Effect of substrate photoexcitation on channel conduction in a modulation-doped Al_xGa_1-xAs/GaAs heterostructure

If a modulation-doped AlGaAs/GaAs heterostructure is illuminated by light, photoexcitation of deep levels in the GaAs substrate leads to some interesting effects. Below 100 K, the heterostructure shows a persistent photoconductivity effect. Moreover, a strong persistent channel depletion is observed at low temperatures when a small negative voltage is applied to the substrate contact (backgate). The latter effect is explained by a double-layer model of GaAs where the GaAs side of the heterostructure consists of (1) a buffer layer and (2) a semi-insulating substrate. Under illumination, most of the applied negative voltage drops across the very thin buffer layer, and the enhanced electric field in the layer exerts a very strong influence on the conducting channel.