Exploration of SiGe HBTs for power amplifiers in the 200 GHz to 500 GHz frequency range

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The continuously increasing demand for mm- and sub-mm-wave applications in, e.g., communications, imaging and sensing poses significant challenges for the corresponding electronic systems in terms of bandwidth, output power, and energy efficiency. An important component in such systems is the power amplifier (PA), which is also a major contributor of energy dissipation. According to present trends, higher bandwidth in future will be achieved by moving to much higher carrier frequencies (than those in present 5G systems) with active antenna phased arrays containing hundreds of PAs for beam steering and massive multiple-input-multiple-output signals. Such highly-integrated high-frequency (HF) systems can only be realized with silicon-germanium (SiGe) BiCMOS technology. Hence, the exploration of both the capability of existing and the requirements for future performance of SiGe heterojunction bipolar transistors (HBTs) with respect to the realization of HF PAs with sufficient output power and high power and spectral efficiency is of great interest. So far, little attention has been paid to frequencies at and beyond 200 GHz, which are relevant, e.g., for high data-rate transmission in processor-to-processor communications, pico- and femto-cell environments, and security screening as well as for biological probing and medical imaging. A major challenge at such frequencies is the generation of sufficient output power, while achieving at the same time also high efficiency. The proposed project therefore aims at (i) the exploration of the maximum achievable power density of SiGe HBT PAs operating in the 200...500 GHz range and the performance limitations related to device physics based on experimental data and ITRS/IRDS predictions; (ii) the investigation of the trade-offs between output power, efficiency, gain, bandwidth, and linearity; (iii) accurate large-signal compact HBT modeling and detailed analysis of non-linear transients and physical effects during PA operation based on mixed-mode numerical device simulation (TCAD) and fabricated circuits; (iv) separate assessment of the harmonic power generated in PAs during nonlinear large-signal operation for, e.g., model verification. The proposed work, as detailed in three well-defined work packages, includes the design, fabrication and experimental characterization of conduction-angle PAs and their building blocks. So far, achieving high output power and efficiency in HF PA designs has been constrained, among others, by conservative approaches due to the lack of deeper insight into HBT operation limits and the corresponding accurate models. The existing barriers will be overcome by pushing transistor operation to its physical limits and by careful PA optimization, considering all possible device layout options based on geometry scalable HBT modeling and important distributed effects such as thermal and substrate coupling.