High Photoresponsivity Multilayer MoS2 Thin-Film Transistors with Local Bottom Gate Structure

Abstract

2D layered transition metal dichalcogenides (TMDs), especially MoS2, has received great attention for next-generation semiconductor devices because their thin film transistors (TFTs) show a nearly ideal subthreshold swing (SS ≈ 70 mV decade-1), high on/off current ratio (I on/I off ≈ 108), and high field-effect mobility (µ eff > 100 cm2 V-1 s-1). In addition, TMDs have the optical characteristic of tuning bandgap from direct to indirect with increasing their layers. Even though photoresponsivity of multilayer MoS2 phototransistor due to its nature of indirect bandgap is lower than that of single layer MoS2, multilayer MoS2 has higher density-of-state and wider spectral response than single layer MoS2. In order to enhance photoresponsivity of multilayer MoS2 phototransistor, we suggest the phototransistor with local bottom gate structure (LBGS), where the bottom gate length of the device is shorter than the channel length. The underlap regions of between gate and channel work like the series resistance in dark state due to ineffective gate modulation at the region. To demonstrate the enhancement of phototransistor performance using LBGS, we previously studied amorphous silicon TFT with LBGS. The photosensitivity enhancement up to 64 times as compared to the conventional a-Si TFT. And then, we used multilayer MoS2 as a channel material to constitute phototransistor with LBGS. The giant photoreponsivity of 342.6 AW-1 at 2 mW cm-1 shows three orders of magnitude larger than that of global gate multilayer MoS2 TFTs. This result of our experiment and simulation reveals that multilayer MoS2 phototransistor with local bottom gate can be used as various photosensing devices.