(a) Schematic representation of back gated GaS FET with tungsten/WS2 electrodes featuring band energy diagram. (b) Raman spectrum of the W electrodes after sulfurization. Phonon modes of WS2 are indicated demonstrating the growth of WS2 on the skin of the electrodes. (c) Optical images of the fabricated devices and electrode gap (inset). The scale bars are 500 and 20 μm, respectively. The fabrication process is outlined in Supplementary Fig. 8. We tested five FETs at different locations and the mobility measurements have been consistent. (d) I/V characteristics of the FET at different gate voltages. (e) I/V phototransistor characteristics with no gate bias.