A thermionic emission device includes an insulating substrate, and one or more grids located thereon. Each grid includes a first, second, third and fourth electrode down-leads located on the periphery thereof, and a thermionic electron emission unit therein. The first and second electrode down-leads are parallel to each other. The third and fourth electrode down-leads are parallel to each other. The first and second electrode down-leads are insulated from the third and fourth electrode down-leads. The thermionic electron emission unit includes a first electrode, a second electrode, and a thermionic electron emitter. The first electrode and the second electrode are separately located and electrically connected to the first electrode down-lead and the third electrode down-lead respectively. The insulating substrate comprises one or more recesses that further insulate the thermionic electron emitters from the substrate.