Photogating and high gain in ReS2 field-effect transistors

Two-dimensional layered transition metal dichalcogenides have shown much promise due to their remarkable electro-optical properties and potential use as photodetectors. We observed photogating in our few-layered (3–4 layers) ReS 2 field-effect transistors (FETs) in which varying the incident optical power shifted the FETs’ threshold voltage. The photogating effect produced a significant gain in the electrical response of the FETs to incident light as measured by the responsivity (R) and external quantum efficiency (EQE). We obtained a maximum R of 45 A/W corresponding to an EQE of ∼10 500% in a four-terminal measurement of the photoconductivity in the ON-state. We attribute both the photogating and the observed gain to the influence of charge traps. An estimate of the device gain based on our observations is calculated to be 5×104.