OPTIMIZATION OF EARLY VOLTAGE FOR COOLED SIGE HBT PRECISION CURRENT SOURCES

The influence of Ge profile shape on the temperature characteristics o f two key analog transistor parameters, Early voltage (V-A) and curren t gain-Early voltage product (beta V-A), in SiGe HBTs have been studie d over the temperature range of 300K-77K using SCORPIO, a transistor s imulation tool calibrated to measured data [I]. 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 pe rformance of SiGe HBT precision current sources, which are strongly in fluenced by the variations in both beta and V-A. Results clearly indic ate that the cryogenic performance of these SiGe current sources can b e significantly improved by using a graded Ge profile instead of a con stant Ge profile in the base region of the HBT.