Vacuum Field Emission Research Articles

Piezo-VFETs: Vacuum Field Emission Transistors Controlled by Piezoelectric MEMS Sensors as an Artificial Mechanoreceptor with High Sensitivity and Low Power Consumption.

As one of the most promising electronic devices in the post-Moore era, nanoscale vacuum field emission transistors (VFETs) have garnered significant attention due to their unique electron transport mechanism featuring ballistic transport within vacuum channels. Existing research on these nanoscale vacuum channel devices has primarily focused on structural design for logic circuits. Studies exploring their application potential in other vital fields, such as sensors based on VFET, are more limited. In this study, for the first time, the design of a vacuum field emission transistor (VFET) coupled with a piezoelectric microelectromechanical (MEMS) sensing unit is proposed as the artificial mechanoreceptor for sensing purposes. With a negative threshold voltage similar to an N-channel depletion-mode metal oxide silicon field effect transistor, the proposed VFET has its continuous current tuned by the piezoelectric potential generated by the sensing unit, amplifying the magnitude of signals resulting from electromechanical coupling. Simulations have been conducted to validate the feasibility of such a configuration. As indictable from the simulation results, the proposed piezoelectric VFET exhibits high sensitivity and an electrically adjustable measurement range. Compared to the traditional combination of piezoelectric MEMS sensors and solid-state field effect transistors (FETs), the piezoelectric VFET design has a significantly reduced power consumption thanks to its continuous current that is orders of magnitude smaller. These findings reveal the immense potential of piezoelectric VFET in sensing applications, building up the basis for using VFETs for simple, effective, and low-power pre-amplification of piezoelectric MEMS sensors and broadening the application scope of VFET in general.

Basic Research on Vacuum Breakdown and Field Emission Characteristics on SUS304 Electrode with Micro-sized Pits

Basic Research on Vacuum Breakdown and Field Emission Characteristics on SUS304 Electrode with Micro-sized Pits

Study on pore structure of foamed cement paste by multi-approach synergetics

In this paper, the pore structure parameters of foamed cement paste with densities of 500, 600, 800, and 1000 kg/m3 were characterized by nuclear magnetic resonance, X-ray computed tomography, vacuum saturation device and field emission scanning electron microscope. The results show that the full pore size of foamed cement paste can be obtained by combining nuclear magnetic resonance and X-ray computed tomography, ranging from 0.001 μm to 4192.31 μm. Compared with the test results of other equipment and the content of bubble volume, the result of vacuum saturation device is closer to the true porosity of foamed cement paste, which ranges from 41.5 % to 71.2 %. The spheroid is the main pore shape of foamed cement paste, accounting for 65.22 % ∼ 75.45 % of the total pore shape. The pore wall thickness increases with the increase of the density of foamed cement paste and its increasing range is 32.81 % ∼ 55.17 % while the quantity shows decreases and its decreasing range is 2.73 % ∼ 32.18 %. On this basis, the change rates of all pore structure parameters of foamed cement paste involved in the test were summarized and discussed. It is found that increasing the density of foamed cement paste will lead to a completely opposite development trend among pore structure parameters. This may be helpful to the modification of foamed cement paste and the change of its application.

The impact of semiconductor surface states on vacuum field emission

This work presents a theoretical analysis of the impact of surface states on vacuum field emission currents in semiconductors. In wide and ultra-wide bandgap semiconductors such as GaN and AlGaN, low electron affinity has been proposed as a benefit for field emission into vacuum. However, in these materials, the surface Fermi level at the surface is pinned well below the conduction band, and the surface depletion barriers due to the surface Fermi level pinning can be comparable to or higher than the electron affinity. Therefore, analysis of field emission requires consideration of not only the vacuum potential barrier set by electron affinity, but also the depletion region near the semiconductor surface. In this paper, we develop analytical models to predict field emission currents with careful consideration of the impact of surface states on the energy band alignment. The results are used to provide guidelines for design of field emitters that could benefit from the low electron affinity of semiconductors such as Al(Ga)N.

