Detailed 77‐K and temperature‐dependent photoluminescence (PL) investigations of heterojunction bipolar transistor (HBT) structures grown by low‐pressure metalorganic vapor‐phase epitaxy are presented. HBT layer sequences with GaAs base and with AlxGa1−xAs base graded in the Al content are investigated. Different peaks belonging to GaAs and AlGaAs layers within the HBT structure are identified. It is shown that the PL spectrum is not only the superposition of single‐layer signals, but also provides additional information about the incorporation of the base dopant Mg into the emitter and collector regions of the HBT during epitaxial growth. The presence of Mg in the collector leads to a signal absent in PL of single‐layer GaAs doped merely with Si. It is interpreted as a free‐to‐bound Mg acceptor transition, and its intensity scales with the amount of diffused Mg. Two additional AlGaAs PL peaks besides that from the usual band‐gap recombination are found to be correlated with Mg in the emitter. Their signal strength is dependent on the Mg concentration level. It will be shown that careful evaluation of the HBT spectra can help to decide whether a certain structure under investigation meets the requirements for further device processing.