Measurements of Franz–Keldysh (FK) effects in GaAs‐AlxGa1−xAs heterostructures by variable angle of incidence spectroscopic ellipsometry are reported. The measured FK effects are due to the built‐in band bending at the surface and heterointerfaces, with no externally applied bias or modulating voltage. The polarity of the FK line shape indicates whether the field is uniform or nonuniform (inhomogeneous) in the direction along the growth axis, while the linewidth is determined by the peak field strength. This information can be used to characterize the doping concentration in the AlxGa1−xAs layer.