While experiment, simulation, and theory all show that the gas breakdown voltage decreases linearly with gap distance for microscale gaps at atmospheric pressure due to the contribution of field emitted electrons, the continuing reduction in device size motivates a more fundamental understanding of gas breakdown scaling for nanoscale gaps. In this study, we measure current–voltage curves for electrodes with different emitter widths for 20–800 nm gaps at atmospheric pressure to measure breakdown voltage and assess electron emission behavior. The breakdown voltage [Formula: see text] depends more strongly on effective gap distance [Formula: see text] than the ratio of the emitter width to the gap distance. For 20 and 800 nm gaps, we measure [Formula: see text] V and [Formula: see text] V. Independent of emitter width, [Formula: see text] decreases linearly with decreasing [Formula: see text] for [Formula: see text] nm; for [Formula: see text] nm, [Formula: see text] decreases less rapidly with decreasing [Formula: see text] which may correspond to a change in the field enhancement factor for smaller gaps. While gas breakdown usually proceeds directly from field emission, as for microscale gaps, some cases exhibit space-charge contribution prior to the transition to breakdown, as demonstrated by orthodoxy tests. Applying nexus theory, we determine that the range of [Formula: see text] studied is close to the transitions between field emission and space-charge-limited current in vacuum and with collisions, necessitating a coupled theoretical solution to more precisely model the electron emission behavior. Implications on device design and an overall assessment of the dependence of emission and breakdown on gap distance are also discussed.

The continuing reduction in device size motivates a more fundamental understanding of breakdown and electron emission for nanoscale gaps. While prior experiments have separately studied breakdown and electron emission in vacuum gaps, no study has comprehensively examined the transitions between these mechanisms. In this study, we measure the current-voltage [Formula: see text] curves for electrodes with different emitter widths for 20–800 nm gaps at vacuum (∼1  μTorr) to measure breakdown voltage and assess electron emission behavior. The breakdown voltage [Formula: see text] increases linearly with increasing gap distance from ∼15 V at 20 nm to ∼220 V at 300 nm and remains nearly constant for larger gaps; [Formula: see text] does not depend strongly on the emitter width. Breakdown can proceed directly from the field emission regime. Nexus theory, which predicts transitions between space-charge limited current (SCLC) and field emission (FE), shows that the experimental conditions are in the Fowler–Nordheim regime and within a factor of 0.7 to the FE-SCLC transition. We also present the results of electrode damage by emission current-induced heating to explain the flattening of [Formula: see text] at larger gaps that was absent in previous experiments for similar gap distances at atmospheric pressure.

The importance of gas discharges for numerous applications with increasingly small device size motivates a more fundamental understanding of breakdown mechanisms. Gas breakdown theories for these gap sizes unify field emission with the Townsend avalanche, which depends on Townsend's first ionization coefficient [Formula: see text]; however, the ratio of the electric field E to gas pressure p for microscale gas breakdown exceeds the range of validity for the typical empirical equation. While some studies have used particle-in-cell simulations to assess [Formula: see text] in this range, they only examined a narrow range of experimental conditions. This work extends this approach to characterize ionization in microscale gaps for N2, Ar, Ne, and He for a broader range of pressure, gap distance d, and applied voltage V. We calculated [Formula: see text] at steady state for [Formula: see text] and p  = 190, 380, and 760 Torr. As expected, [Formula: see text] is not a function of reduced electric field [Formula: see text] for microscale gaps, where the electron mean free path is comparable to d and [Formula: see text] is high at breakdown. For [Formula: see text], [Formula: see text] scales with V and is independent of p. For [Formula: see text], [Formula: see text] approaches the standard empirical relationship for [Formula: see text] and deviates at higher levels because the ionization cross section decreases. We develop a more rigorous semiempirical model for [Formula: see text], albeit not as universal or simple, for a wider range of d and p for different gas species that may be incorporated into field emission-driven breakdown theories to improve their predictive capability.