Gate Electric Field Research Articles

How Photoinduced Gate Screening and Leakage Currents Dynamically Change the Fermi Level in 2D Materials

An in‐depth analysis of physics in 2D materials like transition metal dichalcogenides requires the measurement of many material properties as a function of Fermi level position within the electronic band structure. This is normally done by changing the charge carrier density of the 2D material via the gate electric field effect. Herein, a comparison of gate‐dependent measurements, which are acquired under different measurement conditions, is shown to encounter significant problems due to the temporal evolution of the charging of trap states inside the dielectric layer or at its interfaces. The impact of, e.g., the gate sweep direction and the sweep rate on the overall gate dependence gets especially prominent in optical measurements due to photoexcitation of donor and acceptor states. Under such conditions, the same nominal gate voltage may lead to different gate‐induced charge carrier densities and, hence, Fermi level positions. It is demonstrated that a current flow from or even through the dielectric layer via leakage currents can significantly diminish the gate tunability in optical measurements of 2D materials.

Graphene-based vertical thin film transistors

Vertical field effect transistors (VFETs), where the channel material is sandwiched between source-drain electrodes and the channel length is simply determined by its body thickness, have attracted considerable interest for high performance electronics owning to their intrinsic short channel length. To enable the effective gate modulation and current switching behavior, the electrode of conventional VFET is largely based on perforated metals, in which the gate electrical field could penetrate through. Recently, with the emerge of graphene, a new type of graphene based VFETs has been developed. With finite density of states and the weak electrostatic screening effect, graphene exhibits a field-tunable work-function and partial electrostatic transparency, it can thus function as an “active" contact with tunable graphene-channel junction, enabling entirely new transistor functions or higher device performance not previously possible. In this review, we discuss the research progresses of graphene-based VFET, including the its basic device structure, carrier transport mechanism, device performance and novel properties demonstrated.

Gate-regulated transition temperatures for electron hopping behaviours in silicon junctionless nanowire transistors

We investigate gate-regulated transition temperatures for electron hopping behaviours through discrete ionized dopant atoms in silicon junctionless nanowire transistors. We demonstrate that the localization length of the wave function in the spatial distribution is able to be manipulated by the gate electric field. The transition temperatures regulated as the function of the localization length and the density of states near the Fermi energy level allow us to understand the electron hopping behaviours under the influence of thermal activation energy and Coulomb interaction energy. This is useful for future quantum information processing by single dopant atoms in silicon.

Enhanced Thermoelectric Performance of n-Type Organic Semiconductor via Electric Field Modulated Photo-Thermoelectric Effect.

Modulating photophysical processes is a fundamental way for tuning performance of many organic devices. However, it has not been explored as an effective strategy to manipulate the thermoelectric (TE) conversion of organic semiconductors (OSCs) owing to their critical requirement to carrier concentration (>1018 cm-3 ) and the fact of low exciton separation efficiency in single element OSCs. Here, an electric field modulated photo-thermoelectric (P-TE) effect in an n-type OSC is demonstrated to realize a significant improvement of TE performance. The electrical and spectroscopy characterizations reveal that the electric field gating generates combined modulation of exciton separation, charge screening, and carrier recombination, which produces a more than ten times improvement of photoinduced carrier concentration. These coupled processes contribute to the unconventional Seebeck coefficient (S)-electrical conductivity (σ) trade-off relationship of the photoexcited films, therefore leading to a more than 500% enhancement in the power factor for n-type OTE semiconductors. This work opens a unique way toward state-of-the-art organic P-TE materials for energy harvesting applications.

Asymmetric carrier penetration into hexagonal boron nitride in graphene field-effect transistors

The electronic structure of graphene field-effect transistors with a hexagonal boron nitride (h-BN) gate dielectric was studied in terms of the gate electric field and dielectric thickness using the density functional theory combined with the effective screening medium method. The calculation results showed that the carrier penetration into monolayer h-BN depended on the gate voltage that injected electrons or holes into graphene. The critical voltage for hole penetration was lower than that for electron penetration. At a fixed carrier concentration, carrier penetration soon occurred with increasing h-BN thickness for hole doping.

