Solid-state electronics were developed, in part, because vacuum-tube-based electron-emission electronics systems were fraught with reliability issues, dissipated large amounts of power, and generated inordinate amounts of heat. In addition, the integration capability of vacuum tubes was limited to what could be included within one vacuum-sealed ampule or tube.
The reliability issues of vacuum tubes arise due primarily to the electron emission sources, which must be heated to very high temperatures to efficiently provide electrons by a process called “thermionic emission.” Thermionic emission is a process wherein electric current is passed through a metal filament to heat it and increase its temperature such that electrons gain enough thermal energy for them to overcome the work function of the metal and escape into the free space around the filament. Unfortunately, when a metal filament is heated to the required temperature to enable thermionic emission, metal from the filament can evaporate making the filament brittle and susceptible to burning out and/or cracking.
Unfortunately, semiconductor-based, solid-state electronics devices have a limited temperature range over which they can operate. Silicon-based electronics, for example, are limited to approximately 300° C., while so-called high-temperature electronics based on silicon carbide, gallium nitride, or diamond can operate only up to approximately 450° C. In addition, semiconductor-based devices are highly susceptible to damage from ionizing radiation, which can generate defects in the semiconductor material that can serve as traps that degrade charge-carrier mobility and lifetime.
As a result, renewed interest in integrated electron-emission devices has arisen due to their capability for high-frequency operation, ability to operate at high temperatures, and their inherent resistance to ionizing radiation.
To date, individual electron-emission devices having operating frequencies in the GHz range have been demonstrated, such as those disclosed by Spindt, et al., in “Progress in field-emitter array development for high-frequency operation,” in The Technical Digest of the Electron Devices Meeting—IEDM '93, pp. 749-752 (1993), which is incorporated herein by reference.
In addition, electron-emission devices that can operate at temperatures well above the temperatures at which solid-state transistors fail have also been demonstrated, such as silicon-carbide needle arrays that operate at 500° C., as disclosed by Wang, et al., in “High-temperature stable field-emission of b-doped SiC nanoneedle arrays,” in Nanoscale, Vol. 7, pp. 7585-7592 (2015), which is incorporated herein by reference.
Further, as known in the art, carrier lifetime is less critical for operation of field-emission devices; therefore, such devices are well suited for radiation-hard electronics applications.
Unfortunately, conventional electron-emission devices disclosed to date have been difficult to fabricate individually, much less as part of complex integrated circuits. Furthermore, prior-art field-emission devices have relied upon the use of backplane gating, which makes difficult or precludes independent operation of different devices on the same chip.
The need for an electron-emission technology suitable for integration into high-functionality integrated circuits remains, as yet, unmet in the prior art.