  We theoretically investigate effect of a transverse electric field generated
by side gates and a vertical electric field generated by top, back gates on
energy bands and transport properties of zigzag bilayer graphene ribbons
(Bernal stacking). Using atomistic Tight Binding calculations and Green
function formalism we demonstrate that bandgap is opened when either field is
applied and even enlarged under simultaneous influence of the two fields.
Interestingly, although vertical electric fields are widely used to control
bandgap in bilayer graphene, here we show that transverse fields exhibit more
positive effect in terms of modulating a larger range of bandgap and retaining
good electrical conductance. Seebeck effect is also demonstrated to be enhanced
strongly about 13 times for a zigzag bilayer graphene ribbons with 16 chain
lines. These results may motivate new designs of devices made of bilayer
graphene ribbons using electric gates.
