  We report the synthesis and characterization of a Si-based ternary
semiconductor Mg$_3$Si$_2$Te$_6$, which exhibits a quasi-two-dimensional
structure, where the trigonal Mg$_2$Si$_2$Te$_6$ layers are separated by Mg
ions. Ultraviolet-visible absorption spectroscopy and density functional theory
calculations were performed to investigate the electronic structure. The
experimentally determined direct band gap is 1.39 eV, consistent with the value
of the density function theory calculations. Our results reveal that
Mg$_3$Si$_2$Te$_6$ is a direct gap semiconductor with a relatively narrow gap,
which is a potential candidate for infrared optoelectronic devices.
