  We propose a new concept of two-dimensional (2D) Dirac semiconductor which is
characterized by the emergence of fourfold degenerate band crossings near the
band edge and provide a generic approach to realize this novel semiconductor in
the community of material science. Based on the first-principle calculations
and symmetry analysis, we discover recently synthesised triple-layer (TL)-BiOS2
is such Dirac semiconductor that features Dirac cone at X/Y point, protected by
nonsymmorphic symmetry. Due to sandwich-like structure, each Dirac fermion in
TL-BiOS2 can be regarded as a combination of two Weyl fermions with opposite
chiralities, degenerate in momentum-energy space but separated in real space.
Such Dirac semiconductor carries layer-dependent helical spin textures that
never been reported before. Moreover, novel topological phase transitions are
flexibly achieved in TL-BiOS2: (i) an vertical electric field can drive it into
Weyl semiconductor with switchable spin polarization direction, (ii) an
extensive strain is able to generate ferroelectric polarization and actuate it
into Weyl nodal ring around X point and into another type of four-fold
degenerate point at Y point. Our work extends the Dirac fermion into
semiconductor systems and provides a promising avenue to integrate spintronics
and optoelectronics in topological materials.
