GaN and AlGaN electrical transport properties were investigated in undoped and intentionally doped layers using Hall measurements. GaN/AlGaN films grown by MOCVD exhibited strong variations in resistivity as a result of changing MOCVD growth conditions in controlled experiments. The explanation of high compensation in GaN is connected to carbon impurity acting as a acceptor, substitutional oxygen predicted to act as donor, association of threading dislocations with charge centers, gallium vacancies and VGa-O complexes (deep acceptors). In our experiments we tried to recognize the most efficient source of n- and p-type carriers in undoped GaN and mechanisms of compensation by impurities and defects creation. The influence of AlGaN defects structure, with various Al content, on GaN electrical properties was examined detailed. Highly resistive and highly conductive GaN films were produced as a result of our investigations. GaN/AlGaN structures were applied for HEMTs and photodetectors.