Lateral scaling of the self-aligned extrinsic base in SiGe HBTs

Minimizing R_bb, C_ca, and C_cb is crucial to maximizing the key figure of merit f_max in the SiGe HBT. Through a systematic scaling study of the extrinsic base-emitter sidewall spacer, we explore for the first time the trade-off advantages in minimizing these parasitics in the self-aligned, epitaxial-base SiGe HBT process compared with traditional ion-implanted BJT designs. We demonstrate a wide process window for the spacer thickness that achieves high f_T and f_max without significantly impacting the emitter-base breakdown voltage, the reverse emitter-base leakage, or reliability. By examining the temperature dependence of this leakage current, we establish a transition of the dominant leakage mechanism from impact ionization to band-to-band tunneling as the sidewall spacer is thinned. In addition, we demonstrate an increase in long-term β degradation with spacer thinning. Together, these results confirm the encroachment of extrinsic base doping upon the emitter edge as the limiting factor in the scaling process