  Heating of carriers in an intrinsic graphene under dc electric field is
considered taking into account the intraband energy relaxation due to acoustic
phonon scattering and the interband generation-recombination transitions due to
thermal radiation. The distribution of nonequilibrium carriers is obtained for
the cases when the intercarrier scattering is unessential and when the
carrier-carrier Coulomb scattering effectively establishes the quasiequilibrium
distribution with the temperature and the density of carriers that are
determined by the balance equations. Because of an interplay between weak
energy relaxation and generation-recombination processes a very low threshold
of nonlinear response takes place. The nonlinear current-voltage
characteristics are calculated for the case of the momentum relaxation caused
by the elastic scattering. Obtained current-voltage characteristics show low
threshold of nonlinear behavior and appearance of the second ohmic region, for
strong fields.
