  Recent advancements in nanofabrication have enabled the creation of vacuum
electronic devices with nanoscale free space gaps. These nanoelectronic devices
promise the benefits of cold-field emission and transport through free-space,
such as high nonlinearity and relative insensitivity to temperature and
ionizing radiation, all the while drastically reducing the footprint,
increasing the operating bandwidth and reducing the power consumption of each
device. Furthermore, planarized vacuum nanoelectronics could easily be
integrated at scale similar to typical micro and nanoscale semiconductor
electronics. However, the interplay between different electron emission
mechanisms from these devices are not well understood, and inconsistencies with
pure Fowler-Nordheim emission have been noted by others. In this work, we
systematically study the current-voltage characteristics of planar vacuum
nano-diodes having few-nanometer radii of curvature and free-space gaps between
the emitter and collector. By investigating the current-voltage characteristics
of nearly identical diodes fabricated from two different materials and under
various environmental conditions, such as temperature and atmospheric pressure,
we were able to clearly isolate three distinct emission regimes within a single
device: Schottky, Fowler-Nordheim, and saturation. Our work will enable robust
and accurate modeling of vacuum nanoelectronics which will be critical for
future applications requiring high-speed and low-power electronics capable of
operation in extreme conditions.
