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Graphene is a promising material for various optical and electrical device applications because of its high carrier mobility, broadband photoresponse, and low manufacturing cost. One such application is for infrared (IR) photodetectors (PDs) because conventional quantum-type IR PDs require complex and toxic materials such as HgCdTe and Type II superlattice structures. We have developed high-performance graphene IR PDs, which operate in the middle-wavelength or long-wavelength IR (MWIR or LWIR) regions, based on field-effect transistors (FETs) that use a photogating effect. This effect is induced by photosensitizers located around the graphene to produce a voltage change under incident light, inducing a change in the electric current of the graphene, which is attributed to its high carrier mobility and single-atom thickness. Si, InSb, and LiNbO3 were used as the photosensitizers for the visible to near-IR, MWIR, and LWIR, respectively. The photoresponsivity obtained for each wavelength region was more than 10 times greater than that of conventional PDs. However, graphene FET-based structures inevitably produce a large dark current and require three electrical ports, which significantly degenerates the PD performance, inhibiting the use of readout integrated circuits for the IR image sensors. To address this issue, we have developed graphene photogated diodes (GPDs) with graphene/semiconductor heterojunction structures. The GPDs employ Schottky barrier lowering and carrier density modulation by photogating and have recently realized low dark currents and high responsivities because of the graphene/semiconductor Schottky junction and photogating. These results can contribute to the development of high-performance graphene-based IR image sensors.
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