Impact of Scaling on the Inverse-Mode Operation of SiGe HBTs

The inverse-mode operational regime of silicon germanium (SiGe) heterojunction bipolar transistors (HBTs) has to date been largely ignored and is typically dismissed as a viable possibility for circuit applications due to the general perception of its limited dc and ac performance capabilities. In this paper, the inverse-mode performance of four distinct generations of SiGe HBTs is investigated and is found to improve impressively with generational scaling. The physics behind these scaling-induced improvements is examined in detail using a combination of measurements and calibrated simulations. A novel lateral dependence of the inverse-mode base current is identified and is shown to potentially present new opportunities for even larger improvements in inverse-mode performance in SiGe HBTs. A record peak fT in inverse mode of 25 GHz is reported for a prototype fourth-generation device