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This paper explores the recent history and diversity of this remarkable technology, with emphasis on recent advances in the more traditional device types (traveling-wave tube and klystron), as well as more recent innovations such as the microwave power module, inductive output amplifier, fast-wave devices, ultrahigh-power sources, and RF vacuum microelectronics. These advances can be credited to a combination of device innovation, enhanced understanding gained through improved modeling and design, the introduction of superior materials and sub-assembly components and the development of advanced vacuum processing and manufacturing techniques.

Diamond exhibits very high, but widely varying, secondary-electron yields. In this study, we identified some of the factors that govern the secondary-electron yield from diamond by performing comparative studies on polycrystalline films with different dopants (boron or nitrogen), doping concentrations, and surface terminations. The total electron yield as a function of incident-electron energy and the energy distribution of the emitted secondary electrons showed that both bulk properties and surface chemistry are important in the secondary-electron-emission process. The dopant type and doping concentration affect the transport of secondary electrons through the sample bulk, as well as the electrical conductivity needed to replenish the emitted electrons. Surface adsorbates affect the electron transmission at the surface-vacuum interface because they change the vacuum barrier height. The presence of hydrogen termination at the diamond surface, the extent of the hydrogen coverage, and the coadsorption of hydrocarbon-containing species all correlated with significant yield changes. Extraordinarily high secondary-electron yields (as high as 84) were observed on B-doped diamond samples saturated with surface hydrogen. The secondary electrons were predominantly low-energy quasithermalized electrons residing in the bottom of the diamond conduction band. Two key reasons for the unusually high yields are (1) the wide band gap which allows the low-energy secondary electrons to have long mean-free paths, and (2) the very low or even negative electron affinity at the surface which permits the low-energy quasithermalized electrons that reach the surface to escape into vacuum.

Secondary-electron-emission spectroscopy is used to probe the transport and emission of impact-ionized electrons in single-crystal diamond. By studying the emission from a cesiated C͑100͒ surface having a negative electron affinity ͑NEA͒, the full energy spectrum of the internal electrons is revealed in the measured energy distribution data. The kinetic energy of the electrons and the height of the surface energy barrier are measured relative to the conduction-band minimum E c , which is identified in the spectra. The cesiated diamond surface is observed to be NEA, but the hydrogenated diamond surface ͑commonly believed to be NEA͒ has an electron affinity near zero and slightly positive. Analysis of the very high yield data (␦ max ϳ132) and the sharply peaked energy distribution data indicates that the transport of low-energy electrons is very efficient in C͑100͒. An emission model is deduced that involves the surface properties of the material and the internal energy distribution of the electrons.

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