Design and investigation of dopingless double-gate line tunneling transistor: Analog performance, linearity, and harmonic distortion analysis**Project supported by the Natural Science Research Key Project of Universities of Anhui Province, China (Grant No. KJ2017A502), the Introduced Talent Project of Anhui Science and Technology University, China (Grant No. DQYJ201603), and the Excellent Talents Supported Project of Colleges and Universities, China (Grant No.

The tunnel field-effect transistor (TFET) is proposed by using the advantages of dopingless and line-tunneling technology. The line tunneling is created due to the fact that the gate electric field is aligned with the tunneling direction, which dramatically enhances tunneling area and tunneling current. Moreover, the effects of the structure parameters such as the length between top gate and source electrode, the length between top gate and drain electrode, the distance between bottom gate and drain electrode, and the metal position on the on-state current, electric field and energy band are investigated and optimized. In addition, analog/radio-frequency performance and linearity characteristics are studied. All results demonstrate that the proposed device not only enhances the on/of current ratio and reduces the subthreshold swing, but also offers eight times improvement in cut-off frequency and gain band product as compared with the conventional point tunneling dopingless TFET, at the same time; it shows better linearity and small distortions. This proposed device greatly enhances the potential of applications in dopingless TFET.

Carboxylic Acid‐Functionalized Conjugated Polymer Promoting Diminished Electronic Drift and Amplified Proton Sensitivity of Remote Gates Compared to Nonpolar Surfaces in Aqueous Media

AbstractA systematic analysis is used to understand electrical drift occurring in field‐effect transistor (FET) dissolved‐analyte sensors by investigating its dependence on electrode surface‐solution combinations in a remote‐gate (RG) FET configuration. Water at pH 7 and neat acetonitrile, having different dipoles and polarizabilities, are applied to the RG surface of indium tin oxide, SiO2, hexamethyldisilazane‐modified SiO2, polystyrene, poly(styrene‐co‐acrylic acid), poly(3‐hexylthiophene‐2,5‐diyl) (P3HT), and poly [3‐(3‐carboxypropyl)thiophene‐2,5‐diyl] (PT‐COOH). It is discovered that in some cases a slow reorientation of dipoles at the interface induced by gate electric fields causes severe drift and hysteresis because of induced interface potential changes. Conductive and charged P3HT and PT‐COOH increase electrochemical stability by promoting fast surface equilibrations. It is also demonstrated that pH sensitivity of P3HT (17 mV per pH) is an indication of proton doping. PT‐COOH shows further enhanced pH sensitivity (30 mV per pH). This combination of electrochemical stability and pH response in PT‐COOH are proposed as advantageous for polymer‐based biosensors.

Giant tunability in electrical contacts and doping via inconsiderable normal electric field strength or gating for a high-performance in ultrathin field effect transistors based on 2D BX/graphene (X = P, As) van der Waals heterobilayer

Electrical contacts arising at the van der Waals interface between boron pnictide (h-BP, h-BAs) and graphene monolayers have been systematically investigated using density functional theory. The electronic band structure of the individual monolayers is well preserved in the heterostructures constituted from them, indicating a weak van der Waals (vdW) interaction between them. BP monolayer is found to form n-type Schottky contact with a Schottky barrier height (SBH) of 0.4 eV in BP/graphene van der Waals heterostructure (vdWH), whereas a small p-type SBH of 0.16 eV occurs at BAs/graphene vdWH. The SBHs obtained for BX/graphene are lower than that of other transition metal dichalcogenide based graphene vdWH and electrical properties are found to be significantly tunable/transformable via small applied electric field of ±0.15 V/Å. These insightful results will motivate experimentalists and technologists to design high performance graphene-based hybrid field-effect transistors (FET). Also, the vdWHs are found to show significant robustness, structural integrity and flexibility. The bending modulus of BP (As)/graphene is found to be lower than that in graphene/MoS2. The graphene/h-BN vdWH has been experimentally studied, while the thin films of h-BP have been recently synthesized; thereby substantiating the experimental feasibility in building up the heterostructures proposed in this work.

