An analysis of base bias current effect on SiGeHBTs?

[[abstract]]The anomalous dip in scattering parameter S-11 of SiGe heterojunction bipolar transistors (HBTs) is explained quantitatively for the first time. Our results show that for SiGe HBTs, the input impedance can be represented by a shifted" series RC circuit at low frequencies and a "shifted" parallel RC circuit at high frequencies very accurately. The appearance of the anomalous dip of S-11 in a Smith chart is caused by this inherent ambivalent characteristic of the input impedance. In addition, it is found that under constant collector-emitter voltage (V-CE), an increase of base current (which corresponds to a decrease of base-emitter resistance (r(pi)) and an increase of transconductance (g(m))) enhances the anomalous dip, which can be explained by our proposed theory.[[note]]SC