Investigation of fT-Doubler Technique to Improve RF Performance of Inverse-Mode SiGe HBTs

This letter presents the application of the f_T-doubler technique, for the first time, to improve the unitygain frequency (f_T) of inverse-mode (TM) silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs). An f_T-doubler structure, which used three identical SiGe HBTs with the same emitter area of 0.07 (width) × 0.9 (length) μm², is implemented in a commercial 0.13-μm SiGe-BiCMOS technology platform. A peak f_T of 77 GHz is extrapolated for the TM f_T doubler, whereas a peak f_T of a single TM SiGe HBT is found to be 53 GHz, exhibiting an increase of about 46% in f_T from the f_T-doubler technique. The maximum oscillation frequency of the TM f_T doubler using Mason's unilateral gain is about 158 GHz. Tn addition, small-signal model parameters of the TM f_T doubler are presented, which show the TM f_T-doubler structures can be treated as a single transistor element for highfrequency circuit design.

I. Introduction

The use of silicon–germanium (SiGe) heterojunction bipolar transistors (HBTs) in inverse-mode (IM) operation, where the physical emitter and collector terminals of the device are swapped, has gained attention in the space electronics community due to their inherent tolerance to total ionizing dose (TID) [1], [2] and better resilience to different single-event effects (SEEs) such as single-event transients (SETs) and single-event upset (SEU) [3], [4]. With these benefits, the IM SiGe HBTs have been utilized in various digital [5], analog [6], and radio frequency (RF) circuits [7], [8]. The major drawbacks associated with IM SiGe HBTs, however, include low gain, limited current drive, large parasitic capacitances, and a low unity-gain frequency . Thus, extending the speed and the bandwidth has been a critical issue for potential applications such as RF transceivers and high-speed analog circuits. While aggressive scaling, profile adjustment, and layout modification are a few approaches to improve the performance of IM SiGe HBTs [9], [10], they require complex and costly reoptimization of a fabrication process. As a low-cost alternative, we explore the use of a design technique termed IM -doubler topology to enhance the RF performance of IM SiGe HBTs.