A single-layer MoS₂ phototransistor is fabricated using mechanically-exfoliated MoS₂ nanosheets, and its light-induced electric properties are investigated. The photocurrent generated is solely determined by the illuminated optical power at a constant drain or gate voltage. The switching behavior of photocurrent generation and annihilation can be completed within approximately 50 ms, showing good stability. The single-layer MoS₂ phototransistor exhibits better photoresponsivity compared to a graphene-based device. The unique characteristics of incident-light control, prompt photoswitching, and good photoresponsivity of the MoS₂ phototransistor pave the way for future optoelectronic device applications using single-layer semiconducting materials. The single-layer MoS₂ phototransistor demonstrates a high photoresponsivity of up to 7.5 mA/W under low optical power (80 μW) and medium gate voltage (50 V). The photocurrent generation efficiency depends on the magnitude of photo-generated charges under constant drain or gate voltage. The photoswitching behavior is prompt, with the photocurrent switching between ON and OFF states within 50 ms, and the response rate is lower than that of graphene due to slower carrier transport in MoS₂. The photoresponsivity of the single-layer MoS₂ phototransistor can be tailored by gate voltage. The photoresponsivity increases with gate voltage, reaching 7.5 mA/W at a gate voltage of 50 V. The gate voltage-dependent photoresponsivity is attributed to the n-type doping of MoS₂. The photoswitching behavior is stable, with the ON-OFF switching retained after multiple illumination cycles. The single-layer MoS₂ phototransistor shows good performance in terms of photoresponsivity, photoswitching, and stability. These characteristics make it a promising candidate for optoelectronic devices. The study highlights the potential of single-layer semiconducting materials for future functional device applications in switches, memories, signal amplifiers, and light-related sensors.A single-layer MoS₂ phototransistor is fabricated using mechanically-exfoliated MoS₂ nanosheets, and its light-induced electric properties are investigated. The photocurrent generated is solely determined by the illuminated optical power at a constant drain or gate voltage. The switching behavior of photocurrent generation and annihilation can be completed within approximately 50 ms, showing good stability. The single-layer MoS₂ phototransistor exhibits better photoresponsivity compared to a graphene-based device. The unique characteristics of incident-light control, prompt photoswitching, and good photoresponsivity of the MoS₂ phototransistor pave the way for future optoelectronic device applications using single-layer semiconducting materials. The single-layer MoS₂ phototransistor demonstrates a high photoresponsivity of up to 7.5 mA/W under low optical power (80 μW) and medium gate voltage (50 V). The photocurrent generation efficiency depends on the magnitude of photo-generated charges under constant drain or gate voltage. The photoswitching behavior is prompt, with the photocurrent switching between ON and OFF states within 50 ms, and the response rate is lower than that of graphene due to slower carrier transport in MoS₂. The photoresponsivity of the single-layer MoS₂ phototransistor can be tailored by gate voltage. The photoresponsivity increases with gate voltage, reaching 7.5 mA/W at a gate voltage of 50 V. The gate voltage-dependent photoresponsivity is attributed to the n-type doping of MoS₂. The photoswitching behavior is stable, with the ON-OFF switching retained after multiple illumination cycles. The single-layer MoS₂ phototransistor shows good performance in terms of photoresponsivity, photoswitching, and stability. These characteristics make it a promising candidate for optoelectronic devices. The study highlights the potential of single-layer semiconducting materials for future functional device applications in switches, memories, signal amplifiers, and light-related sensors.