Dielectric-Free Molybdenum Disulfide Transistors with In-Plane Gates

In this work, we realize bilayer molybdenum disulfide (MoS₂) transistors with in-plane gates on sapphire substrates. Through sequential transferring of MoS₂, e-beam lithography, and metal lift-off, a device with channel width/length of 500:400 nm is fabricated. With a 250 nm separation between the in-plane gate and MoS₂ channel, a drain current as high as 37 μA with a clear saturation region is observed. The device shows a high ON/OFF current ratio over 10⁹, small gate bias required for current modulation, and high responsivity about 230 A/W when operated in the phototransistor mode. The gain of the phototransistor reaches 432. A quick calculation from the transfer curve using the formula of conventional transistors gives a field-effect mobility of 6365.9 cm²·V^-1·s^-1, which, although overestimated, still suggests good performance of the device. With an additional MoS₂ layer to isolate the MoS₂ channel from the influence of the substrate, this dielectric-free in-plane gate transistor has exhibited potential in electronics. The high responsivity of the device under relatively low applied voltages is also promising for weak-light detection.