Properties Of Field-effect Transistors Research Articles

Interfacial electrical properties and transport properties of monolayer black AsP alloy in contact with metal

With strong structural stability, ultra-high carrier mobility and controllable direct band gap of semiconductors, the new two-dimensional (2D) monolayer black arsenic-phosphorus (ML b-AsP) alloy has recently attracted much attention in nanoelectronic devices. Through first-principles electronic structure calculations and quantum transport simulations, the interfacial electrical properties of the ML b-AsP/metal (Ag, Al, Au, Fe, Pd, Pt) contacts and transport properties of the integral field effect transistor (FET) configurations constructed with ML b-AsP alloy are investigated first and systematically. The results of electronic structure calculations show that the ML b-AsP alloy is strongly adsorbed to metal substrates and has undergone metallization, which leads to a weak Schottky contact at all the AsP/metal interfaces. Whereas quantum transport simulations show that, with the exception of Pt electrode FET configuration, there are small lateral transport barriers at the electrode/channel interfaces for the other five configurations. The calculations show that the FET configuration with Pt electrode provides better electrical transport performance. In addition, except for the Pd electrode configuration, other configurations exhibit an excellent negative differential conductance (NDC) effect, and some configurations exhibit significant rectifying behavior. These theoretical findings provide guidance, inspiration and insight for the design of future electronic devices based on the ML b-AsP alloy.

Visualizing transport in thiazole flanked isoindigo-based donor–acceptor polymer field-effect transistors

Replacing S with Se in fluorinated isoindigo (IID) copolymers enhances charge transport properties in field-effect transistors (FET). Nonlinear optical imaging from IID FETs helps visualize transport.

Comprehensive understanding of intrinsic mobility and sub-10 nm quantum transportation in Ga2SSe monolayer.

Two-dimensional chalcogenides could play an important role in solving the short channel effect and extending Moore's law in the post-Moore era due to their excellent performances in the spintronics and optoelectronics fields. In this paper, based on theoretical calculations combining density functional theory and non-equilibrium Green's function, we have systematically explored the intrinsic mobility in the Ga2SSe monolayer and quantum transport properties of sub-10 nm Ga2SSe field-effect transistors (FET). Interestingly, the Ga2SSe monolayer presents high intrinsic electron mobility up to 104 cm2 (V s)-1. It is highlighted that the intrinsic mobility in the Ga2SSe monolayer is significantly restrained by phonon scattering, where the out-of-plane acoustic mode and high-frequency optic phonon mode are found predominantly coupled with the electrons. As a result, the n-type doping sub-10 nm Ga2SSe FETs represent distinguished transport properties. In particular, even the gate length is shortened to 3 nm, the on-state current, delay time and power consumption of the n-type doping Ga2SSe FET along the armchair direction can reach the International Technology Roadmap for Semiconductor industry standards for high-performance requirements. Our present study paves the way for the application of Ga2SSe monolayers in ultra-small sized FETs in the post-silicon era.

Dedicated to the memory of V.D. Aleksandrov Effect of annealing in an inert atmosphere on the electrical properties of crystalline pentacene films

The results of a study of the effect of annealing at 150^oC in an inert atmosphere (Ar + 5% H2) on the electrical properties of organic field-effect transistors based on pentacene are presented. Crystalline pentacene films with a thickness of 95±5 nm were obtained using thermal vacuum deposition. The transfer and output characteristics of field-effect transistors before and after annealing for 15 hours are investigated. It was found that as a result of heat treatment, the hole mobility in the saturation regime increased by an average of 30%, and the threshold voltage decreased approximately two times. According to the data of atomic force microscopy, annealing led to a more than twofold decrease in the surface roughness of pentacene films, as well as to a noticeable enlargement of grains, which led to a decrease in the concentration of traps for hole electric transport in the channel of the field-effect transistor. Keywords: pentacene, vacuum thermal deposition, crystalline films, organic field-effect transistors, hole mobility, annealing in an inert atmosphere.

