The applied radiation excites "holes". The condensed composite (c)-bosons formed in the excited channel create a superconducting state with an energy gap. The supercondensate suppresses the non-condensed c-bosons at low temperatures, but it cannot suppress the c-fermions in the base channel, and the small normal current accompanied by the Hall field yields a B-linear Hall resistivity.
@article{arxiv.cond-mat/0402174,
title = {On the zero-resistance states generated by radiation in GaAs/AlGaAs},
author = {S. Fujita and K. Ito},
journal= {arXiv preprint arXiv:cond-mat/0402174},
year = {2007}
}