  The tight-binding model of bilayer graphene is used to find the gap between
the conduction and valence bands, as a function of both the gate voltage and as
the doping by donors or acceptors. The total Hartree energy is minimized and
the equation for the gap is obtained. This equation for the ratio of the gap to
the chemical potential is determined only by the screening constant. Thus the
gap is strictly proportional to the gate voltage or the carrier concentration
in the absence of donors or acceptors. In the opposite case, where the donors
or acceptors are present, the gap demonstrates the asymmetrical behavior on the
electron and hole sides of the gate bias. A comparison with experimental data
obtained by Kuzmenko et al demonstrates the good agreement.
