GaN/Si 3N 4/n-Si and InN/Si 3N 4/n-Si heterojunctions (HJs) were fabricated using plasma-assisted molecular beam epitaxy for a comparison study. Single-crystalline wurtzite structures of GaN and InN epilayers were confirmed by high-resolution X-ray diffraction and thickness of ultrathin Si 3N 4 layer was measured by transmission electron microscopy. n-GaN/Si 3N 4/n-Si HJs show diode-like rectifying current-voltage (I-V) characteristic, while n-InN/Si 3N 4/n-Si HJs show symmetric nonlinear I-V behavior. The I-V characteristics of both HJs were discussed in terms of the band diagram of HJs and the carrier transport mechanism. The activation energies of carrier conduction were estimated to be ∼29 meV for GaN/Si 3N 4/Si and ∼95 meV for InN/Si 3N 4/Si HJs. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.