A highlyefficient electronemitting diode with a flat emission surface has been fabricated using a 600 nm single-crystalline diamond film homoepitaxially grown on high-pressure synthesized (100) diamond by means of chemical-vapor deposition and ion-implantation techniques. The emitter contains a buried injection electrode layer and a hydrogenated diamond surface. When driving voltages ranging from 0.3 to 1.1 kV were applied between them, veryefficient electronemissions were observed. The emission efficiency, defined as the ratio of the emission current to the injection diode current, reached 100&percent; for emission currents on the order of10−7 Aand did not significantly depend on the currents. ©1998 American Institute of Physics.