Nonlinear hot carrier transport features and relatedoptical nonlinearities are investigated under nonuniformillumination and carrier heating in microwave (mw)-biased high-resistivity GaAs crystals. The dynamics of light-interferencepattern-induced nonequilibrium carriers and evolution of internal electric field are studied in mw fieldsextending above a region of negative differential conductivity.We analysed the peculiarities of nonlinear transport in a spatiallymodulated structure at various photoexcited carrierconcentrations, which was kept below its threshold for mw-induced high-field Gunn-domain grating formation. Atlow-illumination levels, we found a three-fold increase in thespace-charge (SC) field due to electron gas heating. At higher lightintensities, enhancement of the internal field is determined bya fast screening of the external mw field and the transportof nonuniformly heated carriers. Hot carrier transport effectsin a region of negative differential conductivity, even atnearly homogeneous but high illumination, lead to SC-wave formation with amplitudes, which may exceed the diffusiveSC-field amplitude by 100 times. The latter effectwas confirmed experimentally using a four-wave mixingtechnique in mw-biased GaAs crystals and short laserpulses.