Vacuum Channel Transistor Research Articles

High-frequency performance in nanoscale vacuum channel transistors with gate-cathode height difference

High-frequency performance in nanoscale vacuum channel transistors with gate-cathode height difference

Planar nanoscale vacuum channel transistors based on resistive switching

Resistance switching (RS) offers promising applications in a variety of areas. In particular, silicon oxide (SiO x ) under RS can serve as electron sources in new types of miniature vacuum electron tubes. In this work, planar nanoscale vacuum channel transistors (NVCTs) with graphene electrodes and RS SiO x electron sources were developed. In each RS-NVCT, the resistance between the ground and the gate underwent high–low–high transitions, which resulted from formation and subsequent rupture of Si conducting filaments. Electrons were emitted from the post-reset Si filaments and the current received by the collector (I C) was well controlled by the gate voltage (V G). The transfer characteristics reveal that I C was quite sensitive to V G when RS occurred. With V G sweeping from 0 to −20 V, the obtained subthreshold swing (SS) of 76 mV dec−1 was quite close to the theoretical limit of the SS of a field effect transistor at room temperature (60 mV dec−1). The largest ON/OFF ratio was of the order of 106. The output characteristics of the devices indicate that the dependence of I C on the collector voltage (V C) weakened at high V C values. These results demonstrate the application potential of RS-NVCTs as either switching devices or amplifiers.

Investigation of silicon-on-insulator back-gate nano vacuum channel transistor array

Recent advances in nanofabrication have made it possible to combine planar solid-state devices with vacuum electronics to create planar nano vacuum channel transistors that offer the advantages of cold-field emission and ballistic transmission. However, the current research is mainly limited to the study of a single field emission transistor, which has problems such as low current and poor gate control capability. To solve the above problems, a multitip field emission array is used in this work, and gate modulation is performed by a back-gate structure to fabricate and process a back-gate nano vacuum transistor array. First, we conducted simulation modeling of the back-gate nano vacuum transistor, investigated the impact of its structural parameters on its performance, and obtained the optimal simulation results. Then, structural parameters of the back-gate nano vacuum channel transistor array (BG-NVCTA) are selected based on the simulation results and fabricated by electron beam lithography on the silicon wafer. The experimental results, agreed well with the simulation results, show that the BG-NVCTA device has excellent gate control characteristics and a high current density. Its anode current is greater than 5 μA, and the transconductance is 1.05 μS when the anode voltage is 5 V.

Effect of water vapor desorption on the performance of gallium nitride field emitter array

We are exploring the potential of gallium nitride (GaN) field emitter arrays in vacuum channel transistors. This study investigated the impact of ultraviolet (UV) light on the emission properties of large arrays of GaN field emitters. Arrays of 150 × 150 emitters were analyzed before and after UV exposure. With a constant collector voltage of 200 V DC, gate voltage sweeps from 0 to 60 V were applied. The initial I–V measurements showed a rapid increase in emission current, indicating a conditioning effect, settling at a stable value of 1.25 μA after three to five sweeps. Remarkably, exposure to UV light resulted in a fivefold increase in the maximum field emission current, reaching an impressive 6 μA. This significant enhancement highlights the potential of UV treatment for improving the performance of GaN-based field emitters. This surge in current can be attributed to the desorption of water vapor caused by the UV light. To compare with the heating-based water desorption technique, another array of 150 × 150 emitters was characterized before and after heating at 400 °C. While the collector voltage remained constant at 200 V DC, the gate voltage was systematically increased from 0 to 75 V in this experiment. This controlled sweeping of the gate voltage provided a precise method for characterizing the field emission properties of the GaN emitters. The I–V measurements revealed that, similar to the UV exposure case, collector current increased by approximately four times after heat treatment at 400 °C for 10 min. This resulted in a maximum field emission current of around 10 μA at 75 V. As with the UV case, this increase can also be attributed to surface desorption, primarily of water.

Structure Optimization of Planar Nanoscale Vacuum Channel Transistor

Due to its unique structure, discoveries in nanoscale vacuum channel transistors (NVCTs) have demonstrated novel vacuum nanoelectronics. In this paper, the structural parameters of planar-type NVCTs were simulated, which illustrated the influence of emitter tip morphology on emission performance. Based on simulations, we successfully fabricated back-gate and side-gate NVCTs, respectively. Furthermore, the electric properties of NVCTs were investigated, showing the potential to realize the high integration of vacuum transistors.

