  In the past two decades another transistor based on conducting polymers,
called the organic electrochemical transistor (ECT) was shown and largely
studied. The main difference between organic ECTs and FETs is the mode and
extent of channel doping: while in FETs the channel only has surface doping
through dipoles, the mixed ionic-electronic conductivity of the channel
material in Organic ECTs enables bulk electrochemical doping. As a result, the
organic ECT maximizes conductance modulation at the expense of speed. Until now
ECTs have been based on conducting polymers, but here we show that MXenes, a
class of 2D materials beyond graphene, have mixed ionic-electronic properties
that enable the realization of electrochemical transistors (ECTs). We show that
the formulas for organic ECTs can be applied to these 2D ECTs and used to
extract parameters like mobility. These MXene ECTs have high transconductance
values but low on-off ratios. We further show that conductance switching data
measured using ECT, in combination with other in-situ ex-situ electrochemical
measurements, is a powerful tool for correlating the change in conductance to
that of redox state: to our knowledge, this is the first report of this
important correlation for MXene films. Many future possibilities exist for
MXenes ECTs, and we think other 2D materials with bandgaps can also form ECTs
with single or heterostructured 2D materials. 2D ECTs can draw great
inspiration and theoretical tools from the field of organic ECTs and have the
potential to considerably extend the capabilities of transistors beyond that of
conducting polymer ECTs, with added properties such as extreme heat resistance,
tolerance for solvents, and higher conductivity for both electrons and ions
than conducting polymers.
