A theoretical study of nonlinear charge transport in polar semiconductors is presented. It is based on a nonequilibrium statistical ensemble formalism which provides a generalized Boltzmann-style nonlinear quantum kinetic theory. The mobility and the diffusion coefficients are obtained and, relating both, a Nernst-Townsend-Einstein relation is derived extended to the nonlinear regime (i.e., outside the Ohmic domain). Numerical calculations are performed considering the particular case of the strongly polar III nitrides, which have application in blue-emitting diodes.997