RF linearity study of SiGe HBTs for low power RFIC design. I

The RF linearity characteristics of a 0.2 μm/120 GHz f_T silicon germanium (SiGe) heterojunction bipolar transistor (HBT) are analyzed using a simplified VBIC model. This model is verified using on-wafer two-tone tests. The study shows that the extrinsic emitter resistance, r_e, has a major impact on transistor linearity through negative feedback and feedback induced cancellation of the third-order intermodulation products generated by the exponential nonlinearity of the transconductance. The base-collector-junction capacitance also contributes to the distortion at high bias currents. The distortion introduced by the weak avalanche current at high collector voltages is also appreciable. The nonlinearity of the base-emitter capacitance (including junction capacitance and diffusion capacitance) and the base current have minimum impact on transistor linearity compared to the effects mentioned above for frequencies under 5 GHz.