The influence of near surface structure termination and surface treatments on the surface electric fields and mobility of modulation doped AlGaAs/GaAs heterostructures (MDH) were investigated. The built-in and surface electric fields were evaluated by photoreflectance spectroscopy, and these values were utilized to simulate the conduction band bending of the MDH. When the capping layer of the MDH was changed, both the built in internal electric field and the surface electric field are decreased, while the electron mobility of the samples is increased. After passivated the surface samples with Si, the surface electric fields were also reduced. Finally, a (NH4)2S x-based treatment of the surface was applied, the surface electric field is annulled, and the conduction band modeling showed an important redistribution of carriers in the films. The electron mobility of the passivated samples does not show any change, neither the internal electric fields, corroborating the close relationship that exist between these two parameters. © 2014 American Vacuum Society.