  We report the discovery of two-dimensional electron gas (2DEG) at the surface
of thermoelectric material In$_4$Se$_3$ by angle-resolved photoemission
spectroscopy. The observed 2DEG exhibits a nearly isotropic band dispersion
with a considerably small effective mass of m* = 0.16 m_0, and its carrier
density shows a significant temperature dependence, leading to unconventional
metal-semiconductor transition at the surface. The observed wide-range thermal
tunability of 2DEG in In$_4$Se$_3$ gives rise to additional degrees of freedom
to better control the surface carriers of semiconductors.
