Threshold Voltage Research Articles

Functional Zwitterionic Polyurethanes as Gate Dielectrics for Organic Field‐Effect Transistors

AbstractThe development of polymeric dielectrics with a high dielectric constant is of great significance for flexible low‐voltage organic field‐effect transistors (OFETs). Herein, functional polyurethanes (PUs) with nitro and sulfobetaine zwitterionic groups are synthesized, and their electrical properties, mechanical properties, and the performances of OFET devices using these zwitterionic PUs as gate dielectrics are studied. Compared with sulfobetaine zwitterion‐containing PUs, nitro‐containing PUs (NO2‐PUs) show higher dielectric constant up to 6.5. Both zwitterionic PUs enhance the OFET performances, while the effect of NO2‐PU is more significant. Devices using NO2‐PU‐15 that contains 15 moL% nitro groups as the dielectric layer show the best performance and a threshold voltage (Vth) of as low as −0.02 V together with two orders’ increase of the mobility is observed, compared with the devices using PU without nitro groups. This study provides a new method to improve the dielectric constant of polymeric dielectrics, which is valuable for flexible/stretchable and wearable electronic devices.

Back-Side Stress to Ease p-MOSFET Degradation on e-MRAM Chips

Abstract Magnetoresistive random access memory process makes great contribution to threshold voltage deterioration of metal-oxide-silicon field-effect transistors, especially on p-type devices. Herein, a method was proposed to reduce threshold voltage degradation by utilizing back-side stress. Through the deposition of tensile material on the back side, positive charges generated by silicon-hydrogen bond breakage were inhibited, resulting in a potential reduction of threshold voltage shift by up to 20%. In addition, it was found that the method could only relieve silicon-hydrogen bond breakage physically, failing to provide a complete cure. However, it holds significant potential for applications where additional thermal budget is undesired. Furthermore, it was also concluded that the method used in this work was irreversible, with its effect sustained to the chip package phase, and it ensures competitive reliability of the resulting magnetic tunnel junction devices.

Increased Static Charge‐Induced Threshold Voltage Shifts and Memristor Activity in Pentacene OFETs Comprising Polystyrene‐Based Gate Dielectrics Containing Electroactive Small Molecule Crystallites

AbstractTop‐contact bottom‐gate pentacene OFETs are fabricated with single layer dielectrics comprised of either polystyrene (PS), poly(4‐methylstyrene) (P4MS), or poly(4‐tert‐butylstyrene) (P4TBS). The polystyrenes are blended with varying concentrations of two different small molecules, dibenzotetrathiafulvalene (DBTTF) and 2,8‐difluoro‐5,11‐bis(triethylsilylethynyl)anthradithiophene (diF‐TES‐ADT), to form small, separated crystallites contained throughout the polymer dielectric layer. The OFET characteristics of these devices are investigated and their threshold voltage shifts are measured after −70 V static charging for 5 min. Two‐terminal measurements are conducted using multiple different gate biases in the range of −50 to +50 V to investigate memristor behavior in the devices. OFETs containing DBTTF exhibited ΔVth increases as large as 330% relative to control OFETs containing no DBTTF, while OFETs containing at least 7.5 wt.% DBTTF exhibited memristor activity, with currents ranging from 20 nA to 44 µA depending on the applied bias. This work demonstrates that including small, separated crystallites in polymer dielectrics enhances their charge storage ability and can be promising for creating nonbinary memory devices for data processing. Additionally, the observed memristor activity indicates the OFETs in this work can be used in development of neuromorphic systems that aim to mimic the synaptic behavior of the human nervous system.

