Photoreflectance (PR) and reflectance have been applied to characterize undoped and modulation‐doped heterostructures of AlGaAs/GaAs grown by metal‐organic chemical vapor deposition. The PR spectra were taken on these samples after sequential etching steps in a phosphoric acid etch to study the effects of the surface electric field, the heterointerface, and the two‐dimensional electron gas. PR spectra were also taken with an external electric field applied through a transparent gate electrode. The results show that the oscillations appearing near the bandgap energy of GaAs are Franz–Keldysh oscillations originating from the large surface electric field. The surface electric field of the heterostructures can be modified through the application of an external electric field or by etching. The oscillation period is observed to increase with increasing reverse bias or with etching of the GaAs cap layer and the PR features disappear at a forward bias of 0.45 V. The very sharp features associated with the GaAs bandgap energy after etching have also been verified to be Franz–Keldysh oscillations and the presence of a two‐dimensional electron gas cannot be confirmed with PR.