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

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An analysis of the effects of base bias current ($I_{\text{B}}$) and small-signal intrinsic base resistance ($R_{\text{bin}}$) 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 [$\mathop{\mathrm{Re}}(Z_{\text{in}})$] and the equivalent output impedance [$\mathop{\mathrm{Re}}(Z_{\text{out}})$] increase with the increase of $R_{\text{bin}}$. Therefore, an increase of $R_{\text{bin}}$ (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 $S_{11}$ and $S_{22}$ of these HBTs more prominent. These phenomena can be explained by our derived analytical expressions of $Z_{\text{in}}$ and $Z_{\text{out}}$. In addition, for relatively smaller $R_{\text{bin}}$, it was found that under constant collector–emitter voltage ($V_{\text{CE}}$), an increase of $I_{\text{B}}$ [which corresponds to a decrease of base–emitter resistance ($r_{\pi}$) and an increase of trans-conductance ($g_{\text{m}}$)] enhances the anomalous dip. However, for relatively larger $R_{\text{bin}}$, it was found that under constant $V_{\text{CE}}$, an increase of $I_{\text{B}}$ obscures the anomalous dip. These phenomena can also be explained by our proposed theory.
2006-05-15