Contents I. Introduction
Due to their high-power and high-frequency (HF) operation, vacuum electron radiation sources underpin radar, communication, and particle accelerator systems [1]. Nevertheless, large working volumes, high-temperature operation, and slow reaction times, due to the need for thermionic cathodes, have limited the application of vacuum electron radiation sources in some fields. Solid-state electron radiation sources have attracted increasing attention of late. They have developed rapidly and have replaced their vacuum counterparts in many applications. Their simple miniaturization, low working voltages, room-temperature operation, and ready integration are particularly attractive. However, solid-state electron radiation sources have some limitations; their poor anti-interference performance, deleterious responsiveness to incident radiation, and low output power in the millimeter-wave and terahertz frequency bands are perhaps some of the most critical issues plaguing solid-state radiation sources to date. Vacuum microelectronic (VME) devices combine vacuum and solid-state electronics [2]. They have many advantages inherited from both the vacuum and solid-state electron devices upon which they are based, including HF operation, low-temperature operation, and simple integration. Central to such VME radiation sources is the derivation of a room-temperature, rapidly time-responding electron beam based on field emission cold cathode sources. Carbon nanotubes (CNTs) have proved to be a leading field emission material [3]. Such nanoengineered devices mediate high emission current densities, impressive chemical stability, mechanical strength, and temporal stability [4], making CNTs well suited to underpin the next generation of VME radiation sources [5]–[9].
