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The sensitivity of carrier concentration and mobility of Al0.25Ga0.75As/GaAs modulation-doped structures to light exposure has been studied. The light sensitivity of both parameters was found to be strongly dependent on the growth temperature of the Al0.25Ga0.75As which was varied between 580Â°C and 695Â°C. A growth temperature of about 610Â°C was observed to produce the least light sensitivity, while a growth temperature of 660Â°C resulted in the most sensitivity to light. The corresponding minimum and maximum changes in carrier concentration, measured at 10 K, were 5% and 52% of the dark values, respectively, while changes in mobility were 2% and 40%, respectively. A persistent photoconductivity effect was also observed in the samples. The minimum in this effect was found to be a 1% change in 10 K carrier concentration at a growth temperature of 610Â°C.