  We develop a theory of nonlinear response to an electric field of a
two-dimensional electron gas (2DEG) placed in a classically strong magnetic
field. The latter leads to a non-linear current-voltage characteristic at a
relatively weak electric field. The origin of the non-linearity is two-fold:
the formation of a non-equilibrium electron distribution function, and the
geometrical resonance in the inter-Landau-levels transitions rates. We find the
dependence of the current-voltage characteristics on the electron relaxation
rates in the 2DEG.
