The influence of Ge grading on the bias and temperature characteristics ofSiGeHBT's for precision analog circuits

This paper analyzes the effects of Ge profile shape on the bias and tempera ture characteristics of advanced UHV/CVD SiGe heterojunction bipolar transi stors (HBT's). The widely used bandgap reference (BGR) design equation and a more general analytical expression incorporating Ge grading developed in this work are used to compare silicon devices to their SiGe counterparts. T heory, device measurements, and SPICE simulations are used to investigate t he impact of Ge grading on SiGe HBT precision voltage references, It is con cluded that conventional SPICE can be used to account for Ge grading effect s in SiGe HBT modeling. Sufficient Ge grading can have a significant impact on the accuracy of precision voltage references, particularly at reduced t emperatures, and thus warrants attention.