Thermionic electron emission in the 1D edge-to-edge limit

Thermionic emission is a tunneling phenomenon, which depicts that electrons on the surface of a conductor can be pulled out into the vacuum when they are subjected to high electrical tensions while being heated hot enough to overtake their work functions. This principle has led to the great success of the so-called vacuum tubes in the early 20th century. To date, major challenges still remain in the miniaturization of a vacuum channel transistor for on-chip integration in modern solid-state integrated circuits. Here, by introducing nano-sized vacuum gaps (∼ 200 nm) in a van der Waals heterostructure, we successfully fabricated a one-dimensional (1D) edge-to-edge thermionic emission vacuum tube using graphene as the filament. With the increasing collector voltage, the emitted current exhibits a typical rectifying behavior, with the maximum emission current reaching 200 pA and an ON–OFF ratio of 103. In addition, it is found that the maximum emission current is proportional to the number of the layers of graphene. Our results expand the research of nano-sized vacuum tubes to an unexplored physical limit of 1D edge-to-edge emission, and hold great promise for future nano-electronic systems based on it.

Effect of room air exposure on the field emission performance of UV light irradiated Si-gated field emitter arrays

Field emission cathodes are promising candidates in nanoscale vacuum channel transistors and are used in microwave vacuum electron devices. Prior research has shown that UV light exposure as well as 350 °C vacuum bake can desorb water vapor from Si field emission tips, resulting in lower work function and improved emission performance. However, after long exposure to room air (greater than 24 h), the improved performance is lost as water adsorbs on the tips. In this study, experiments were carried on two sets of 1000 × 1000 Si-gated field emitter arrays to determine the length of time that emitters can be exposed to room air without degradation. First, the samples were exposed to UV light irradiation in vacuum, and the I–V curves were measured. Then, the samples were exposed to room air with a relative humidity ranging from 30% to 40% for varying times (5, 6, 8, 12, 24, and 48 h) and then tested again under high vacuum. It was found that the emission current did not degrade after room air exposure of 5 h. However, at 6 h of exposure, degradation started to occur, and after 24 h, the emission current went back to the original, pre-UV exposure case. In a separate experiment, UV irradiated samples were stored in nitrogen for 72 h, with a 10% degradation in current. These results demonstrate that field emission devices with improved performance resulting from water desorption can be handled in air up to 5 h, depending upon humidity and stored in nitrogen for 72 h while maintaining improved performance.

Particle tracking simulation of an air channel transistor

Advances in nanofabrication techniques have underpinned the recent growing interest in vacuum channel transistors due to their ability to showcase ballistic transport and immunity to most radiations. However, the geometry of the electrodes plays an important role in the overall performance and efficiency of the device. Several studies on the geometry of the source electrode have been carried out because of its role in enhancement of the local electric field, which triggers electron tunneling, yet the geometry of the drain electrode has been neglected. Here, we propose a new planar vacuum channel transistor with gold electrodes. Our study shows that the unique geometrical design of the device leads to high collection efficiency. Furthermore, it reveals the importance of the geometry of the drain on device performance. Emission characteristics of the device and the effect of geometrical parameters such as channel length and source geometry on its performance have also been investigated.

Space–charge limited current in nanodiodes: Ballistic, collisional, and dynamical effects

This Perspective reviews the fundamental physics of space–charge interactions that are important in various media: vacuum gap, air gap, liquids, and solids including quantum materials. It outlines the critical and recent developments since a previous review paper on diode physics [Zhang et al. Appl. Phys. Rev. 4, 011304 (2017)] with particular emphasis on various theoretical aspects of the space–charge limited current (SCLC) model: physics at the nano-scale, time-dependent, and transient behaviors; higher-dimensional models; and transitions between electron emission mechanisms and material properties. While many studies focus on steady-state SCLC, the increasing importance of fast-rise time electric pulses, high frequency microwave and terahertz sources, and ultrafast lasers has motivated theoretical investigations in time-dependent SCLC. We particularly focus on recent studies in discrete particle effects, temporal phenomena, time-dependent photoemission to SCLC, and AC beam loading. Due to the reduction in the physical size and complicated geometries, we report recent studies in multi-dimensional SCLC, including finite particle effects, protrusive SCLC, novel techniques for exotic geometries, and fractional models. Due to the importance of using SCLC models in determining the mobility of organic materials, this paper shows the transition of the SCLC model between classical bulk solids and recent two-dimensional (2D) Dirac materials. Next, we describe some selected applications of SCLC in nanodiodes, including nanoscale vacuum-channel transistors, microplasma transistors, thermionic energy converters, and multipactor. Finally, we conclude by highlighting future directions in theoretical modeling and applications of SCLC.

