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.
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