In this article, we report on the growth by molecular beam epitaxy of a series of modulation-doped AlGaAs/AlAs/GaAs structures designed to work as quantum well infrared photodetectors in the 3-5 mum transmission atmospheric window. Growth conditions based on particular aspects of the surface reconstructions observed in the constituent layers are used. The performance of the modulation doped samples is analyzed by a number of characterization techniques and compared with that of a detector with doping in the well and otherwise the same structure. Responsivity measurements performed at various bias voltages reveal a strong photovoltaic behavior of the detectors. The value of the zero volts responsivity and of the external voltage required to quench the photovoltaic signal are highly dependent on the doping profile. A controlled photovoltaic operation may then be achieved by a proper design of the dopant location. (C) 2003 American Vacuum Society. [DOI: 10.1116/1.1562643].