  We study the problem of impurities and mid-gap states in a biased graphene
bilayer. We show that the properties of the bound states, such as localization
lengths and binding energies, can be controlled externally by an electric field
effect. Moreover, the band gap is renormalized and impurity bands are created
at finite impurity concentrations. Using the coherent potential approximation
we calculate the electronic density of states and its dependence on the applied
bias voltage.