Numerical analysis on the performance enhancement in AlGaN/GaN vertical CAVET with InGaN/AlN/InGaN hybrid current blocking layer

Abstract In this paper, we propose an enhanced superjunction AlGaN/GaN current aperture vertical electron transistor (CAVET) that integrates a novel hybrid InGaN/AlN/InGaN current blocking layer (CBL) situated above the P-column, directly beneath the channel. The incorporation of InGaN substantially elevates the device's saturation output current by introducing a subchannel beneath the AlGaN/GaN heterojunction conducting channel.Nonetheless, the introduction of AlN engenders a two-dimensional hole gas (2DHG) of opposite polarisation direction via the GaN/AlN heterojunction, which aids in depleting the two-dimensional electron gas (2DEG) within the channel, thereby mitigating the issue of negative threshold voltage (Vth) bias occasioned by the subchannel introduced by InGaN. The outcomes demonstrate that the maxim m saturation output current (u u Id) for the proposed architecture is 361.5 mA/mm, with a Vth of 1.63V, marking increases of 98.6% and 0.08%over the traditional superjunction CAVET, respectively. Simultaneously, the breakdownvoltage (Vbr) of the designed device is recorded at 12064 V, and the specific on-resistance(Ron,sp) is 6.45 Ω·mm, showcasing a enhancement figure of merit (FOM) of 32.9% incomparison to the traditional superjunction CAVET.

Manipulating conductivity and noise for transitioning between stochastic and inverse stochastic resonances in liquid–crystal electroconvection

Noise can play a constructive role in nature and various engineering systems. Over the past four decades, noise-induced stochastic resonances (SRs) have been extensively documented, showing enhancement in system performance. Additionally, inverse SR has been observed in various systems. Typically, these resonances were studied independently. A transition between these resonances was recently observed in an alternating current-driven liquid–crystal electroconvection (EC) system using combined amplitude and phase noises. This study uses internal (material) and external (noise) parameters to demonstrate the control of this transition. Specifically, the nonmonotonic threshold voltage behavior of the EC system, indicative of the resonances, was numerically examined using additional parameters. Experimental tests were conducted to confirm the effects of these parameters. The findings reveal that the transition between these resonances can be systematically controlled to meet specific needs, whether desirable or undesirable system performances. Notably, this study illustrates how to modify the behavior of both resonances in colored noise by adjusting its cutoff frequency and steepness and phase noise, which is often overlooked. Moreover, this study provides valuable insights for various noise-related applications.

Numerical simulation study on discharge characteristics of the dual hollow cathode plasma source using magnetic mirror field

Abstract A two-dimensional fluid model of a double hollow cathode discharge structure constrained by a magnetic mirror field is established. The discharge is ignited by a hot cathode arc, and the length of the discharge arc column fills the entire central region of the double hollow cathode structure under the action of the magnetic field, with the electrons self-sustainedly oscillating on the axis. The electron density within the hollow cathode tube does not increase all the time with the arc voltage, the enhancement of the wall ion current by the arc voltage is very obvious, and the hollow cathode discharge is a result of the mutual enhancement of the arc voltage and the hollow cathode voltage. Hollow cathode sputtering needs to reach a certain voltage threshold to ensure that the inner wall has sufficient ion energy and that there exists an optimal radius range to maintain the discharge chamber to achieve a high plasma density and to favor hollow cathode sputtering.

Study on stable configurations and snap through of the rectangular VS bi-stable laminates with curvilinear fiber alignment

With advances in fiber placement technology, variable stiffness bistable laminates (VSBL) composited by curvilinear fiber format become a subject of intense interest for morphing structures due to their lower weight, adjustable stiffness and a richer range of stable configurations compared to straight fiber bistable laminate. Merely conducting research on square laminates is not enough allowing for practical application scenarios. This article investigates the influence of geometrical parameters of VSBL on the stable state configuration and static snap through actuated by MFC systematically. A quadratic variable curvature polynomial is used to fit the transverse displacement according to classical laminate theory. By means of von-Karman geometrical relationship and the Rayleigh-Ritz method, stable configurations of the VS bistable laminate are solved. By comparing with finite element results, the reliability and precision of the current results are verified. It shows that changing fiber orientation leads to significant effects on the steady state configuration and critical bifurcation points. Theoretical analysis of VS bistable laminates containing the smart piezoelectric material MFC is followed by analytical modeling. The voltage threshold required for actuating snap through of two stable states is acquired by smart material layer MFC. The specific reasons for the differences in the configurations of VS bistable laminates are illustrated by analyzing distribution of the stiffness of VSBL in the plane.

