The deep‐level spectroscopy is applied to LPE‐grown heterostructures of the AlxGa1−xAs system. It is found that a specific DX level which has been attributed to the incorporation of Sn up to now comes from the Ge doping. The Ge‐related DX level is used to record the Ge donor distribution within the Sn‐doped Al0.35Ga0.65As layer of Sn/Ge/Ge‐doped double‐heterostructures. It is suggested that the high‐ohmic layer often present in Ge containing junctions is due to the site conversion of Ge atoms and diffusion in the electric field of the junction. A novel method is introduced for very sensitive probing the actual position of the pn junction relative to the heterointerface in heterojunctions using the specific properties of the Ge‐related deep level in AlxGa1−xAs.