SiGe heterojunction bipolar transistors

The authors review recent progress in SiGe-based heterojunction bipolar transistors (HBTs), their potential leverage, and some of the challenges for large-scale integration. It is observed that the advent of low-temperature epitaxial processes that allow the growth of strained Si1-xGex epitaxial layers have made possible the first Si-based double heterojunction HBTs in an integrated process. The extra degree of freedom available for device profile optimization can be used either to enhance fT or to shift the peak fT to lower current density or to lower the base sheet resistance. Improved low-temperature operation is also expected. Today's HBTs can be further optimized by fine-tuning the GE profiles.