Design and Simulation of a High-Performance Tunneling Field Effect Transistor-Based Biosensor Using a Heterojunction Electron-Hole Bilayer

Abstract This study introduces a novel dielectrically-modulated heterojunction electron-hole bilayer tunnel field-effect transistor (DM-HEHBTFET) for bio-sensing applications. The device features a Ga0.85Sb0.15As/Ga0.8In0.2As heterojunction and a p-type pocket in the channel, achieving a remarkably low threshold voltage (VT) of 20 mV, an average subthreshold slope (SS) of 5.7 mV/dec, and a leakage current (IOFF) as low as 5 × 10⁻¹¹ A/μm. The staggered bandgap in the heterostructures enhances electric field control, enabling lower gate voltage operation. Furthermore, the strategically positioned nanogap cavities in non-overlapping regions of the top and bottom gates effectively mitigate gate control issues over the channel, ensuring improved device performance. A modified design, the modified DM-HEHBTFET, is also proposed, featuring source and drain regions engineered with Ga0.85Sb0.15As/Ga0.8In0.2As heterojunctions. This design mitigates leakage current and improves the average subthreshold slope (SS). For biomolecules with a dielectric constant of 12, the modified biosensor exhibits a drain current sensitivity (Scurrent) of 2.6e4, average SS=2.7 mV/dec, and IOFF=1e-12 A/μm. The device's performance is assessed by examining steric hindrance and band tailing effects. The modified biosensor outperforms recent DM-TFET biosensors, making it a promising candidate for low-power, high-switching speed bio-sensing.

Carrier modulation and effective passivation of tin oxide thin-film transistors by organic surface doping

Doping is a useful technique for metal oxide thin-film transistors (TFTs) to adjust the threshold voltage and charge carrier density. However, a notable drawback is the disruption of the microstructure caused by doping crystalline lattice, leading to a partial decrease in charge carrier mobility. In this work, we suggest a surface doping technique that modifies the carrier concentration and passivates the device surface while preserving the channel layer lattice structure through the use of organic dopant molecules. It is shown that tin oxide (SnO2) TFTs doped in this manner typically exhibit improved electrical characteristics, particularly greater mobility and a noticeably lower threshold voltage, without negatively affecting the devices on/off current ratio. Furthermore, compared to pristine devices, bias stress stability and long-term durability are also enhanced. These findings suggest that surface doping may find use in high-performance oxide semiconductor devices and circuits.

Low-voltage organic field-effect transistors by using solution-processable high-κ inorganic-polymer hybrid dielectrics

Organic field-effect transistors (OFETs) incorporating hybrid high-κ inorganic Al2O3 and polymer dielectrics, including polyvinyl alcohol (PVA), polystyrene (PS), or polymethyl methacrylate (PMMA), through solution-processing techniques were fabricated. The analyses revealed that the high surface energy and hydrophilicity property of Al2O3 and PVA, and the relatively hydrophobic property of PS surface, hindered the performance of corresponding OFETs. The Al2O3/PMMA-based OFET achieved the optimized performance, with a threshold voltage of −2.7 V, a hole carrier mobility of 0.056 cm2/Vs, and a current on/off ratio of 1.0 × 104 at a low operating voltage of −5 V. Through analyzing the characteristics of leakage current, capacitance, contact resistance, and trap density of OFETs, we found that the PMMA-engaged films possessed the optimized electrical properties. The introduction of PMMA eliminated the interfacial trapping, thereby lowering the threshold voltage and enhancing the performance of the device. The COMSOL Multiphysics simulation was conducted to confirm the physical mechanism. The strategy of utilizing Al2O3/PMMA hybrid dielectric could simultaneously ensure the low operating voltage and good performance of OFET, while guaranteeing the low leakage current by the thick PMMA.

