The present work describes a comprehensive design and scalable micro-fabrication technique for the production of novel 3D cathodes, consisting of chemical-vapor-deposition-grown high-density multi-wall carbon nanotubes along the walls of alumina microchannels. Under high DC and AC electric fields, the 3D cathodes displayed significantly high and moderately stable emission current of approximately 5.25 and approximately 14 mA, respectively. The inherent advantages of the 3D microchannel geometry for cold cathodes are higher emitter area, less ion bombardment and robustness under high voltage conditions. The 3D cathodes are envisioned to offer an entirely new class of miniature electron sources for high current vacuum microelectronics.