Photoreflectance of GaAs/Ga1-xAlxAs heterojunction bipolar transistor structures

We have studied the photoreflectance spectra at 300 K from a number of GaAs/Ga1AlAs heterojunction bipolar transistor (HBT) structures grown by molecular beam epitaxy and metal-organic chemical vapor deposition. From the observed Franz-Keldysh oscillations we have been able to evaluate the built-in dc electric fields, F , in the Ga Al As emitter as well as the n GaAs collector region. In dc 1-x x addition, the Ga1AlAs band gap (and hence Al composition) has been determined. The obtained values of Fd are in good agreement with numerically-computed values for the analyzed HBT structures, thus making it possible to deduce doping levels in these structures. The GaAlAs FKO have been correlated with device performance.