Gate modulation of barrier height of unipolar vertically stacked monolayer ReS2/MoS2 heterojunction

This study investigates vertically stacked CVD grown ReS2/MoS2 unipolar heterostructure device as Field Effect Transistor (FET) device wherein ReS2 on top acts as drain and MoS2 at bottom acts as source. The electrical measurements of ReS2/MoS2 FET device were carried out and variation in Ids (drain current) Vs Vds (drain voltage) for different Vgs (gate voltage) revealing the n-type device characteristics. Furthermore, the threshold voltage was calculated at the gate bias voltage corresponding to maximum transconductance (gm) value which is ~ 12 V. The mobility of the proposed ReS2/MoS2 heterojunction FET device was calculated as 60.97 cm2 V−1 s−1. The band structure of the fabricated vDW heterostructure was extracted utilizing ultraviolet photoelectron spectroscopy and the UV–visible spectroscopy revealing the formation of 2D electron gas (2DEG) at the ReS2/MoS2 interface which explains the high carrier mobility of the fabricated FET. The field effect behavior is studied by the modulation of the barrier height across heterojunction and detailed explanation is presented in terms of the charge transport across the heterojunction.

Comprehensive analysis of charge carriers dynamics through the honeycomb structure of graphite thin films and polymer graphite with applications in cold field emission and scanning tunneling microscopy

Polymer graphite electron sources have performed satisfactorily as field emission emitters and scanning tunneling microscopy probes in the past few years. However, the emission process was characterized by limited total emission currents. This paper introduces the elemental, vibrational, electronic structure, and optical analysis of polymer graphite and glass-graphite composite field emission cathodes to study these limitations. Moreover, the field emission characteristics are studied including the changes in the potential energy barrier of the used materials and structures. Among the studied structures, the cathodes prepared from graphite thin films deposited on a micropointed glass substrate (film-GMF) showed superior performance as random field emission arrays. This includes obtaining much higher emission current values ≈ 20 times) and lower threshold voltages ≈ 1/2) compared to the results obtained from polymer graphite samples. The enhancement factor in such emitters is believed to be the three-dimensional honeycomb structure of graphite. Moreover, the study includes applying graphite coatings to tungsten nano-field emission cathodes and scanning tunneling microscopy probes, which improves the performance of such cathodes/probes in both microscopic techniques.

The effect of temperature on the electrical characteristics of zigzag and armchair black phosphorus based 2D MOSFET

Abstract A first time comparative study of the thermal dependence of vital electrical characteristics of 2D MOSFETs based on black phosphorus for both zigzag and armchair orientations is presented in this paper. It is seen that a higher in-plane thermal conductivity in zigzag direction results in a much better on state current performance which comes at the cost of orders of magnitude increase in gate leakage and a reduced on to off state current ratio. The effect of temperature on threshold voltage (VTH), short channel effects like drain induced barrier lowering (DIBL), subthreshold swing (SS), Schottky barrier height ΦSB and transconductance behavior in both zigzag and armchair orientations is thoroughly discussed and the inherent physical mechanisms resulting the variations are also presented. Though increase in temperature is found to deteriorate the SS and drain conductance but at the same time, it is found to improve the short channel performance of the devices under consideration.

Thickness dependence and crystallization properties of amorphous GeTe thin films on silicon dioxide

Radio frequency magnetron sputtering was used to prepare the amorphous GeTe thin films on silicon dioxide and the thickness effects on the crystallization behavior were investigated. With the film thickness reducing, the crystallization temperature, crystallization activation energy, amorphous and crystalline resistance increase remarkably, indicating the great improvement in thermal stability and power consumption. Ozawa’s model was used to estimate the crystallization kinetics of GeTe thin films, it shows that nucleation and grain growth occur simultaneously, and grain growth dominates ultimately. XRD analysis demonstrated that the grain size can be reduced and the crystallization process of GeTe thin film can be inhibited with the film thickness decreasing. Furthermore, the thinner film has smaller resistance drift index and surface roughness, which are beneficial to improve the reliability of storage device. T-type phase change memory devices based on 25 nm GeTe thin film were fabricated by 0.13 μm CMOS technology, and the current–voltage and resistance-voltage characteristics demonstrate the excellent electrical performance, including the fast resistance switching between SET and RESET processes, low threshold current and voltage. All the results proved the strong dependency relationships between the crystallization properties and film thickness of GeTe thin film, which paves the way for developing high-density phase change memory in the fields of big data and artificial intelligence.