Stacked conductive metal–organic framework nanorods for high-performance vacuum electronic devices

Metal-organic frameworks (MOFs) possessing many unique features have been utilized in several fields in recent years. However, their application in field emission (FE) vacuum electronic device is hindered by their poor electrical conductivity. Herein, a novel conductive MOF of Cu-catecholate (Cu-CAT) with the nanorod length of 200 nm and conductivity of 0.01 S/cm is grown on the graphite paper (GP). Under an applied electric field, a large number of electrons can be emitted from the nanoscale emitter tips of MOF surface to the anode. The great field emission performance of Cu-CAT@GP cold cathode film including a low turn-on field of 0.59 V/μm and ultra-high field enhancement factor of 29622, even comparable to most carbon-based materials that have been widely investigated in FE studies, is achieved in this work. Meanwhile, Cu-CAT@GP film has a good electrical stability with a current attenuation of 9.4% in 2 h. The findings reveal the cathode film fabricated by conductive MOF can be a promising candidate of cold electron source for vacuum electronic applications.

Systematic Field Electron Emission Investigations of Vapor‐Phase‐Grown Sb2Te3 Nanosheets

Micron‐sized Sb2Te3 nanosheets are grown on p‐type Si substrate by facile vapor‐phase route. Physicochemical properties of the Sb2Te3 nanosheets are revealed using X‐ray diffraction (XRD), scanning electron microscope (SEM), atomic force microscopy (AFM), transmission electron microscope (TEM), Raman spectroscopy, and X‐ray photoelectron spectroscopy (XPS). Owing to their high aspect ratio, field emission (FE) characteristics of the Sb2Te3 nanosheets grown on Si substrate (termed as a “planar emitter”) are investigated. Furthermore, the FE behavior of a single Sb2Te3 nanosheet attached to a blunt tungsten needle is studied. The single nanosheet emitter exhibited superior emission behavior in contrast to the planar emitter. The Fowler–Nordheim (F–N) plot for the single nanosheet emitter exhibits linear nature, whereas for the planar emitter it shows deviation from linearity. An attempt has been made to reveal the FE properties of a single Sb2Te3 nanosheet in an “in‐plane” configuration, similar to the vacuum field emission transistor (VFET). In this case, a maximum emission current of 1 μA is observed at 290 V, which is comparable to other VFETs. The observed results clearly imply the potential of the single nanosheet emitters due to topological insulators, like Sb2Te3, for the development of new generation nanoscale vacuum electronic devices.

Vacuum field emission transistors with small gate-cathode overlapping areas: a simulation study

Vacuum field emission transistors with small gate-cathode overlapping areas: a simulation study

Nanoscale Vacuum Field Emission Triode With a Double Gate Structure

A planar lateral Vacuum Field Emission Triode (VFET) with a double gate structure is proposed to improve the gate modulation characteristics in this letter. The preparation of this double gate structure is obtained by using the mature bottom gate VFET manufacturing process without any additional dedicated masks. The emission characteristics of the fabricated VFET under vacuum and atmosphere condition are measured. The experimental results demonstrate that both the bottom gate and the top gate can effectively control the emission electrons in the vacuum channel with width of about 110–200 nm which is successfully prepared by the electro-forming process. The operating mechanism of the device is basically the same as that of the plane-bottom gate VFET, and the device has achieved good field electron emission characteristics and double-gate modulation characteristics. Meanwhile, the vacuum channel is covered by the top gate insulator layer, which relaxing the vacuum requirement and improving the collection efficiency of emitted electrons by drain electrode about 50% in air.

Fabrication and field emission properties of vertical, tapered GaN nanowires etched via phosphoric acid

The controlled fabrication of vertical, tapered, and high-aspect ratio GaN nanowires via a two-step top-down process consisting of an inductively coupled plasma reactive ion etch followed by a hot, 85% H3PO4 crystallographic wet etch is explored. The vertical nanowires are oriented in the direction and are bound by sidewalls comprising of semipolar planes which are at a 12° angle from the [0001] axis. High temperature H3PO4 etching between 60 °C and 95 °C result in smooth semipolar faceting with no visible micro-faceting, whereas a 50 °C etch reveals a micro-faceted etch evolution. High-angle annular dark-field scanning transmission electron microscopy imaging confirms nanowire tip dimensions down to 8–12 nanometers. The activation energy associated with the etch process is 0.90 ± 0.09 eV, which is consistent with a reaction-rate limited dissolution process. The exposure of the type planes is consistent with etching barrier index calculations. The field emission properties of the nanowires were investigated via a nanoprobe in a scanning electron microscope as well as by a vacuum field emission electron microscope. The measurements show a gap size dependent turn-on voltage, with a maximum current of 33 nA and turn-on field of 1.92 V nm−1 for a 50 nm gap, and uniform emission across the array.

