Characterization of GaAs/Ga1−xAlxAs heterojunction bipolar transistor structures using photoreflectance

We have studied the photoreflectance spectra at 300 K from a number of GaAs/Ga1−x AlxAs 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 fields Fdc in the Ga1−x Alx As emitter as well as the n−‐GaAs collector region. In addition, the Ga1−x Alx As band gap (and hence Al composition) has been determined. The obtained values of Fdc are in good agreement with numerically computed values for the analyzed HBT structures, thus making it possible to deduce doping levels in these sections.