The authors have developed a high-current electron emitter device using the highly-uniform device fabrication process and phosphorus-doped n-type diamond. Diamond is a highly electron emitting material, and phosphorus-doped n-type diamond has an especially high electron-emission property. The threshold voltage for electron emission from the sharp emitter tip of n-type diamond was lower than that of p-type diamond. Emission properties changed also according to surface conditions. Contrary to the case of p-type diamond electron emitter, in the case of n-type diamond electron emitter, an emitter with an oxygen-terminated surface has a higher emission property than that with a hydrogen-terminated surface. The authors have developed a composite substrate on which n-type diamond can be grown epitaxially to have larger areas than conventional diamond single crystals, This development has consequently led to the successful development of a highly-uniform n-type diamond electron emitter device. Formed at each tip of the diamond emitter, was a nanometer-size coated electrode whose edge was as near as 200 nm from the apex of the emitter tip. The coated electrodes enabled both low-voltage electron emission from n-type diamond and high conductivity of electrodes. By using this n-type diamond electron emitter device, the electron emission current from a 1-mm2 emission area has reached 1103 mA. The electron emission yield was also as high as 99%, The development of this high-current electron emitter device offers a new approach to many vacuum devices such as microwave transmitter tubes and electron-beam processing machines.