Spin Seebeck effect in bipolar magnetic semiconductor: A case of magnetic MoS2 nanotube

Bipolar magnetic semiconductors (BMSs) are a new member of spintornic materials. In BMSs, one can obtain 100% spin-polarized currents by means of the gate voltage. However, most of previous studies focused on their applications in spintronics instead of spin caloritronics. Herein, we show that BMS is an intrinsic model for spin Seebeck effect (SSE). Without any gate voltage and electric field, currents with opposite spin orientation are generated and flow in opposite directions with almost equal magnitude when simply applying a temperature bias. This is also due to the special electronic structure of BMS where the conduction and valence bands near the Fermi level belong to opposite spin orientation. Based on density function theory and non-equilibrium Green’s function methods, we confirm the thermal-induced SSE in BMS using a case of magnetic MoS2 nanotube. The magnitude of spin current in zigzag tube is almost four times higher than that in armchair tube. BMS is promising candidates for spin caloritronic applications.

The Effect of Doping on Different FET Structures: MOSFET, TFET and FinFET

MOSFET have been scaled down over the past few years in order to give rise to high circuit density and increase the speed of circuit. But scaling of MOSFET leads to issues such as poor control gate over the current which depends on gate voltage. Many short channel effects (SCE) influence the circuit performance and leads to the indeterminist response of drain current. These effects can be decreased by gate excitation or by using multiple gates and by offering better control gate the device parameters. In Single gate MOSFET, gate electric field decreases but multigate MOSFET or FinFET provides better control over drain current. In this paper, different FET structures such as MOSFET, TFET and FINFET are designed at 22nm channel length and effect of doping had been evaluated and studied. To evaluate the performance donor concentration is kept constant and acceptor concentration is varied.

Electronic and ionic conduction in ligand-free lead selenide colloidal quantum dots

We fabricated a ligand-free lead selenide (PbSe) colloidal quantum dot (CQD) film and probed its electrical properties using a field-effect transistor (FET) at low temperature. The contribution of the ionic conduction to the electrical properties in the PbSe CQD film became significant after ligand removal using ammonium sulfide [(NH4)2S], evidenced by temperature-dependent FET mobility, threshold voltage, and current hysteresis depending on gate scan direction. The enhanced FET mobility and increased carrier concentration at a lower temperature are attributed to reduced ionic conduction, suppressing the gate electric field screening at the PbSe/SiO2 interface.

Impact of transverse and vertical gate electric field on vibrational and electronic properties of MoS2

Selectivity of the electric field direction plays a vital role in modulating the phonon characteristics as well as electrical properties in low-dimensional materials. A comprehensive study on the effects of the direction-dependent electric field on MoS2 sample is reported herewith. The field-induced changes in the phonon characteristics and electronic band structure have been systematically investigated based on field responsive Raman and photoluminescence measurements. The atomistic insights obtained from density functional theory calculations have been correlated with the experimental observations to elucidate the underlying mechanism. The applied transverse electric field is found to be significantly more efficacious than the electric field applied vertically in altering the phonon signatures and bandgap in MoS2, where the electrostrictive response is found to arise from the field-induced alteration in metal–chalcogen interatomic bonds.

Эффективный интерференционный механизм управления проводимостью элементов молекулярной наноэлектроники

Исследованы структурные модели молекулярных квантовых проводников, транспортные свойства которых могут эффективно управляться за счет интерференционного изменения туннельной прозрачности полем затвора.

