Small-signal intrinsic base resistance effect on InP-InGaAs, InGaP-GaAs and SiGe HBTs

In this paper, a comprehensive analysis of the effect of small-signal intrinsic base resistance (R_bin) on the RF performances of InP-InGaAs (Gao et al., 2004), InGaP-GaAs (Bousnina et al., 2004), and SiGe HBTs are demonstrated for the first time. It was found that for these HBTs, both the real part of the equivalent input impedance (Re(Z_in)) and the real part of the equivalent output impedance (Re(Z_out)) increase with the increase of the intrinsic base resistance R_bin. Therefore, an increase of R _bin (i.e. reducing the base doping, the base width, and the base edge length in the base contact side) makes the kink phenomena of both the scattering parameter S₁₁ and S₂₂ of these HBTs more obvious (Lu et al., 2001). These phenomena can be explained perfectly by our derived complete expressions of Z_in and Z _out at low and high frequencies. In addition, for relatively smaller R_bin, it was 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 trans-conductance (g_m)) enhances the anomalous dip. While for relatively larger R_bin, it was found that under constant V_CE, an increase of base current obscures the anomalous dip. These phenomena can also be explained by our proposed theory