  Layered semiconductors show promise as channel materials for field-effect
transistors (FETs). Usually, such devices incorporate solid back or top gate
dielectrics. Here, we explore de-ionized (DI) water as a solution top gate for
field-effect switching of layered semiconductors including SnS2, MoS2, and
black phosphorus. The DI water gate is easily fabricated, can sustain rapid
bias changes, and its efficient coupling to layered materials provides high
on-off current ratios, a near-ideal sub-threshold swing, and enhanced
shortchannel behavior even for FETs with thick, bulk-like channels. Screening
by the high-k solution gate eliminates hysteresis due to surface and interface
trap states and substantially enhances the field-effect mobility. The onset of
water electrolysis sets the ultimate limit to DI water gating at large negative
gate bias. Measurements in this regime show promise for aqueous sensing,
demonstrated here by the amperometric detection of glucose in aqueous solution.
DI water gating of layered semiconductors can be harnessed in research on novel
materials and devices, and it may with further development find broad
applications in microelectronics and sensing.
