  Reliable and precise measurements of the relative energy of band edges in
semiconductors are needed to determine band gaps and band offsets, as well as
to establish the band diagram of devices and heterostructures. These
measurements are particularly important in the field of two-dimensional
materials, in which many new semiconducting systems are becoming available
through exfoliation of bulk crystals. For two-dimensional semiconductors,
however, commonly employed techniques suffer from difficulties rooted either in
the physics of these systems, or of technical nature. The very large exciton
binding energy, for instance, prevents the band gap to be determined from a
simple spectral analysis of photoluminescence, and the limited lateral size of
atomically thin crystals makes the use of conventional scanning tunneling
spectroscopy cumbersome. Ionic gate spectroscopy is a newly developed technique
that exploits ionic gate field-effect transistors to determine quantitatively
the relative alignment of band edges of two-dimensional semiconductors in a
straightforward way, directly from transport measurements (i.e., from the
transistor electrical characteristics). The technique relies on the extremely
large geometrical capacitance of ionic gated devices that -- under suitable
conditions -- enables a change in gate voltage to be directly related to a
shift in chemical potential. Here we present an overview of ionic gate
spectroscopy, and illustrate its relevance with applications to different
two-dimensional semiconducting transition metal dichalcogenides and van der
Waals heterostructures.