Optimization of a Hetero-Structure Vertical Tunnel FET for Enhanced Electrical Performance and Effects of Temperature Variation on RF/Linearity Parameters

A new hetero-structure vertical tunnel field effect transistor is proposed and investigated using Sentaurus Technology Computer-aided Design simulation. Since the direction of the gate electric field is in alignment with the direction of tunneling of electrons, a higher on state current is attained. Optimization of the geometric dimension of the proposed structure is carried out to yield its optimum performance. As temperature variation affects the device performance, temperature analysis of the proposed structure is performed in which both the off state leakage current and the average subthreshold swing increases with temperature. Further investigation on the influence of temperature variation is carried out in terms of Analog/RF parameters such as transconductance, gate capacitance, cut-off frequency, transconductance frequency product and linearity figure of merits. It is observed that the proposed structure attains superior linearity performance and lower distortion at higher gate voltage with rise in temperature and is less affected by temperature variation at lower gate voltage.

Concurrence of quantum anomalous Hall and topological Hall effects in magnetic topological insulator sandwich heterostructures.

The quantum anomalous Hall (QAH) effect is a consequence of non-zero Berry curvature in momentum space. The QAH insulator harbours dissipation-free chiral edge states in the absence of an external magnetic field. However, the topological Hall (TH) effect, a hallmark of chiral spin textures, is a consequence of real-space Berry curvature. Here, by inserting a topological insulator (TI) layer between two magnetic TI layers, we realized the concurrence of the TH effect and the QAH effect through electric-field gating. The TH effect is probed by bulk carriers, whereas the QAH effect is characterized by chiral edge states. The appearance of the TH effect in the QAH insulating regime is a consequence of chiral magnetic domain walls that result from the gate-induced Dzyaloshinskii-Moriya interaction and occurs during the magnetization reversal process in the magnetic TI sandwich samples. The coexistence of chiral edge states and chiral spin textures provides a platform for proof-of-concept dissipationless spin-textured spintronic applications.

Valley-Layer Coupling: A New Design Principle for Valleytronics.

The current valleytronics research is based on the paradigm of time-reversal-connected valleys in two-dimensional (2D) hexagonal materials, which forbids the fully electric generation of valley polarization by a gate field. Here, we go beyond the existing paradigm to explore 2D systems with a novel valley-layer coupling (VLC) mechanism, where the electronic states in the emergent valleys have a valley-contrasted layer polarization. The VLC enables a direct coupling between a valley and a gate electric field. We analyze the symmetry requirements for a system to host VLC, demonstrate our idea via first-principles calculations and model analysis of a concrete 2D material example, and show that an electric, continuous, wide-range, and switchable control of valley polarization can be achieved by VLC. Furthermore, we find that systems with VLC can exhibit other interesting physics, such as valley-contrasting linear dichroism and optical selection of the valley and the electric polarization of interlayer excitons. Our finding opens a new direction for valleytronics and 2D materials research.

Enhancement of Room-Temperature Effective Spin Diffusion Length in a Si-Based Spin MOSFET With an Inversion Channel

We have demonstrated a Si-based bottom-gate-type spin metal-oxide-semiconductor field-effect transistor (spin MOSFET) with an electron inversion channel at room temperature and showed the enhancement of the room-temperature effective spin diffusion length by “spin drift”. Our spin MOSFET was fabricated on a (001)-oriented silicon-on-insulator (SOI) substrate with a p -type ultrathin (8 nm) Si layer, in which the channel length was $1.0~\mu \text{m}$ , the spin injector/detector electrodes were ferromagnetic multilayer (from top to bottom) Fe(4nm)/Mg(1nm)/MgO(1nm) tunnel junctions, and a 200-nm-thick buried oxide layer was used for the gate dielectric. Using various gate electric fields and source-drain electron currents, two types of spin-dependent transport signals were measured: spin-valve signals with an in-plane magnetic field and Hanle signals with an out-of-plane magnetic field. Clear Hanle signals and spin-valve signals were obtained for various bias conditions. We analyzed the Hanle signals and the change of the spin-valve signals based on our original formulas that take into account the distribution of the lateral electric field along the electron transport, and revealed that the spin drift can enhance the effective spin diffusion length by the lateral electric field parallel to the electron transport in the inversion channel. The effective spin diffusion length becomes 3–13 times larger than the intrinsic spin diffusion length ${\lambda }_{\mathrm{ S}}= 0.89\mu \text{m}$ owing to the increase in the lateral electric field. It was confirmed that the spin drift is very useful to achieve larger spin-valve signals in spin MOSFETs with an electron inversion channel.