Complementary Vacuum Field Emission Transistor

A complementary vacuum field emission device structure is proposed, and its operation is analyzed by multiphysics simulation. A freely moving double-clamped cantilever, which balances the electrostatic attraction force and elastic restoration force, is used as the source electrode while the drain electrode is fixed. The electron-emitting cathode is formed on the source for the n-type device and on the drain for the p-type. Thus, complementary current–voltage characteristics are obtained only with Fowler–Nordheim tunneling electron transport. The impact of various design parameters such as vacuum gap, cantilever beam thickness, gate width, and tip offset on the drain and gate leakage currents is investigated. Inverter logic operation is verified successfully using complementary devices.

Electrical Characteristics of Vertical GaN Nanowire Vacuum Field Emitter Devices

The electrical characteristics of simulated vertical gallium nitride (GaN) nanowire (NW) vacuum field emission diodes (VFEDs) and GaN NW vacuum field emission transistors (VFETs) with flat- and sharp-tip GaN NW field emitters (FEs) are presented by investigating their dependence on geometry and material parameters. For a flat-tip GaN NW VFED, the turn-on voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TO</sub> ) was decreased by decreasing the GaN NW diameter ( D), the anode to emitter distance (d), the work function of the anode metal (φ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</sub> ), or by increasing the GaN NW height ( h). Variations in doping concentration (N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</sub> ) of the GaN NW do not impact V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TO</sub> . The sharp-tip GaN NW VFEDs exhibited decreased V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TO</sub> as the degree of sharpness ( d <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sh</sub> ) was increased from 0 to 0.5 μm. The saturation emitter current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sat</sub> ) was only affected by variations in N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</sub> , and was increased for increased N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</sub> . The transfer (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> - V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">GS</sub> ) characteristics of sharp-tip VFETs exhibited reduced threshold voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TH</sub> ), increased transconductance (G <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">tm, max</sub> ), and ON-/ OFF-current ratio, compared with the flat-tip ones. The output (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> - V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> ) characteristics of both types of devices showed nonlinear behavior at the low V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> regime.

Ultralow Voltage GaN Vacuum Nanodiodes in Air.

The III-nitride semiconductors have many attractive properties for field-emission vacuum electronics, including high thermal and chemical stability, low electron affinity, and high breakdown fields. Here, we report top-down fabricated gallium nitride (GaN)-based nanoscale vacuum electron diodes operable in air, with record ultralow turn-on voltages down to ∼0.24 V and stable high field-emission currents, tested up to several microamps for single-emitter devices. We leverage a scalable, top-down GaN nanofabrication method leading to damage-free and smooth surfaces. Gap-dependent and pressure-dependent studies provide new insights into the design of future, integrated nanogap vacuum electron devices. The results show promise for a new class of high-performance and robust, on-chip, III-nitride-based vacuum nanoelectronics operable in air or reduced vacuum.

Analysis of the electron emission characteristics and working mechanism of a planar bottom gate vacuum field emission triode with a nanoscale channel.

A planar lateral vacuum field emission triode (VFET) with a nanoscale channel of 80-90 nm was fabricated on a silicon wafer. The nanoscale channel of this vacuum triode was generated by via the electro-forming process (EFP). With the use of a wedge-waist-type conductive film, the distribution of Joule heat during EFP could be guided, which helped to realize the controllable preparation of a nanoscale fissure that served as the vacuum channel. Experimental results revealed that the fabricated device demonstrated good emission characteristics in vacuum and could be operated normally under atmospheric conditions. The triode emission characteristics under atmospheric pressure were preliminarily realized. The field-assisted thermal electron emission for low operating voltage and the Fowler-Nordheim tunneling emission mechanism for voltages greater than the turn-on voltage were used to explain the emission mechanism of the proposed VFET.

A nanoscale vacuum field emission gated diode with an umbrella cathode.

A nanoscale field emission vacuum channel gated diode structure is proposed and a tungsten cathode with an umbrella-like geometry and sharp vertical edge is fabricated. The edge of the suspended cathode becomes the field emission surface. Unlike in the traditional transistor with the gate typically located between the source and the drain, the bottom silicon plate becomes the gate here and the anode terminal is located between the umbrella cathode and the gate. The fabricated devices show excellent diode characteristics and the gated diode structure is attractive for extremely low gate leakage.

