Current gain rolloff in graded-base SiGe heterojunction bipolar transistors

The authors report the experimental observation of a novel effect in SiGe heterojunction bipolar transistors (HBTs) with graded bases which results in a significant emitter-base bias dependence of the current gain. The nonideal collector current is caused by the interaction of the bias dependence of the emitter-base space-charge region width and the exponential dependence of the collector current on the germanium concentration at the edge of the space-charge region. The resulting current gain rolloff must be taken into account for accurate modeling of bipolar transistors with bandgap grading in the base.< >