Compared with traditional bulk materials, 2D materials have many extraordinary properties in the field of optoelectronic devices, such as tunable bandgap and absence of dangling bonds. Although their excellent light-matter interaction, a low optical absorption which proportional to the thickness has considered as a major limitation. One possible way to overcome this limitation is to increase the light absorption using plasmonic nanostructures. These nanostructures interact with incident light and generate localized surface plasmon resonance (LSPR), thereby the light absorption can be significantly enhanced near the structure. The magnitude of LSPR can be highly intensified when two metallic nanostructures are placed with a small insulating gap, and called gap-mode plasmon. Here, we apply a gap-mode plasmon structure to the Schottky junction of Au/MoS2 to compensate for the low absorption. The magnitude of gap-mode plasmon is generally known to be inversely proportional to the gap distance, so that the atomically thin thickness of 2D materials can be considered as a good candidate for gap spacer. Owing to the gap-mode plasmon structure, the photoresponsivity of the device is enhanced approximately 11.6 times from 25 to 290 A/W without degradation of the
photoresponse time. In order to verify the presence of gap-mode plasmon, surface enhanced Raman spectroscopy, absorption spectroscopy and numerical simulation are also performed. Unlike the conventional Schottky junction photodiode, the device exhibits its highest responsivity in the forward-biased condition because of the traps and photoresistor feature of MoS2.