## 3 Results and Discussion
where the channel length, \(L\), is 1 mm, the channel width, \(W\), is 1.2 mm, and the gate capacitance, \(C_{g}\), is 115 aF/\(\upmu\)m\({}^{2}\) for a 300 nm thick SiO\({}_{2}\) layer [9]. The mobility of the device was calculated to be 65.2 cm\({}^{2}\) V\({}^{-1}\)s\({}^{-1}\), which is comparable with previous results of similar back-gated FET devices [18]. However, the mobility was lower than that obtained from the top-gated FET. This difference in the mobility was due to the existence of trap states within the SiO\({}_{2}\) dielectric layer at the bottom gate. Moreover, the device exhibited an on/off current ratio in the order of \(10^{3}\), and a threshold gate bias of about \(-5\) V, whereas the MoS\({}_{2}\) PFET was positively biased under 4 V. This indicates that a large negative V\({}_{g}\) (less than or equal to \(-5\) V) is needed to tune the electron-rich layers of the bottom untreated MoS\({}_{2}\) layers into hole-rich layers. The energy band structures of the device under a negative gate with applied bias are shown in Figure 4d-f, where the conduction band (CB) and Fermi level (E\({}_{\text{F}}\)) of multilayer MoS\({}_{2}\) were assumed to be 4.2 eV and 4.7 eV, respectively. From Figure 4d, the work function of Au was found to be about 5.1 eV. Although the E\({}_{\text{F}}\) of the MoS\({}_{2}\) was shifted toward the valence band (VB) due to \(p\)-type doping, the E\({}_{\text{F}}\) of Au still lay on the top half of the MoS\({}_{2}\) band. This led to a large Schottky barrier for holes along the Au/MoS\({}_{2}\) interface. As shown in Figure 4e, for V\({}_{\text{g(th)}}\)\(<\) V\({}_{\text{g}}\)\(<\) 0, holes could only pass through the barrier at a high V\({}_{\text{ds}}\) due to the bending of the MoS\({}_{2}\) channel. As shown in Figure 4f, as the negative V\({}_{\text{g}}\) increased, for V\({}_{\text{g}}\)\(<\) V\({}_{\text{g(th)}}\)\(<\) 0, the hole barrier became thinner and allowed holes to penetrate through. This was consistent with the threshold gate bias (\(-5\) V) obtained in Figure 4c. The drain current was greatly enhanced under a negative gate bias less than \(-5\) V.  
MoS\({}_{2}\) is sensitive to light illumination and is able to generate photoexcited electron-hole pairs from incident light. Figure 5a displays the photoinduced I\({}_{\text{ds}}\)-V\({}_{\text{ds}}\) output curves of the MoS\({}_{2}\) PFET excited by ultraviolet (\(\lambda\) = 365 nm) light (UV lamp SLUV-8, intensity 1407 \(\upmu\)W/cm\({}^{2}\)). The photocurrent generated at V\({}_{\text{g}}\) from \(-10\) to 0 V suggests that the MoS\({}_{2}\) PFET can be used as a phototransistor for UV light  
Figure 4: (a) Schematic diagram of the back-gated field-effect transistor (FET). Source and drain metal contacts are deposited on the untreated MoS\({}_{2}\) region. F atoms are incorporated into the upper regions of the multilayer MoS\({}_{2}\). (b) I\({}_{\text{ds}}\)–V\({}_{\text{ds}}\) graph of the FET device at different gate bias values ranging from 0 to \(-10\) V. (c) I\({}_{\text{ds}}\)–V\({}_{\text{g}}\) graph displaying transfer characteristic of the MoS\({}_{2}\)\(p\)-type FET for V\({}_{\text{ds}}\) = 4 V. (d–f) Band diagram of Au/MoS\({}_{2}\) (d) under equilibrium condition, (e) with V\({}_{\text{g(th)}}\)\(<\) V\({}_{\text{g}}\)\(<\) 0, and (f) with V\({}_{\text{g}}\)\(<\) V\({}_{\text{g(th)}}\)\(<\) 0.  
detection. Based on the photocurrent generated under different values of V\({}_{\text{g}}\), the photoresponsivity as a function of V\({}_{\text{g}}\) is plotted in Figure 5b. The photoresponsivity increased from 0.45 A/W under zero gate voltage to 9.3 A/W under the gate voltage of 10 V, with an incident light power of 16 \(\mu\)W and V\({}_{\text{ds}}\) set at 4 V. The gate voltage-dependent photoresponsivity in the MoS\({}_{2}\) FET was due to the _p_-type doping of MoS\({}_{2}\). Under the negative V\({}_{\text{g}}\), E\({}_{\text{F}}\) moved from the CB to the VB of the _p_-MoS\({}_{2}\), forming a smaller Schottky barrier between the CB of MoS\({}_{2}\) and the E\({}_{\text{F}}\) of the Au electrode.