Analysis of Fully Integrated SiGe Heterojunction Phototransistors: Performance Optimization and Design Tradeoffs

The present work investigates the performance of SiGe heterojunction phototransistors (HPTs) fabricated in a commercially available 130-nm SiGe BiCMOS (8XP) technology, with emphasis on their unique ability to function not only as photodetectors, but also as the core active devices within an optical receiver. Detailed characterization demonstrates that electrical and optical figures-of-merit track closely, highlighting the transistor-like nature of SiGe HPTs and enabling compact modeling within standard circuit frameworks. The dependence of responsivity on external base resistance and bias conditions is quantified, showing that responsivity is not a fixed parameter, but can be engineered through circuit-level choices. Geometry and layout tradeoffs are also explored across multiple SiGe HPT variants, where optical window size and contact configuration determine the balance between responsivity, bandwidth, and fill factor. The results highlight the expanded design space available when SiGe HPTs are integrated directly into optical receiver front ends, offering application-specific tradeoffs not accessible with conventional photodiode (PD)-plus-transimpedance amplifier (TIA) architectures.