  Electronic localization is numerically studied in disordered bilayer graphene
with an electric-field induced energy gap. Bilayer graphene is a zero-gap
semiconductor, in which an energy gap can be opened and controlled by an
external electric field perpendicular to the layer plane. We found that, in the
smooth disorder potential not mixing the states in different valleys (K and K'
points), the gap opening causes a phase transition at which the electronic
localization length diverges. We show that this can be interpreted as the
integer quantum Hall transition at each single valley, even though the magnetic
field is absent.
