An Analysis of Base Bias Current and Intrinsic Base Resistance Effects on InP–InGaAs, InGaP–GaAs, and SiGe Heterojunction Bipolar Transistors

Abstract

An analysis of the effects of base bias current (IB) and small-signal intrinsic base resistance (Rbin) on the RF performances of various HBTs, including InP–InGaAs, InGaP–GaAs, and SiGe HBTs, are demonstrated. It was found that for these HBTs, both the real part of the equivalent input impedance [Re(Zin)] and the equivalent output impedance [Re(Zout)] increase with the increase of Rbin. Therefore, an increase of Rbin (i.e., reducing the base doping, reducing the base width, and increasing the equivalent distance between the emitter–base junction and the base contact) makes the kink phenomena of both the scattering parameters S11 and S22 of these HBTs more prominent. These phenomena can be explained by our derived analytical expressions of Zin and Zout. In addition, for relatively smaller Rbin, it was found that under constant collector–emitter voltage (VCE), an increase of IB [which corresponds to a decrease of base–emitter resistance (rπ) and an increase of trans-conductance (gm)] enhances the anomalous dip. However, for relatively larger Rbin, it was found that under constant VCE, an increase of IB obscures the anomalous dip. These phenomena can also be explained by our proposed theory.