  We have addressed the microscopic transport mechanism at the switching or
on-off transition in transition metal dichalcogenide (TMDC) field-effect
transistors (FET), which has been a controversial topic in TMDC electronics,
especially at room temperature. With simultaneous measurement of channel
conductivity and its slow time-dependent fluctuation (or noise) in ultra-thin
WSe2 and MoS2 FETs on insulating SiO2 substrates, where noise arises from
McWhorter-type carrier number fluctuations, we establish that the switching in
conventional backgated TMDC FETs is a classical percolation transition in a
medium of inhomogeneous carrier density distribution. From the experimentally
observed exponents in the scaling of noise magnitude with conductivity, we
observe unambiguous signatures of percolation in random resistor network,
particularly in WSe2 FETs close to switching, which crosses over to continuum
percolation at a higher doping level. We demonstrate a powerful experimental
probe to the microscopic nature of near-threshold electrical transport in TMDC
FETs, irrespective of the material detail, device geometry or carrier mobility,
which can be extended to other classes of 2D material-based devices as well.
