  The Nernst effect, a transverse thermoelectric phenomenon, has attracted
significant attention for its potential in energy conversion, thermoelectrics,
and spintronics. However, achieving high performance and versatility at low
temperatures remains elusive. Here, we demonstrate a large and electrically
tunable Nernst effect by combining graphene's electrical properties with indium
selenide's semiconducting nature in a field-effect geometry. Our results
establish a novel platform for exploring and manipulating this thermoelectric
effect, showcasing the first electrical tunability with an on/off ratio of
10^3. Moreover, photocurrent measurements reveal a stronger photo-Nernst signal
in the Gr/InSe heterostructure compared to individual components. Remarkably,
we observe a record-high Nernst coefficient of 66.4 {\mu}V K^(-1) T^(-1) at
ultra-low temperatures and low magnetic fields, paving the way toward
applications in quantum information and low-temperature emergent phenomena.