Ultrahigh Anisotropic Transport Properties of Black Phosphorus Field Effect Transistors Realized by Edge Contact

AbstractHighly anisotropic black phosphorus (BP) has recently attracted significant interest for electronic and optoelectronic devices. To date, in‐plane anisotropic properties of BP field effect transistors (FETs) have been reported only with top contact. However, the 2D top contact geometry is unable to measure the in‐plane electrical conductance precisely, due to the presence of the out‐of‐plane conductance, resulting in underestimation of anisotropy. Here, 1D edge contact method is employed to measure the in‐plane conductance precisely along the armchair and zigzag directions of BP without the contribution of out‐of‐plane conductance. The conductance and mobility anisotropies for BP FETs are measured with edge contact at 300 K to be ≈5.5 and ≈7.5, respectively. The results further show that the mobility of BP FETs with edge contact weakly depends on temperature, indicating that the edge roughness scattering limits the mobility. In contrast, the mobility of BP FETs with top contact strongly depends on temperature, showing that the impurity scattering and phonon scattering limit the mobility at below and above 150 K, respectively. Finally, a scattering phase diagram is demonstrated to understand the role of different scattering mechanisms on the modulation of mobility anisotropy in BP FETs with edge and top contacts.

NeuroSOFM: A Neuromorphic Self-Organizing Feature Map Heterogeneously Integrating RRAM and FeFET

Many currently available hardware implementations of the unsupervised self-organizing feature map (SOFM) algorithm utilize complementary metal–oxide–semiconductor (CMOS)-only circuits that often compromise key behaviors of the SOFM algorithm due to complexity. We propose a neuromorphic architecture harnessing the unique properties of ferroelectric field-effect transistors (FeFETs) and gated-resistive random access memory (RRAM) for in-memory computing to implement the SOFM algorithm. The FeFET-based synapse, organized in a novel circuit, is able to compute the input-weight Euclidean error in memory via the saturation drain current. The self-decaying states of the gated-RRAM allow for a self-decaying neighborhood and learning rate implementation to allow for convergence and lifelong learning. This novel architecture is able to successfully cluster benchmarks (RGB colors and MNIST handwritten digits) and real-life datasets, such as COVID-19 patient chest X-rays completely unsupervised. The architecture also demonstrates a significant amount of robustness to device variability and damaged neurons. In addition, the proposed architecture is completely parallelized and provides a power-efficient platform for implementing the SOFM algorithm.

Morphology and Charge Transport Properties of P(NDI2OD-T2)/Polystyrene Blends

While the strategy of blending commodity insulating polymers with conjugated polymers has been previously applied to modify the properties of organic field-effect transistors (OFETs), comprehensive studies on the evolution of morphology and OFET performance with film composition are scarce. In this contribution, films comprising the n-type semiconducting polymer P(NDI2OD-T2) and the insulating polymer polystyrene (PS) are explored in detail. In particular, the electrical properties and film morphology of blends with a P(NDI2OD-T2) concentration ranging from 0.125 to 100 wt % are studied. Two regimes characterized by distinct dependencies of device mobility as a function of P(NDI2OD-T2) content are observed: Above 12.5 wt % P(NDI2OD-T2), the OFET mobility exhibits a weak dependence on P(NDI2OD-T2) concentration despite the presence of strong lateral phase separation, while below 12.5%, the device mobility is observed to decrease strongly with decreasing P(NDI2OD-T2) concentration attributed to disrupted surface connectivity of the P(NDI2OD-T2) phase. UV–vis absorption spectroscopy and grazing incidence wide-angle X-ray scattering (GIWAXS) measurements indicate that the aggregation and crystalline packing are not strongly affected by the presence of PS. Atomic force microscopy and scanning transmission X-ray spectro-microscopy reveal lateral phase separation for blends with compositions of 87.5 to 25 wt % P(NDI2OD-T2). Enrichment of P(NDI2OD-T2) at the film surface is confirmed by near-edge X-ray absorption fine structure spectroscopy, with a high P(NDI2OD-T2) surface composition (>80 wt %) maintained down to an overall blend composition of 12.5 wt % P(NDI2OD-T2). The strong decrease in OFET mobility for P(NDI2OD-T2) concentration less than 25 wt % is attributed to a loss of P(NDI2OD-T2) surface connectivity at low P(NDI2OD-T2) loading. We also observe a transition from face-on to edge-on dominated configuration at low P(NDI2OD-T2) content with GIWAXS, consistent with an edge-on surface layer that persists down to monolayer surface coverage.

