In some devices based on GaAs/AlxGa1-xAs heterostructures, the AlxGa1-xAs plays the role of a wide band gap "insulator". These devices are therefore excellent systems for studying charge trapping in AlxGa1-xAs. It is a poorly understood property of AlxGa1-xAs that incorporation of any n-type dopant results in the formation of a deep electron trap, the DX center. Recent experiments on heterostructure devices have probed both thermal and athermal (hot electron) capture and emission by the DX center. By observing the trapping behavior as the composition (Al mole fraction) of the alloy is varied, the relationship between the trap level and the band structure of the host material has been clarified. A remarkable result is the observation of electron trapping at alloy compositions where the trap state is resonant with the conduction band. © 1987.