  We report a persistent increase or decrease in the two-dimensional electron
density of AlAs or GaAs quantum wells flanked by AlGaAs barriers, brought about
by illuminating the samples at T ~ 4 K while simultaneously applying a voltage
bias between a back gate and the two-dimensional electron gas. Control of the
final carrier density is achieved by tuning the back gate bias during
illumination. Furthermore, the strength of the persistent photoconductivity
depends on the Al mole fraction in the back Al_{x}Ga_{1-x}As barrier, and is
largest at $x \simeq 0.4$.