Effects of N‐Ion Implantation on the Electrical and Photoelectronic Properties of MoS2 Field Effect Transistors

Ion implantation is widely used in the semiconductor industry as an important method of controlling the performance of semiconductor materials. As a 2D material with fantastic physical characteristics, MoS2 has been receiving extensive attention for its potential applications in electronic devices. N element is an impurity objectively existing in semiconductors. It may come from the semiconductor manufacturing process or the environment. Therefore, the effect of N element implantation on the material, electrical, and photoelectronic properties of MoS2 field effect transistor (FET) is reported. The results of Raman and photoluminescence (PL) shows that N‐ion implant caused the expansion of the MoS2 lattice and the disappearance of the characteristic peaks of PL. The prepared MoS2 FET exhibits a slow photoelectric response to 254 nm UV, and a fast photoelectric response to 650 and 532 nm lasers. After ion implant and annealing, the electrical performance and photocurrent of the MoS2 FET are significantly degraded. The research has a guiding role in impurity control in the semiconductor manufacturing process.

Large difference of doping behavior of tetracyanoquinodimethane (TCNQ) and 2,3,5,6-Tetrafluoro-7,7,8,8- tetracyanoquinodimethane (F4-TCNQ) on field effect transistor with channel of atomic layer MoS2

We examined the mechanism of the large difference of the doping effect of two molecules of tetracyanoquinodimethane(TCNQ) and 2,3,5,6-Tetrafluoro-7,7,8,8- tetracyanoquinodimethane (F4-TCNQ) on the field effect transistor (FET) property, which is composed of the atomically thin MoS2 channel. The electron-acceptor behaviors of the two molecules are detected by examining the threshold voltage shift (ΔVth) of the drain current vs gate voltage curve plot. It shows only a mild shift of ∼4 V for the TCNQ but it shows a large value of ∼33 V for the F4-TCNQ molecule. The similar rapid change with small amount of F4-TCNQ deposition was detected as the Fermi level shift in the ultra violet photoemission spectroscopy (UPS). The DFT calculation shows the flat lying configuration for both molecules after the structural optimization, and the difference of the charge transfer to the substrate is not large enough to explain the difference of theΔVth. Instead, we want to propose a model in which the bonding of the F atom of the F4-TCNQ molecule at the S-defect contribute to the initial rapid variation of theΔVth.

A First-Principles Study of Transport Properties in Armchair Germanene Nanoribbon Field Effect Transistors Subject to Metallic Dopants

Density functional theory (DFT) in combination with non-equilibrium Green’s function (NEGF) formalism are utilized to reveal the transport properties of armchair Germanene nanoribbon field effect transistors (AGeNR-FETs) subject to metallic dopants. Due to the semiconducting nature of p-type and n-type in the AGeNRs in the presence of In and Ag/Au dopants, these atoms are used at the source/drain electrodes to make FETs and Tunneling-FETs (TFETs). The transport properties of these transistors subject to Ag, Au and In atoms are analyzed with the aid of I–VDS, I–VG characteristics and transmission spectrum. It is shown that a negative differential resistance (NDR) is observed for each structure which occurs at low bias voltages. In particular, the maximum NDR with Ip/Iv = 14.78 is observed for the AGeNR-FET. From the transfer characteristics at VDS = 0.7 V it is also observed that depending on the type of impurity and the transistor, the minimum current decreases or increases, Dirac point shifts, and on-state current decreases. In addition, the highest Ion/Ioff ratio in FET is related to Ag-AGeNR structure with the value of 49 while for TFET it is related to In-Ag-AGeNR structure with a value of 6.4.

Electron-to Hole Transport Change Induced by Solvent Vapor Annealing of Naphthalene Diimide Doped with Poly(3-Hexylthiophene).

