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

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Abstract
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 °C range, it can become important as the temperature drops, perhaps affecting the operation of SiGe circuits in the 77 K regime

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