An analysis of the bias dependence of scattering parameters S11 and S22 of SiGe heterojunction bipolar transistors (HBTs)

The anomalous dip in scattering parameter S₁₁ 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. The appearance of the anomalous dip of S₁₁ in a Smith chart is caused by this inherent ambivalent characteristic of the input impedance. It is found that under constant collector-emitter voltage (V_CE), an increase of base current (which corresponds to a decrease of r_π and an increase of g_m) enhances the anomalous dip. In addition, the anomalous dip in S₁₁ of SiGe HBTs can also be interpreted in terms of poles and zeros.