  In this work we introduce a low-energy Hamiltonian for single layer and
bilayer black phosphorus that describes the electronic states at the vicinity
of the gamma point. The model is based on a recently proposed tight-binding
description for electron and hole bands close to the Fermi level. We calculate
expressions for the Landau level spectrum as function of magnetic field and in
the case of bilayer black phosphorus we investigate the effect of an external
bias on the electronic band gap. The results showcase the highly anisotropic
character of black phosphorus and in particular for bilayer BP, the presence of
bias allows for a field-induced semiconductor-metal transition.