Mathematical Modelling of Electrically Controlled Filters of Microwave, subTHz and THz–bands on the Base of Grapheneand-Dielectric Multilayer Structure

Report is devoted to investigations of graphene meta–surfaces for the transmission of radiation induced by plasmons in subTHz and THz ranges, cell of which consists of structures based on graphene ring and graphene nano–tape. It create regimes of radiation transmission – transparency windows induced by electric dipole resonances. Resonant frequency of transparency window can be dynamically tuned in wide band of subTHz and THz bands by changing the chemical potential (Fermi energy) of graphene by applying external electric field (gating) instead of re–fabricating of structures. Questions of possibilities of electronic controlled filters creating of subTHz and THz bands grounded on different configurations of graphene meta–surfaces are discussed; their characteristics and frequency dependencies are investigated. Mathematical modelling and electrodynamic calculation of the filters characteristics of subTHz and THz bands grounded on multilayer structures of “graphene–dielectric” type are carried out. From results of mathematical modelling it follows that periodic layered microstructures “graphene–dielectric” type can be used for creation of subTHz and THz bands broadband filters of planar construction, controlling by electric field and fast tuning at small changes in Fermi energy level of graphene.

Oxide semiconductor thin-film transistors with nano-splitting and field-surrounding channels fabricated by subwavelength photolithography

Oxide semiconductors feature high tunability of carrier concentrations under the control of electric field. In thin film transistors (TFTs), applying dual gate has been reported to be efficient in enhancing the coupling between the gate field and the channel accumulation. In this work, we demonstrate nano-splitting and field-surrounding semiconducting channels (based on InGaZnO) in TFTs, which is fabricated by facile subwavelength photolithography. In such TFTs, semiconducting channels have 200 nm gaps parallel to the drain field, are wrapped by oxide insulators and thus the gate field. The devices show enhanced performance as compared with dual-gate and single gate TFTs, exhibiting higher drain current and steeper subthreshold swing. The maximum transconductance gm is 27.9% higher than dual gate TFT and 73.1% higher than single gate TFT. According to device simulation, the improvement of the wrapping insulators device correlates with the three-dimensional accumulation of carriers and increased gate electric field near the semiconductor-dielectric interface. These surrounded-channel effects become noticeable in the device with the gap distance less than 1 μm, with gate electric field squeezing in the submicron gaps. The proposed approach offers an alternative way to take advantage of the oxide semiconductors and their application in TFTs with related circuits.

Investigation of a Ge-Source Vertical TFET With Delta-Doped Layer

In this article, a $\delta $ -doped germanium-source vertical TFET (Ge-source vTFET) is proposed and investigated by Synopsis TCAD simulation. Higher ON-state current is obtained as the electron tunneling takes place in a direction parallel to the gate electric field. The incorporation of the $\delta $ -doped layer in the germanium source region further reduces the OFF-state leakage current. The impact on the variation of the device geometric dimensions has been studied for various electrical parameters and the dimensions are optimized accordingly through extensive simulations. A very high current ratio of the order ~1011 with a substantially low average subthreshold swing of 21.2 mV/decade is achieved. Benchmarking of the proposed Ge-source vTFET with other novel TFETs is carried out which produced better results in terms of high ON -state current and SSavg. The results obtained for the proposed vTFET make it a potential candidate for ultralow-power applications.