Phototransistors based on two dimensional semiconductors have drawn increasing attention in recent years. GaSe is a typical semiconductor with a layered structure. In this work, the ultrathin GaSe nanosheets were exfoliated from commercially available crystals using a micromechanical cleavage technique. Then, the nanosheets were used to fabricate field effect transistors (FETs) on Si/SiO2 substrates with interdigitated electrodes. The electrical and optoelectronic properties of the FET were characterized. The phototransistor based on a GaSe nanosheet had a high photoresponsivity (∼2200 mA/W) and a high Iphoto/Idark (photoresponse current over dark current) ratio of almost 103.

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