The interface properties of Zr-Si-N/hydrogen-terminated diamond (H-diamond) metal insulator semiconductor field transistors (MISFETs) with and without Al₂O₃ protective layer were studied. The Al₂O₃ protection layer and Zr-Si-N insulation layer were deposited by atomic layer deposition (ALD) and radio frequency (RF) sputter methods, respectively. The transfer characteristics of the MISFETs show that the gate threshold voltage varies from -2.5 V to 3 V with and without Al₂O₃ layer, which indicates that the devices switch from normally off to normally on operation. The output and transfer properties reveal the preservation of hydrogen termination because of the Al₂O₃ layer.