensation technique is proposed: first, via a weighted subtraction of two currents, one proportional to the base-emitter junction of Si BJT and the other proportional to the base-emitter junction of SiGe heterojunction bipolar transistor, major nonlinear temperature-dependent terms are canceled; second, via the addition of a proportional to the absolute temperature current, the remaining linear temperature-dependent term is canceled. As a result of this two-step temperature compensation technique, an output voltage proportional to the difference of the bandgap voltages of Si and SiGe can be obtained. The proposed compensation technique is utilized to implement a precision reference circuit in IBM's SiGe BiCMOS 8HP technology. Measurement results verify the stability of the circuit against temperature variations. As the generated output voltage of the circuit is proportional to the difference of Si and SiGe bandgap voltages, the proposed circuit can be utilized to experimentally evaluate the Ge-induced bandgap offset in a given SiGe technology platform.