A diamond semiconductor device with a carbide interlayer includes a diamond layer having a semiconducting diamond region of first conductivity type therein and an insulated gate structure on a face of the diamond layer. The relatively thin carbide interfacial layer is provided between the insulated gate structure and the diamond layer in order to inhibit the formation of electrically active defects, such as interface states at the face. By inhibiting the formation of interface states at the face, the carbide interfacial layer suppresses parasitic leakage of charge carriers from the diamond layer to the insulated gate structure. The carbide interfacial layer can be intrinsic silicon carbide or an intrinsic refractory metal carbide (e.g., TiC or WC) or the layer can be of opposite conductivity type to thereby form a P--N heterojunction with the diamond layer. The carbide interfacial layer and the insulated gate structure can be used in a variety of diamond electronic devices such as MIS capacitors, enhancement-mode and buried-channel insulated-gate field effect transistors (IGFETs), surface-channel and buried-channel charge-coupled devices (CCDs), detectors, heterojunction devices, and other related field effect devices. Related fabrication methods are also disclosed.