Herein we report on fabrication and properties of organic field-effect transistors (OFETs) based on the spray-coated films of N,N′-dioctyl naphthalene diimide (NDIC8) doped with 2.4 wt% of poly (3-hexylthiophene) (P3HT). OFETs with the untreated NDIC8:P3HT films revealed electron conductivity [μe* = 5 × 10–4 cm2×(Vs)−1]. After the annealing in chloroform vapor the NDIC8:P3HT films revealed the hole transport only [μh* = 0.9 × 10–4 cm2×(Vs)−1]. Due to the chemical nature and energy levels, the hole transport was not expected for NDIC8-based system. Polarized optical- and scanning electron microscopies indicated that the solvent vapor annealing of the NDIC8:P3HT films caused a transition of their fine-grained morphology to the network of branched, dendritic crystallites. Grazing incidence wide-angle X-ray scattering studies indicated that the above transition was accompanied by a change in the crystal structure of NDIC8. The isotropic crystal structure of NDIC8 in the untreated film was identical to the known crystal structure of the bulk NDIC8. After the solvent annealing the crystal structure of NDIC8 changed to a not-yet-reported polymorph, that, unlike in the untreated film, was partially oriented with respect to the OFET substrate.

Electronic Structure and Quantum Transport Properties of 2D SiP: A First-Principles Study

With the rapid evolution of microelectronics, the field of integrated circuits is facing unprecedented challenges. Traditional silicon-based transistors cannot maintain the advantages of high performance during the process of further ultra-scaling due to severe short-channel effects. Two-dimensional (2D) materials are potential channel materials that can replace silicon. Herein, 2D SiP is predicted to have a band gap of 1.49 eV with anisotropic electronic properties by means of first-principles calculations, which is suitable as a channel candidate of transistors. Hence, we investigate the ballistic transport properties of 2D SiP double-gate metal oxide semiconductor field-effect transistors (MOSFETs) by using ab initio quantum transport simulations. Despite anisotropic electronic properties of 2D SiP, the performances of monolayer SiP MOSFETs have weak directional dependence due to high valley degeneracy. The n-MOSFETs with 10-nm gate length can fulfill the high-performance requirements of the International Roadmap for Devices and Systems 2020 Edition (IRDS 2020). However, the p-MOSFETs cannot fulfill the demands of IRDS 2020 because of heavy hole effective masses. Considering the appropriate on-current of 1292 μA/μm for SiP n-MOSFETs, 2D SiP could be utilized as a potential channel material in the next-generation FETs.Graphic abstract

High-performance n-type transistors based on CVD-grown large-domain trilayer WSe2

Atomically thin layered tungsten diselenide (WSe2) has attracted tremendous research attention for its potential applications in next-generation electronics. This article reports the synthesis method of high-quality monolayer to trilayer WSe2 by molten-salt-assisted chemical vapor deposition. With the optimization of different types of molten salts and depths of corundum boat, large trilayer WSe2 films can be grown with domain size up to 80 µm for the first time. A systematic study of the electrical properties of the n-type field-effect transistor has been carried out based on WSe2 with the above three different layer thicknesses. The trilayer WSe2 devices exhibit higher drive current, mobility, on/off ratio, and lower contact resistance than both bilayer and monolayer counterparts. Moreover, short channel transistors using the trilayer WSe2 with a channel length of 230 nm have been fabricated, exhibiting an excellent on/off ratio up to 108 and a high current density of 187 µA/μm. This facile synthesis of high-quality large-area multilayer WSe2 provides a pathway for future high-performance two-dimensional electronic devices.

Influence of swift heavy ion irradiation on sensing properties of nickel-(NRs-Ni3HHTP2) metal-organic framework

Repercussion of Swift Heavy Ion (SHI) irradiation on nickel-based nanorods of Metal-Organic Framework (NRs-Ni3HHTP2 MOF) for enhancement in the properties of ChemFET-based gas sensor has been investigated. Nanorods of Ni3HHTP2-MOF were synthesized by chemical method and exposed to C12+ ions irradiation with fluence 1 × 1011 ion/cm2 and 1 × 1012 ion/cm2. The structural, spectroscopic, morphological, and optical characterizations were carried out using x-ray diffraction (XRD), fourier transfer infrared spectroscopy (FTIR), atomic force microscopy (AFM) with scanning electron microscopy (SEM), and UV-visible spectroscopy, respectively, whereas the bandgap was calculated from the Tauc’s plot. The synthesized nanorods of Ni3HHTP2 MOF were drop-casted on gold-coated microelectrodes on silicon/silicon dioxide (Si/SiO2) substrate, where silicon layer serves as a gate and gold microelectrodes on silicon/silicon dioxide (Si/SiO2) substrate as a source and drain. The transmutations in material properties due to SHI irradiations were serviceable for enhancing field-effect transistor (transfer and output) properties and sensing properties. After Swift Heavy Ion (SHI) irradiation (1 × 1011 ion/cm2), it shows excellent response and recovery time i.e., 20 and 23 s, respectively, for 1 ppm SO2 concentration at room temp (RT) with a lower detection limit of 0.625 ppm.

