GaN/Si3N4/n-Si and InN/Si3N4/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 Si3N4 layer was measured by transmission electron microscopy. n-GaN/Si3N4/n-Si HJs show diode-like rectifying current–voltage (I–V) characteristic, while n-InN/Si3N4/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/Si3N4/Si and ∼95 meV for InN/Si3N4/Si HJs.