Modeling of U-shaped and plugged emitter resistance of high speed SiGe HBTs

In this paper, we investigate the emitter resistance R_e in SiGe HBTs with speeds up to 280GHz, using a U-shaped polysilicon emitter. We observed that R_e increased with lateral scaling, thereby degrading f_T. Although a negligible component in the past, in this experiment R_e * C_cb transit time delay is playing a more significant role in limiting f_T. R_e was modeled to explain the increase due to lateral scaling, and was shown to result from the plugging of the emitter opening by the emitter polysilicon. Furthermore, process experiments were conducted to investigate the effect of emitter polysilicon thickness, sidewall height, and emitter i-layer thickness.