The Impact of Ge Grading on the Bias and Temperature Characteristics of SiGe HBT Precision Voltage References

We analyze the effects that the Ge profile shape has upon the bias and temperature characteristics of SiGe HBT's. The widely used bandgap reference (BGR) design equation and a more general analytical expression we developed incorporating Ge grading are used to compare silicon devices to their SiGe counterparts. Measurement and simulation show that although the Ge-ramp effect is negligible in the -55 to 85 OC range, it can become important as the temperature drops, perhaps affecting the operation of SiGe circuits in the 77 K regime.