Optimization of Early Voltage for Cooled SiGe HBT Precision Current Sources

Abstract
The influence of Ge profile shape on the temperature characteristics of two key analog transistor parameters, Early voltage (V_A) and current gain-Early voltage product (βV_A), in SiGe HBTs have been studied over the temperature range of 300K-77K using SCORPIO, a transistor simulation tool calibrated to measured data [1]. A new version of SPICE that accounts for the temperature dependence of V_A was used to model the various SiGe HBTs simulated and thereby evaluate the cryogenic performance of SiGe HBT precision current sources, which are strongly influenced by the variations in both β and V_A. Results clearly indicate that the cryogenic performance of these SiGe current sources can be significantly improved by using a graded Ge profile instead of a constant Ge profile in the base region of the HBT.