▎ 摘　　要

We consider the heating of carriers in an intrinsic graphene in a strong dc electric field. The intraband energy relaxation due to acoustic phonon scattering and the interband generation-recombination transitions due to thermal radiation are taken into account. The distributions of nonequilibrium carriers are obtained for the cases when the carrier-carrier scattering is unessential and when the intercarrier Coulomb scattering effectively establishes the quasiequilibrium distribution with the temperature and the density of carriers determined by the balance equations. Due to an interplay between weak energy relaxation and generation-recombination processes, the nonlinear response is characterized by a very low threshold electric field. The nonlinear current-voltage characteristics as well as the field-dependent carrier concentration are calculated for the case of the momentum relaxation associated with the elastic scattering. The obtained current-voltage characteristics exhibit a low threshold of nonlinearity and an appearance of the second ohmic region, for strong fields.