Electrical properties of n-ZnO/n-GaN isotype heterostructures obtained by rf-sputtering of ZnO films on GaN layers grown by metal-organic vapour phase epitaxy are discussed. Current-voltage (I-V) characteristics of the n-ZnO/n-GaN diodes revealed highly rectifying behavior with forward and reverse currents ∼1.43×10-2 A/cm2 and ∼2.4×10-4 A/cm2, respectively, at ±5 V. From the Arrhenius plot built using temperature dependent current-voltage characteristics (I-V-T) an activation energy 0.125 eV was derived for the reverse bias leakage current path, and 0.62 eV for the band offset from forward bias measurements. From electron-beam induced current measurements the minority carrier diffusion length in ZnO was estimated in the range 0.125-0.175 mm, depending on excitation conditions. The temperature dependent EBIC measurements yielded an activation energy of 0.462 ± 0.073 V.