Temperature effects on gated silicon field emission array performance

Silicon field emitter arrays (Si FEAs) are being explored as an electron source for vacuum channel transistors for high temperature electronics. Arrays of 1000 × 1000 silicon tip based gated field emitters were studied by measuring their electrical characteristics up to 40 V of DC gate bias with a 1.3 mA emission current at different temperatures from 25 to 400 °C. At ∼350 °C, residual gas analyzer measurements show that water desorption and carbon dioxide partial pressures increase significantly, the gate to emitter leakage current decreases by more than ten times, and the collector current increases by more than ten times. These improvements remained after heat-treatment but were then lost once the device was exposed to the atmosphere for several days. The improvements could be recovered upon additional baking suggesting that adsorbates (primarily water) on the surface affected field emission and surface leakage. It was also found that after heat-treatment, the electrical characteristics of the devices exhibited <3% variation in collector current at 40 V, which (without exposure to the atmosphere) can be termed as a weak temperature dependence. These results suggest that Si FEAs could be viable as a high temperature transistor.

Comment on "Design and circuit simulation of nanoscale vacuum channel transistors" by J. Xu, Y. Qin, Y. Shi, Y. Yang and X. Zhang, Nanoscale Adv., 2020, 2, 3582.

These comments aim to correct some apparent weaknesses in the theory of field electron emission given in a recent paper about nanoscale vacuum channel transistors, and to improve the presentation of this theory. In particular, it is argued that a “simplified” formula stated in the paper should not be used, because this formula is known to under-predict emission current densities by a large factor (typically around 300 for an emitting surface with local work function 4.5 eV). Thus, the “simplified” formula may significantly under-predict the practical performance of a nanoscale vacuum channel transistor.

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...

Negative Capacitance Vacuum Channel Transistors for Low Operating Voltage.

This study proposes negative capacitance vacuum channel transistors. The proposed negative capacitance vacuum channel transistors in which a ferroelectric capacitor is connected in series to the gate of the vacuum channel transistors have the following two advantages: first, adding a ferroelectric capacitor in series with a gate capacitor makes the turn-on voltage lower and on–off transition steeper without causing hysteresis effects. Second, the capacitance matching between a ferroelectric capacitor and a vacuum channel transistor becomes simplified because the capacitance of a vacuum channel transistor as seen from a ferroelectric capacitor is constant.

Study on electrical properties and structure optimization of side-gate nanoscale vacuum channel transistor

In this paper, a nanoscale vacuum channel transistor (NVCT) with a side-gate structure is fabricated by standard electron beam lithography. The states of the proposed NVCTs could be effectively modulated by side-gate bias, exhibiting a drive current (>400 nA), low work voltage (<20 V) and high on/off current ratio (>103). Moreover, we further optimize the shapes of the electrodes, demonstrating that the electric performance is closely related to the structure parameters. The side-gate NVCT offers an alternative method to fulfill the on-chip vacuum devices with high integration.

Design and circuit simulation of nanoscale vacuum channel transistors.

Nanoscale vacuum channel transistors (NVCTs) are promising candidates in electronics due to their high frequency, fast response and high reliability, and have attracted considerable attention for structural design and optimization. However, conventional modeling for vacuum devices tends to focus on the work function or electric field distribution for an individual structure. Therefore, it is desirable for a new simulation method to explore the function circuits of NVCTs, e.g. high-speed logic circuits. In this study, a complete simulation of the fabrication, structure design and circuit simulation of NVCTs is demonstrated. First, the fabrication process was designed to be compatible with current semiconductor technology. Then, the “fabricated” structure was directly employed to investigate the influence of the structure parameters on the electrical performance. Furthermore, we explore the possibility of implementing an invert circuit with a single optimal NVCT. To the best of our knowledge, this is the first demonstration of a vacuum-state invertor with a circuit-simulation module in which NVCT functions as a conventional triode or FET. These simulation results illustrate the feasibility of integrating NVCTs into functional circuits and provide a theoretical method for future on-chip vacuum transistors applied in logic or radio-frequency (RF) devices.

Vertical Field Emission Air-Channel Diodes and Transistors.

