The measured data of diamond SBDs is used for the work: A fully covered (FC) Schottky contact structure, realized by retaining the Al hard mask during cyclic deep diamond etching, is demonstrated to modulate the electric field distribution and enhance the breakdown performance of diamond Schottky barrier diodes (SBDs). The devices were fabricated on oxygen-terminated p-type diamond with a 6.5 μm-6 thick drift layer defined by multi-cycle ICP-RIE deep etching. Compared with conventional pseudo-vertical SBDs, the FC-SBD exhibits comparable forward characteristics, with a reduced specific on-resistance Ron,sp of 0.12 Ω · cm2, attributed to improved current spreading and reduced lateral resistance. Temperature-dependent analysis based on the thermionic emission model reveals nonideal metal–semiconductor interfaces in both devices, with similar interface-state densities on the order of 1013 eV−1cm−2. Despite the comparable interface properties, the FC-SBD achieves a significantly enhanced breakdown voltage of 453 V, compared to 375 V for the conventional device. Simulation results attribute this improvement to suppressed electric-field crowding at the Schottky-contact edge and a more uniform electric-field distribution within the diamond mesa. The proposed FC structure provides a simple and effective strategy for improving the high-voltage performance of diamond SBDs without additional interface engineering.