High Mobility, High Carrier Density SnSe₂ Field-Effect Transistors with Ultralow Subthreshold Swing and Gate-Controlled Photoconductance Switching

2D and layered semiconductors are considered as promising electronic materials, particularly for applications that require high carrier mobility and efficient field-effect switching combined with mechanical flexibility. To date, however, the highest mobility has been realized primarily at low carrier concentration. Here, it is shown that few-layer/multilayer SnSe₂ gated by a solution top gate combines very high room-temperature electron mobility (up to 800 cm² V⁻¹s⁻¹), along with large on-off current ratios (>10⁵) and a subthreshold swing below the thermodynamic limit (50 mV per decade) in field-effect devices, at exceptionally large sheet carrier concentrations of ≈10¹³ cm⁻². Observed mobility enhancements upon partial depletion of the channel point to near-surface defects or impurities as the mobility-limiting scattering centers. Under illumination, the resulting gap states give rise to gate-controlled switching between positive and negative photoconductance. The results qualify SnSe₂ as a promising layered semiconductor for flexible and wearable electronics, as well as for the realization of advanced approaches to photodetection.