## 3 Results and Discussion  
The electrical characterization of the PdSe\({}_{2}\) back-gated field-effect transistor is reported in Figure 2. The drain-source current-voltage, I\({}_{\text{ds}}\)-V\({}_{\text{ds}}\), output characteristics (Figure 2a), measured in a high vacuum and at room temperature, are linear and demonstrate the formation of a good ohmic contact between PdSe\({}_{2}\) and Ti. The I\({}_{\text{ds}}\)-V\({}_{\text{gs}}\) transfer curve, reported in Figure 2b, was measured by fixing a bias V\({}_{\text{ds}}\) = 100 mV between the source and the drain, and sweeping the gate bias V\({}_{\text{gs}}\) forward from \(-\)80 V to \(+\)80 V and then backward from \(+\)80 V to \(-\)80 V. The transfer curve has a hysteretic behavior, often observed in 2D material-based field effect transistors (FET), and it can be explained in terms of adsorbates on the channel surface, trap states in the dielectric or at the dielectric/channel interface or due to intrinsic defects [7; 14; 15; 16]. In particular, trap states can be filled (or emptied) during the gate bias sweep and can participate in the gating effect, originating the hysteresis [17; 18]. Figure 2: PdSe\({}_{2}\) back-gated FET characterization in high vacuum. (**a**) Output curves I\({}_{\text{ds}}\)–V\({}_{\text{ds}}\); (**b**) transfer curve I\({}_{\text{ds}}\)–V\({}_{\text{gs}}\). Comparison of transfer curves of the same device at atmospheric pressure and at high vacuum: (**c**) transfer curve measured in high vacuum for a forward gate bias sweep (from \(-\)50 V to +50 V); (**d**) transfer curve measured at 760 Torr; (**e**) transfer curve measured after keeping the transistor at 760 Torr for 8 h; (**f**) transfer curve measured in high vacuum again. The measured curve shows a clear ambipolar behavior and a current modulation of about one order of magnitude. The observed ambipolar behavior gives an indication that we are dealing with a semiconducting material characterized by a small band gap. We note that the band gap amplitude is expected to depend on the number of layers. According to the nanosheet characterization, the PdSe\({}_{2}\) layer under investigation is about 15-nm thick, i.e., about 25 layers [10], which should correspond to a bandgap less than 100 meV [19]. The transfer curve reported in Figure 2b shows a conductance minimum at about V\({}_{\text{gs}}\) = 0 V that is indicative of the midgap alignment of the Fermi levels of the contacts. These curves also allowed us to obtain an estimation of the field-effect mobility, using the following equation:  
\[\mu=\frac{\text{L}}{\text{W}}\bigg{(}\frac{1}{C_{\text{SiO2}}\cdot\text{V}_{ \text{ds}}}\bigg{)}\frac{\text{d}\text{I}_{\text{ds}}}{\text{dV}_{\text{gs}}} \tag{1}\]  
where L and W are the channel length and width, respectively, and C\({}_{\text{SiO2}}\) is the capacitance per unit area of SiO\({}_{2}\). We can calculate \(C_{\text{SiO2}}=\frac{(\epsilon_{0}\cdot\epsilon_{\text{SiO2}})}{\text{t}_{ \text{SiO2}}}=0.11\cdot 10^{-3}\) F/m\({}^{2}\), considering that \(\epsilon_{0}\) is the vacuum permittivity, \(\epsilon_{\text{SiO2}}\) = 3.9 is the relative permittivity of SiO\({}_{2}\) and t\({}_{\text{SiO2}}\) = 300 nm is the thickness of SiO\({}_{2}\). Then, evaluating the slope \(\frac{\text{d}\text{I}_{\text{ds}}}{\text{dV}_{\text{gs}}}\) of the I\({}_{\text{ds}}\) -V\({}_{\text{gs}}\) curve in the linear region, we obtain the field-effect mobility, \(\mu\approx\) 3.0 cm\({}^{2}\)V\({}^{-1}\)s\({}^{-1}\), within the range of values typically reported (0.01-100 cm\({}^{2}\)V\({}^{-1}\)s\({}^{-1}\)) for TMD-based FETs on SiO\({}_{2}\)[20; 21; 22; 23; 24; 25]. In Figure 2c-f, we report the evolution of the transfer characteristics measured in the presence of different pressure conditions, i.e., at atmospheric pressure and in a high vacuum (10\({}^{-6}\) Torr), in order to verify the effects of the pressure on the transistor properties. We first show the curve measured in a high vacuum (Figure 2c) and then the curve measured at 760 Torr (Figure 2d).