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Trilayered MoS ₂ metal-semiconductor-metal (MSM) photodetectors (PDs) exhibit the photogain is up to 24 with high responsivity (~1.04 A/W). This is because photocarriers generated at the MoS2 between two Au electrodes drift toward the metal-semiconductor interfaces due to band bending under applied bias and are then trapped at the surface state sites at the Au/MoS ₂ interfaces, giving rise to the decrease in Schottky barrier height. Moreover, MoS ₂ MSM PDs show fast operation speed; as the contact spacing reduces from 8 to 4 μm, the rise time and fall time of PDs reduce from 70 to 40 μs and from 110 to 50 μs, respectively. Trilayered MoS ₂ MSM PDs can operate even after 2-MeV proton illumination with ~10 ¹¹ cm ^-2 fluences, indicating the high radiation tolerance. This work demonstrates that trilayer MoS ₂ opens up a new dimension for 2-D nanomaterial applications in harsh electronics.