We report on the characterization of wide-band-gap heterojunction diodes based on the p-ZnO/n-GaN material system. The layer structure consists of 11 µm GaN on sapphire substrates and As-doped ZnO film of thickness 0.4 µm obtained by plasma-assisted molecular beam epitaxy (PA-MBE). The quality of the heterojunction was examined by x-ray diffraction, atomic force microscopy and scanning electron microscopy. The arsenic concentration in ZnO, measured by secondary ion mass spectroscopy (SIMS), is 5 × 10²⁰ cm⁻³. The maximum forward-to-reverse current ratio I _F/I _R is of about 10⁵ in the applied voltage ±3 V, a very good result for this type of heterojunction.