  2DEG states of Al/$\delta$-GaAs structures were investigated in the
persistent tunneling photoconductivity (PTPC) regime at low temperatures.
"Thickening" of the unoccupied subbands to the ground state of 2DEG was
observed at $T=4.2$~$K$. It was found that there is a uniform shift of the
subband levels at $T=77$~$K$. The behavior of the persistent 2D states in
tunneling spectra after various illumination and at different temperatures
allows us to assume that there are two PTPC mechanisms, namely, the broadening
of the potential well profile of the $\delta$-layer and photoionization of the
DX-centers.
