The suppression of parasitic conductivity at the substrate/MBE regrowth interface in GaN/AlGaN heterostructures by carbon δ-doping is reported. Parasitic conductivity results from silicon adhesion at the GaN substrate surface; its removal before loading the substrates into the UHV growth chamber seems to be impossible. This contamination and the resulting parasitic conductivity is particularly detrimental when growing on unintentionally doped substrates since it masks the 2D transport properties in lateral transport devices even at cryogenic temperatures. The formation of this parasitic channel can be impeded by compensating the silicon-induced charges through carbon δ-doping. In consequence, the intrinsic 2D channel properties can be studied in low-temperature magneto-transport measurements.