We have studied the photoreflectance spectra at 300 K from a number of GaAs/Ga1−xAlxAs heterojunction bipolar transistor (HBT) structures grown by molecular beam epitaxy and metalorganic chemical vapor deposition. From the observed Franz–Keldysh oscillations we have been able to evaluate the built‐in dc electric fieldsFdcin the Ga1−xAlxAs emitter as well as then−‐GaAs collector region. In addition, the Ga1−xAlxAs band gap (and hence Al composition) has been determined. The obtained values ofFdcare in good agreement with numerically computed values for the analyzed HBT structures, thus making it possible to deduce doping levels in these sections.