Control of majority carrier type and concentration in transition metal dichalcogenides (TMDs) is an important goal for engineering and improving TMD-based devices. Monolayer and few-layer molybdenum disulphide (MoS2) is an n-type semiconductor due to the presence of electron-donating native defects whose distribution is strongly dependent on the processing history and ambient environment. However, the spatial heterogeneity of the charge carrier concentration has not yet been studied in MoS2 when implemented in devices such as field-effect transistors (FETs). Here, we present a method to extract the spatial distribution of charge carriers using Kelvin probe force microscopy of MoS2 FETs in operando. The carrier concentration in monolayer MoS2 exfoliated on SiO2/Si ranges from 1.2×1012 cm−2  to 2.3×1012 cm−2, corresponding to a three-dimensional concentration of 1018 cm−3 to 2.5×1018 cm−3. A comparable carrier concentration is obtained for few-layer MoS2, while for thicker MoS2 (>50 nm) it is an order of magnitude lower (2×1017 cm−3–4×1017 cm−3). This finding is consistent with an increased concentration of electron-donating sulfur vacancies at surfaces compared to the bulk. Thus, the reported method for measuring the carrier concentration may advance strategies for doping and improve understanding of devices and defects in 2D materials.

Understanding the nature of the barrier height in a two-dimensional semiconductor/metal interface is an important step for embedding layered materials in future electronic devices. We present direct measurement of the Schottky barrier height and its lowering in the transition metal dichalcogenide (TMD)/metal interface of a field effect transistor. It is found that the barrier height at the gold/ single-layer molybdenum disulfide (MoS2) interfaces decreases with increasing drain voltage, and this lowering reaches 0.5–1 V We also show that increase of the gate voltage induces additional barrier lowering.

Gap states and Fermi level pinning play an important role in all semiconductor devices, but even more in transition metal dichalcogenide-based devices due to their high surface to volume ratio and the absence of intralayer dangling bonds.

The discovery of layered materials, including transition metal dichalcogenides (TMD), gives rise to a variety of novel nanoelectronic devices, including fast switching field-effect transistors (FET), assembled heterostructures, flexible electronics, etc. Molybdenum disulfide (MoS2), a transition metal dichalcogenides semiconductor, is considered an auspicious candidate for the post-silicon era due to its outstanding chemical and thermal stability. We present a Kelvin probe force microscopy (KPFM) study of a MoS2 FET device, showing direct evidence for pinch-off formation in the channel by in situ monitoring of the electrostatic potential distribution along the conducting channel of the transistor. In addition, we present a systematic comparison between a monolayer MoS2 FET and a few-layer MoS2 FET regarding gating effects, electric field distribution, depletion region, and pinch-off formation in such devices.

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