On the Use of a SiGe Spike in the Emitter to Improve the $f_{T}\hbox{xBV}_{\rm CEO}$ Product of High-Speed SiGe HBTs

Aggressive vertical scaling of SiGe HBTs has yielded impressive values for the cut-off frequencies (f
_T
), but these HBTs often suffer from too high current gains. This leads to low values for the open-base breakdown voltage (BV
_CEO
). In this letter we demonstrate the use of a SiGe spike in the emitter as a practical method to increase the base current. Hence, the breakdown voltage is increased. At the same time, the device RF performance is not affected, which leads to a significant improvement in the f
_T
xBV
_CEO
product