  We study the geometric and electronic structures of silicene monolayer using
density functional theory based calculations. The electronic structures of
silicene show that it is a semi-metal and the charge carriers in silicene
behave like massless Dirac-Fermions since it possesses linear dispersion around
Dirac point. Our results show that the band gap in silicene monolayer can be
opened up at Fermi level due to an external electric field by breaking the
inversion symmetry. The presence of buckling in geometric structure of silicene
plays an important role in breaking the inversion symmetry. We also show that
the band gap varies linearly with the strength of external electric field.
Further, the value of band gap can be tuned over a wide range.
