Boosting External Quantum Efficiency beyond 100% in a Nanosecond-Scale Fast Photovoltaic MoS2-Si Photodiode

As photodetectors continue to scale toward smaller device dimensions, achieving high sensitivity without sacrificing response speed remains a significant challenge for conventional silicon photodiodes. Here, we report a CMOS-compatible vertical 2D/3D hybrid photodiode consisting of trilayer molybdenum disulfide (MoS2) on p-type silicon (Si) with monolayer graphene as a transparent top electrode. The atomic-scale thickness of trilayer MoS2 (∼2.1 nm) enables a strong internal electric field and a fully depleted junction under low reverse bias. Consequently, the device exhibits high responsivity (1.0-1.4 A W-1) and external quantum efficiency (EQE) exceeding 100% across the visible range (420-660 nm), with a maximum EQE of 320% at -1 V, while maintaining a nanosecond-scale response. Power-law analysis, C-V characteristics, and nanosecond time-resolved measurements suggest that the observed gain is associated with electric-field-driven carrier multiplication, while long-lived trap-assisted photogating is unlikely to be the dominant gain mechanism.