Diode ideality factors of 2.0–8.0 have been reported in GaN-based p-n junctions. These values are much higher than the expected values of 1.0–2.0 as per the Sah-Noyce-Shockley theory. We propose a fundamentally new model for the high ideality factors obtained in GaN-based diodes. This model is based on the effect of moderately doped unipolar heterojunctions as well as metal–semiconductor junctions in series with the p-n junction. A relation for the effective ideality factor of a system of junctions is developed. A detailed experimental study is performed on diodes fabricated from two different structures, a bulk GaN p-n junction structure and a p-n junction structure incorporating a p-type AlGaN/GaN superlattice. Bulk GaN p-n junction diode displays an ideality factor of 6.9, whereas the one with the superlattice structure displays an ideality factor of 4.0. In addition, device simulation results further strengthen the model by showing that moderately doped unipolar heterojunctions are rectifying and increase the effective ideality factor of a p-n junction structure.