On the RF Properties of Weakly Saturated SiGe HBTs and Their Potential Use in Ultralow-Voltage Circuits

We investigate, for the first time, the feasibility of operating silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs) in a weakly saturated bias regime to enable ultralow-voltage RF front-end design. Measured dc, ac, and RF characteristics of third-generation high-performance SiGe HBTs operating in weak saturation are presented. Robust RF operation of 0.12 × 6.0 μm² SiGe HBTs are demonstrated in a common-emitter configuration at collector-to-emitter voltages above 0.15 V. A noise figure of 1.33 dB and an input third-order intercept point above -8 dBm for a 3-GHz input tone are achieved at 0.30 V. These results have potential implications for RF circuits used in severely power-constrained systems.