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) μm
²
, 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.