  We analyze the driven Harper model, which appears in the problem of
tight-binding electrons in the Hall configuration (normal to the lattice plane
magnetic field plus in-plane electric field). The presence of an electric field
extends the celebrated Harper model, which is parametrized by the Peierls
phase, into the driven Harper model, which is additionally parametrized by two
Bloch frequencies associated with the two components of the electric field. We
show that the eigenstates of the driven Harper model are either extended or
localized, depending on the commensurability of Bloch frequencies. This results
holds for both rational and irrational values of the Peierls phase. In the case
of incommensurate Bloch frequencies we provide an estimate for the
wave-function localization length.