Fine-tuning Cesium lead chloride perovskite field-effect transistors for sensing applications: Bridging numerical modeling and experimental validation

This study presents a comprehensive approach to fine-tuning Cesium Lead Chloride Perovskite Field-Effect Transistors (CsPbCl3-FETs) for sensing applications by bridging numerical modeling with experimental validation. By combining finite element methods in COMSOL Multiphysics for optimization, we tailored FET parameters such as oxide and perovskite thin film thickness. The fabricated FET, with a 200 nm semiconductor layer and 30 nm oxide thickness, was strategically chosen to operate in a non-depletion mode, maximizing mobility while minimizing power consumption. Experimental results closely aligned with numerical simulations, showcasing a threshold voltage of 0.50 V±0.07 V and an impressive on/off current ratio of 1.50 x 104 ± 0.3 x 104. Notably, the perovskite FET exhibited remarkable carrier mobility in saturation mode, reaching 5.40 cm2/V-s ± 0.8 cm2/V-s, outperforming other attempts in the literature. This work underscores the potential of CsPbCl3 FETs for high-performance sensing applications, offering insights into optimizing device parameters for enhanced functionality and efficiency.

High-performance single-crystalline In2O3 field effect transistor toward three-dimensional large-scale integration circuits

Formation of a single crystalline oxide semiconductor on an insulating film as a channel material capable of three-dimensional (3D) stacking would enable 3D very-large-scale integration circuits. This study presents a technique for forming single-crystalline In2O3 having no grain boundaries in a channel formation region on an insulating film using the (001) plane of c-axis-aligned crystalline indium gallium zinc oxide as a seed. Vertical field-effect transistors using the single-crystalline In2O3 had an off-state current of 10−21 A μm−1 and electrical characteristics were improved compared with those using non-single-crystalline In2O3: the subthreshold slope was improved from 95.7 to 86.7 mV dec.−1, the threshold voltage showing normally-off characteristics (0.10 V) was obtained, the threshold voltage standard deviation was improved from 0.11 to 0.05 V, the on-state current was improved from 22.5 to 28.8 μA, and a 17-digit on/off ratio was obtained at 27 °C.

MR fluid-Based threshold-feedback overload-protection system for miniature turbine generator

Abstract Miniature turbine generators, which can supply power to miniature mechatronical system by converting natural energy, are promising for future applications in micro-aircraft, ammunition, and missiles. However, turbine generators are at the risk of rotor wear and circuit failure under high wind-penetration conditions. As the feature size decreases, conventional sensors can hardly be integrated in microsystems, making control more difficult. In addition, microspace demands higher practicality of control strategies. Existing complex control units of large-scale systems are not applicable to microsystems. Therefore, a safe and robust control strategy that incorporates practical applications should be considered. This study aims to bridge the gap between the phase-transition properties of magnetorheological (MR) materials and control methods in a microspace. A MR fluid-based threshold-feedback overload-protection strategy is proposed for the miniature turbine generators. This strategy realises the sensorless control of miniature turbine generators by directly capturing the output frequency. To accurately describe the control characteristic of the overload-protection system, a dynamic model of the rotating shaft is proposed. A series of threshold voltages (Ur = 4.0, 5.0 and 6.0 V) is used to test the controllability of the overload-protection system. Experimental results show that the rotational speed of the miniature turbine generator is effectively controlled under hurricane-force wind conditions (v=40 m/s). Consequently, this study has developed a control strategy to solve the overload failure of miniature turbine generators. Under low wind speeds, the miniature turbine generator starts reliably. When the wind speed exceeds the threshold value, the miniature turbine generator is protected from overload failure. We believe that this work is invaluable for the functional expansion and performance improvement of miniature turbine generators.

