Field-effect transistor (FET)-based biosensors are promising platforms for point-of-care diagnostics because of their low power requirements, miniaturization capabilities, and high sensitivity toward charged biomolecules. In particular, light-induced FET (L-FET) facilitate photo-generated carrier signal amplification using an external light source, enabling high detection sensitivity from minimal sample volumes. This study proposed the first L-FET biosensor that integrated single-crystal ReS₂ with biomolecules to achieve effective photo-generated carrier signal amplification. ReS₂ maintains its single-layer electronic characteristics even in bulk, thus minimizing signal variability across devices. The ReS₂ current channel is established via mechanical exfoliation onto Au electrodes on a SiO₂ substrate, and photo-generated carrier amplification was induced using a photoconductive effect via laser illumination. Signal variation was assessed using 2-channel I-V measurements following the biomolecular complex formation. The results demonstrated an approximately two times increase in the photo-generated carrier amplification ratio. The detection range for exosomes targeting CD9 protein within human serum was determined to be 10²-10⁷ exosomes/mL, with a limit of detection (LOD) of 9.79 × 10³ exosomes/mL, thereby demonstrating stable detection even at low concentrations. The proposed L-FET biosensor design which incorporated direct bioconjugation with semiconductor devices offered a fundamental advance for early diagnostic platforms. The device's simplified architecture and signal amplification mechanism exhibited advantages for large-scale production, enhancing its commercial viability, and highlighting its potential to impact the evolution of next-generation medical diagnostic technologies enormously. This study introduced a new paradigm for FET-based biosensors, positioning this platform as a candidate for diverse biomarker detection and early disease diagnostic applications.