THE INFLUENCE OF EMITTER-BASE JUNCTION DESIGN ON COLLECTOR SATURATION CURRENT, IDEALITY FACTOR, EARLY VOLTAGE, AND DEVICE SWITCHING SPEED OF SI/SIGE HBTS

In advanced Si/SiGe HBT's the base is doped much higher than emitter and collector. Base outdiffusion becomes a problem because of the formation of parasitic barriers that degrade device performance. The simulations and experiments of this paper show that a strong correlation exists between (a) the drop of the collector saturation current, (b) an increase of its ideality factor and (c) a rise of the switching time due to an additional emitter delay which can no longer be neglected. Curves of these three parameters as a function of Si/SiGe heterointerface position and outdiffusion at the base-emitter interface have been calculated and indicate that only a few nm shift may cause severe device degradation. An important result is that the collector current ideality factor or the inverse Early voltage is a very sensitive indicator for the quality of the emitter-base interface. Application of these results have yielded experimental SiGe HBT's with transit frequencies above 60 GHz.