We investigated the effects of passivation on the electrical characteristics of molybdenum disulfide (MoS_2) field effect transistors (FETs) under nitrogen, vacuum, and oxygen environments. When the MoS_2 FETs were exposed to oxygen, the on-current decreased and the threshold voltage shifted in the positive gate bias direction as a result of electrons being trapped by the adsorbed oxygen at the MoS_2 surface. In contrast, the electrical properties of the MoS_2 FETs changed only slightly in the different environments when a passivation layer was created using polymethyl methacrylate (PMMA). Specifically, the carrier concentration of unpassivated devices was reduced to 6.5 × 10~(15) cm~(-2) in oxygen from 16.3 × 10~(15) cm~(-2) in nitrogen environment. However, in PMMA-passivated devices, the carrier concentration remained nearly unchanged in the range of 1-3 × 10~(15) cm ~(-2) regardless of the environment. Our study suggests that surface passivation is important for MoS2-based electronic devices.
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