We report on novel transport and optical data for GaN/AlGaN modulation doped heterostructures grown by Metal Organic Chemical Vapor Deposition (MOCVD). Variable temperature galvanomagnetic, resistivity, photoluminescence and photoconductivity measurements have been performed. Evidence for potential fluctuations is provided by the observation of weakly localized transport at low temperatures, together with a negative magnetoresistance due to disorder in the interface region. Additional evidence for a built-in electric field caused by the fluctuations near the heterointerface region is given by the observation of photoconductivity dips resonant with free excitons, indicating free exciton ionization. It is concluded that interface roughness, dislocations, and similar structural defects, have a strong influence on the transport properties of the two-dimensional electron gas in these structures.