Fabrication of vertically well-aligned NiSi2 nanoneedle arrays with enhanced field emission properties

In this study, we demonstrated the controllable fabrication of periodic arrays of vertically well-aligned, fully-silicided NiSi2 nanoneedles with sharp nanotips on (100)Si substrates. The new approach proposed here eliminates the need for complicated and costly photolithographic processes and the use of toxic chemicals. All the produced vertical Si and NiSi2 nanoneedles were identified to be single crystalline and with the same geometric morphology and the same axial orientation of [100]. Additionally, the produced tapered NiSi2 nanoneedle array possessed excellent electron emission properties with a very low turn-on field of 0.85 V/μm, which is superior to many reported one-dimensional metal silicide nanostructures. Such an enhancement in the field emission can be attributed to the lower effective work function, sharp nanotips, single-crystalline structure, and good vertical alignment. The combination of the facile approach proposed here and superior electron emission performances make the well-ordered vertical NiSi2 nanoneedles promising candidates for vacuum nanoelectronics and field emission display applications.

Nanoscale Complementary Vacuum Field Emission Transistor

Nanoscale vacuum channel transistors based on field emission have gained attention recently, and device demonstrations using various material systems have been reported. Whereas solid-state electro...

Reducing the gate current in vacuum channel field-emission transistors using a finger gate

A nanofinger gate vacuum field-emission transistor with a vertical channel (FGVFET) is proposed herein. The reduction of the gate leakage current is investigated to obtain an optimum structure. The proposed three-terminal metal–insulator–metal device with a 43-nm vertical vacuum channel is capable of operating in air ambient and provides a high anode drive current (101 µA), while both the gate and anode voltages are small at about 5 V. Meanwhile, the gate leakage current of the FGVFET is reduced by about sevenfold compared with conventional structures. Also, this vacuum transistor exhibits a low threshold voltage (0.55 V) that is comparable to modern solid-state devices. As a result, a significant cutoff frequency (fT) of 1.13 THz is obtained. Other electrical characteristics of the FGVFET, such as the on–off current ratio and transconductance, are also calculated. The introduced modification could be applied to other vacuum vertical-channel transistors to provide a new class of high-speed low-power transistors for digital applications.

AUTOELECTRONIC CATHODES BASED ON ARRAYS OF NIOBIUM-OXIDE COLUMNAR NANOSTRUCTURES FOR FIELD EMISSION DISPLAYS

The article discusses the prospects of creating controlled field-effect cathodes based on arrays of columnar oxide niobium nanostructures for field emission displays. Geometrical models of field-emission cathodes and vacuum elements have been developed and investigated. The distribution of the electric field in the vacuum device at various distances between the cathode and the anode, the applied voltages between them, the shape and microgeometry of the cathodes were obtained. The optimal geometric parameters of nanostructured autoelectronic cathodes and matrices of these were calculated based on the simulation. The technological route has been developed for the production of autoelectronic cathode matrices based on arrays of niobium-oxide columnar nanostructures formed by electrochemical anodization of Al/Nb thin-film system. The samples of controlled arrays of autoelectronic cathodes were fabricated and the current-voltage characteristics with interelectrode gap of 2, 5 and 10 μm in various electric modes with change in the electric field strength from 3 to 85 V/μm were studied. At 2 μm gap between the anode and cathode, the emission occurs at minimum threshold voltages, but it is characterized by limited current values. The increasing in the interelectrode gap allows rising the emission currents, however, the threshold voltages increase. In the pulsed mode, the large emission currents are achieved. The threshold voltage of autoelectronic cathode matrices with interelectrode gap of 5 μm was 9.16 V, the maximum currents reached 350 μA at voltage of 22.5 V. In the pulsed mode, the emission arose at 11.06 V, the maximum current reached 1500 μA at 40 V.

Development of technology for the formation of vacuum field emission cells using focused ion beams

The article presents the results of the development of technology for the formation of vacuum field emission structures. Experimental studies of local ion-stimulated deposition of W and C and ion-beam etching were carried out and their effects on the formation of final structures were studied. The technological process of manufacturing nanoscale field emission structures was developed, and experimental samples were fabricated. It is shown that the use of the method of focused ion beams (FIB) demonstrates its advantages compared with other methods.

Investigation of local ion-stimulated carbon deposition to create vacuum field emission diodes

This paper presents the results of experimental studies of the influence of the technological parameters of a focused ion beam (FIB) on the process of local ion-stimulated deposition of carbon and tungsten when creating elements of vacuum nanoelectronics. The dependences are obtained illustrating the influence of the time of the FIB exposure at a point on the geometric parameters of the structures. Experimental samples of vacuum field-emission diodes based on semiconductor-metal-dielectric structures were fabricated by ion-stimulated carbon deposition. A technological process for creating field-emission diodes has been developed. The prospects of applying the FIB method for creating structures of vacuum field emission nanoelectronics are demonstrated.