## 3 Results and Discussion  
The output and transfer curves of the devices with four types of metallic electrodes are shown in Figure 4. We can find that the devices are n-type depletion FETs as the current appeared in the negative coordinates in the transfer curve with the drain-source voltage of 10 V, 20 V and 30 V, as shown in Figure 4e-h. This result is consistent with previous studies. Meanwhile, we obtained the on/off ratio, threshold voltage (V\({}_{\text{th}}\)) from the curves and calculated the carrier mobility [15] through the Equation (1). Where \(W\) denotes the channel width, \(L\) denotes channel length, \(C_{i}\) stands for the capacitance per unit area of silicon oxide. \[\mu_{carrier}=\frac{W}{L}\text{C}_{\text{i}}V_{DS}I_{DS}(V_{\text{GS}}-V_{th}) \tag{1}\]  
The characteristics of the devices are as shown in Table 1. We can find that the device with silver electrode has the relative ideal performance with a carrier mobility of 23.15 cm\({}^{2}\)/Vs, the largest on/off ratio of \(1.23\times 10^{4}\) and the lowest threshold voltage of 2.21 V. This phenomenon may attributed to the fact that the work function of silver (4.26 eV) is close to the conduction band of molybdenum disulfide. Figure 3: Microscope images of FET devices with the electrode of (**a**) Au; (**b**) Ag; (**c**) Al; (**d**) Cu. Figure 2: Molybdenum disulfide (MoS\({}_{2}\)) FET devices structure. Figure 4: Output curve of (**a**) Au, (**b**) Ag, (**c**) Al, (**d**) Cu and transfer curve of (**e**) Au, (**f**) Ag, (**g**) Al, (**h**) Cu. The property of metal-semiconductor contact is essential to modern electronics and optoelectronics [17]. According to the thermionic emission theory, the mean free path of the majority of carriers (electrons) in n-type semiconductor-metal contact is much larger than the Schottky barrier width, so the collision of electrons in the barrier region can be neglected. The influence of the barrier shape is not prominent, and the Schottky barrier height (SBH) plays a decisive role here. SBH represents the energy required for a single charge carrier to transfer between semiconductor and metal. It fundamentally determines the efficiency of charge transfer and directly affects device performance [18]. Meanwhile, SBH is derived from the difference of work functions between metal and semiconductor, which can lead to discrepancies in barrier height and charge injection quantity [19]. All of these factors will eventually cause an obvious disparity in on-state current, which is presented on the transfer characteristic curve. Figure 5 is the on-state current of MoS\({}_{2}\) devices with four metallic electrodes at three types of drain-source voltage. The difference in the on-state currents in the transfer curve is less than 0.63 orders of magnitude that cannot fit with the gaps in the work functions of the four metals. As mentioned above, electrons in semiconductors can enter metals through barriers as long as they have enough energy to cross barriers. Similarly, electrons in metals can also enter semiconductors. Based on the above conditions, the calculation of current can be reduced to the calculation of the number of carriers crossing the barrier, as shown in Equation (2) [16], where A is the Richardson constant (120 A/cm\({}^{2}\)-K\({}^{2}\)), T is the thermodynamic temperature (300 K), K\({}_{0}\) is the Boltzmann constant (\(1.380\times 10^{-23}\) J/K), V\({}_{\text{DS}}\) is the drain-source voltage (10 V), \(q\Phi_{SB}\) is SBH, q is the electron charge (\(1.602\times 10^{-19}\) C), and J is the current density between semiconductor and metal. It is known that the \begin{table}
\begin{tabular}{c c c c} \hline
**Electrode** & **Mobility (cm2/Vs)** & **On/Off Ratio** & **Threshold Voltage(V)** \\ \hline Au & 21.01 & \(2.18\times 10^{1}\) & –3 \\ Ag & 23.15 & \(1.23\times 10^{4}\) & 2.21 \\ Al & 5.35 & \(4.14\times 10^{0}\) & –55 \\ Cu & 40.52 & \(2.25\times 10^{2}\) & –55 \\ \hline \end{tabular}
\end{table} 
Table 1: Characteristic data of the MoS\({}_{2}\)-FET devices (V\({}_{\text{DS}}\) = 10 V).