Reversible Switching of Light‐Gated Organic Transistors Employing Dihydroazulene/Vinylheptafulvene Photo‐/Thermochromic Molecules

AbstractAn innovative possibility to introduce additional functionality to organic field‐effect transistors (OFETs) is to employ photochromic molecules, which undergo reversible isomerization under applied stimuli such as irradiation with specific wavelengths. As a result, the transistors not only can be switched on/off by the applied voltages, they can also be programmed by alternate triggers, such as light. Here, reversible switching of OFETs is presented by blending various dihydroazulene/vinylheptafulvene photoswitches into polythiophene‐based conjugated polymers. In result, the transfer characteristics of the transistors are altered significantly through UV irradiation. In contrast to current literature on different photoswitches such as spiropyrans or diarylethenes, the backreaction is induced thermally and not via visible light irradiation and reproducibly yields the pristine transistor characteristics. This reversible switching upon alternating UV irradiation and thermal annealing is quantified by figures of merit such as the magnitude of drain current, threshold voltage, and subthreshold swing. Irradiating the devices with different doses of UV light shows that the magnitude of switching directly depends on the respective UV dose, hence enabling a multi‐level electronic system. Furthermore, long‐term cyclability over 100 steps of repeated UV light exposure and thermal annealing is demonstrated.

Synergistic effects of anionic surfactant doping on dielectric and electro-optical properties of nematic liquid crystals by cyano-ionic interactions

Modern research has long focused on controlling and optimizing the dielectric, electro-optical and optical attributes of liquid crystals (LCs) to increase their technological value. Numerous approaches have been investigated for this reason in an effort to overcome various issues such as high response time, high driving voltage requirements, low dielectric anisotropy and limited operating temperature range, etc. The strategy utilizing binary/multicomponent systems is the most investigated and seems to have the most potential out of all the other techniques. In keeping with the same subject, the influence of anionic surfactant on the dielectric, electro-optical, and optical properties of nematic LC (NLC) has been examined. Precisely, dodecane-1-sulfonic acid sodium salt (SDS) which is an anionic surfactant was dispersed in a room temperature NLC E7 at various concentrations. Our findings reveal a significant alteration in the electro-optical behavior resulting from the introduction of an anionic surfactant, extending well beyond the instance of electrode polarization, albeit with a detrimental impact on performance within the low-frequency range as a result of increased ionic contribution. The main findings of this study include a 17.5%% increase in dielectric anisotropy, an appreciable reduction of about 30%% in threshold voltage and rotational viscosity, and approximately 64% reduction in response time in the host NLC due to the addition of anionic salt in it. A significant photoluminescence quenching of about 64%% is observed after dispersion of 3.0% by weight anionic surfactant in E7. These findings offer potential pathways for the development of future technologies in various domains, including photonic devices, biological imaging, light-emitting diodes (LEDs), and luminescent markers.

Impact of Channel Thickness and Doping Concentration for Normally-Off Operation in Sn-Doped β-Ga2O3 Phototransistors.

We demonstrate a Sn-doped monoclinic gallium oxide (β-Ga2O3)-based deep ultraviolet (DUV) phototransistor with high area coverage and manufacturing efficiency. The threshold voltage (VT) switches between negative and positive depending on the β-Ga2O3 channel thickness and doping concentration. Channel depletion and Ga diffusion during manufacturing significantly influence device characteristics, as validated through computer-aided design (TCAD) simulations, which agree with the experimental results. We achieved enhancement-mode (e-mode) operation in <10 nm-thick channels, enabling a zero VG to achieve a low dark current (1.84 pA) in a fully depleted equilibrium. Quantum confinement in thin β-Ga2O3 layers enhances UV detection (down to 210 nm) by widening the band gap. Compared with bulk materials, dimensionally constrained optical absorption reduces electron-phonon interactions and phonon scattering, leading to faster optical responses. Decreasing β-Ga2O3 channel thickness reduces VT and VG, enhancing power efficiency, dark current, and the photo-to-dark current ratio under dark and illuminated conditions. These results can guide the fabrication of tailored Ga2O3-based DUV phototransistors.