Vacuum channel transistors are potential candidates for low-loss and high-speed electronic devices beyond complementary metal-oxide-semiconductors (CMOS). When the nanoscale transport distance is smaller than the mean free path (MFP) in atmospheric pressure, a transistor can work in air owing to the immunity of carrier collision. The nature of a vacuum channel allows devices to function in a high-temperature radiation environment. This research intended to investigate gate location in a vertical vacuum channel transistor. The influence of scattering under different ambient pressure levels was evaluated using a transport distance of about 60 nm, around the range of MFP in air. The finite element model suggests that gate electrodes should be near emitters in vertical vacuum channel transistors because the electrodes exhibit high-drive currents and low-subthreshold swings. The particle trajectory model indicates that collected electron flow (electric current) performs like a typical metal oxide semiconductor field effect-transistor (MOSFET), and that gate voltage plays a role in enhancing emission electrons. The results of the measurement on vertical diodes show that current and voltage under reduced pressure and filled with CO2 are different from those under atmospheric pressure. This result implies that this design can be used for gas and pressure sensing.

Nanoscale vacuum channel transistor with in-plane collection structure

High quality nanoscale vacuum channel transistors (NVCTs) enable carriers to transport ballistically through the vacuum nanogap, achieving high speed and frequency characteristic which are essential for on-chip vacuum electronic devices. However, the studies to date have been largely confined to explore the common electrical performance, while the fast response characteristic of NVCTs remains a challenge. We report the fabrication of metal-based NVCT, with sub-100 nm vacuum channel and specific designed in-plane collection structure which can enhance the emission or collection efficiency of the electrons simultaneously. Importantly, the demonstration of a rise/fall time of less than 100 ns is achieved, which is compatible with those high-quality solid-state transistors based on low-dimensional materials. Moreover, the device can also retain excellent electrical performance, exhibiting a high drive current (>10 μA), low work voltage (<10 V) and high on/off current ratio (>104). The verification of fast temporal response of NVCT makes a significant step towards on-chip vacuum electronics with high speed and integration.

Nanoscale vacuum channel transistors fabricated on silicon carbide wafers

Vacuum tubes were central to the early development of electronics, but were replaced, decades ago, by semiconductor transistors. Vacuum channel devices, however, offer inherently faster operation and better noise immunity due to the nature of their channel. They are also stable in harsh environments such as radiation and high temperature. However, to be a plausible alternative to solid-state electronics, nanoscale vacuum channel devices need to be fabricated on the wafer scale using established integrated circuit manufacturing techniques. Here, we show that nanoscale vacuum channel transistors can be fabricated on 150 mm silicon carbide wafers. Our devices have a vertical surround-gate configuration and we show that their drive current scales linearly with the number of emitters on the source pad. The silicon carbide vacuum devices are also compared to identically sized silicon vacuum channel transistors, which reveals that the silicon carbide devices offer superior long-term stability. Nanoscale vacuum channel transistors, which have a vertical surround-gate configuration, can be fabricated on 150 mm silicon carbide wafers using conventional integrated circuit processing technology.

Nanoscale Vacuum Channel Transistor

Vacuum tubes that sparked the electronics era had given way to semiconductor transistors. Despite their faster operation and better immunity to noise and radiation compared to the transistors, the vacuum device technology became extinct due to the high power consumption, integration difficulties, and short lifetime of the vacuum tubes. We combine the best of vacuum tubes and modern silicon nanofabrication technology here. The surround gate nanoscale vacuum channel transistor consists of sharp source and drain electrodes separated by sub-50 nm vacuum channel with a source to gate distance of 10 nm. This transistor performs at a low voltage (<5 V) and provides a high drive current (>3 microamperes). The nanoscale vacuum channel transistor can be a possible alternative to semiconductor transistors beyond Moore's law.

Field-Induced Crystalline-to-Amorphous Phase Transformation on the Si Nano-Apex and the Achieving of Highly Reliable Si Nano-Cathodes.

Nano-scale vacuum channel transistors possess merits of higher cutoff frequency and greater gain power as compared with the conventional solid-state transistors. The improvement in cathode reliability is one of the major challenges to obtain high performance vacuum channel transistors. We report the experimental findings and the physical insight into the field induced crystalline-to-amorphous phase transformation on the surface of the Si nano-cathode. The crystalline Si tip apex deformed to amorphous structure at a low macroscopic field (0.6~1.65 V/nm) with an ultra-low emission current (1~10 pA). First-principle calculation suggests that the strong electrostatic force exerting on the electrons in the surface lattices would take the account for the field-induced atomic migration that result in an amorphization. The arsenic-dopant in the Si surface lattice would increase the inner stress as well as the electron density, leading to a lower amorphization field. Highly reliable Si nano-cathodes were obtained by employing diamond like carbon coating to enhance the electron emission and thus decrease the surface charge accumulation. The findings are crucial for developing highly reliable Si-based nano-scale vacuum channel transistors and have the significance for future Si nano-electronic devices with narrow separation.