  The photoconductivity of GaAs structures delta-doped by Sn has been
investigated for wavelengths lambda= 650-1200 nm in the temperature interval T=
4.2-300 K. The electron densities and mobilities, before and after
illumination, have been determined by magnetoresistance, Shubnikov-de Haas
effect and Hall effect measurements, in high magnetic fields. For the heavily
doped structures (n_H> 2x10^13 cm^-2) we observe under illumination by light
with wavelengths larger than the band-gap wavelength of the host material
(lambda= 815 nm at T= 4.2 K) first positive (PPPC) and then negative (NPPC)
persistent photoconductivity. The NPPC is attributed to the ionisation of DX
centres and PPPC is explained by the excitation of electrons from Cr impurity
states in the substrate. For lambda< 815 nm in addition the excitation of
electron over the band gap of GaAs contributes to the PPPC. For the lightly
doped structures (n_H<= 2x10^13 cm^-2) the photoconductivity effect is always
positive.
