The influence of Ge grading on the bias and temperature characteristics of SiGe HBTs for precision analog circuits

This paper analyzes the effects of Ge profile shape on the bias and temperature characteristics of advanced UHV/CVD SiGe heterojunction bipolar transistors (HBTs). 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. Theory, device measurements, and SPICE simulations are used to investigate the impact of Ge grading on SiGe HBT precision voltage references. It is concluded that conventional SPICE can be used to account for Ge grading effects in SiGe HBT modeling. Sufficient Ge grading can have a significant impact on the accuracy of precision voltage references, particularly at reduced temperatures, and thus warrants attention.