Revealing the Active Role of the Gate Electrode in Weak-Light Detection

The contribution of gate materials to the photoresponse of thin-film field-effect photodetectors has long been overlooked, with prior studies focusing primarily on photoconductive and photogating effects within the sensing layer. Here, we show that under weak illumination, photocarriers primarily originate from the silicon gate rather than the MoS₂ channel. Absorption spectra confirm that light is mainly absorbed by the gate, driving a negative photocurrent (NPC). The NPC magnitude and slope vary with illumination intensity and V_DS, suggesting transport dominated by Si/SiO₂ interface traps. NPC persists when MoS₂ is replaced with Au/Ti, reinforcing the gate-driven mechanism. At higher powers, band bending reverses due to competing photovoltaic and trap-induced potentials. These results highlight the active role of the gate and offer strategies for device optimization.