Electrical contacts in monolayer Ga2O3 field-effect tansistors

Monolayer (ML) Ga2O3 with wide bandgap and ultra-high electron mobility has gained extensive interests due to its great potential in next-generation electronic and solar-blind optoelectronic applications. Here, we perform a comprehensive investigation on the electronic properties of the ML Ga2O3 field-effect transistors (FETs) with different metal electrodes spanning a wide work function using energy band structure and quantum transport (QT) calculations. The results indicate all the investigated metals form n-type Ohmic contact with ML Ga2O3 in vertical direction, while high Schottky barrier heights (SBHs) exist for holes except for Sc due to its metallization and re-construction at the contact interface. In addition, the existence of tunneling barrier (TB) at the interface of ML Ga2O3/Ag, Au, Pd, and Pt systems reduce the electron injection efficiency to 20.09%, 13.03%, 34.70%, and 26.29%, respectively. Using a back-gated FET configuration, Al electrode exhibits the highest performance with Ohmic contact property and absence of tunneling barrier, thus is believed to act as the best candidate electrode material for ML Ga2O3 transistors. In addition, the results indicate that the current of ML Ga2O3 transistor is dominated by electrons rather than holes. Our study offers a theoretical foundation for the electrode selection for ML Ga2O3 devices.

High mobility field-effect transistors based on MoS2 crystals grown by the flux method

Two-dimensional (2D) molybdenum disulphide (MoS2) transition metal dichalcogenides (TMDs) have great potential for use in optical and electronic device applications; however, the performance of MoS2 is limited by its crystal quality, which serves as a measure of the defects and grain boundaries in the grown material. Therefore, the high-quality growth of MoS2 crystals continues to be a critical issue. In this context, we propose the formation of high-quality MoS2 crystals via the flux method. The resulting electrical properties demonstrate the significant impact of crystal morphology on the performance of MoS2 field-effect transistors. MoS2 made with a relatively higher concentration of sulphur (a molar ratio of 2.2) and at a cooling rate of 2.5 °C h−1 yielded good quality and optimally sized crystals. The room-temperature and low-temperature (77 K) electrical transport properties of MoS2 field-effect transistors (FETs) were studied in detail, with and without the use of a hexagonal boron nitride (h-BN) dielectric to address the mobility degradation issue due to scattering at the SiO2/2D material interface. A maximum field-effect mobility of 113 cm2 V−1 s−1 was achieved at 77 K for the MoS2/h-BN FET following high-quality crystal formation by the flux method. Our results confirm the achievement of large-scale high-quality crystal growth with reduced defect density using the flux method and are key to achieving higher mobility in MoS2 FET devices in parallel with commercially accessible MoS2 crystals.

Controlling Structural and Energetic Disorder in High-Mobility Polymer Semiconductors via Doping with Nitroaromatics

Molecular doping has emerged as a powerful strategy to tune the charge transport properties of organic field-effect transistors (OFETs). However, the limited tool-box of molecular dopants and unres...

Impact of device scaling on the electrical properties of MoS2 field-effect transistors

Two-dimensional semiconducting materials are considered as ideal candidates for ultimate device scaling. However, a systematic study on the performance and variability impact of scaling the different device dimensions is still lacking. Here we investigate the scaling behavior across 1300 devices fabricated on large-area grown MoS2 material with channel length down to 30 nm, contact length down to 13 nm and capacitive effective oxide thickness (CET) down to 1.9 nm. These devices show best-in-class performance with transconductance of 185 μS/μm and a minimum subthreshold swing (SS) of 86 mV/dec. We find that scaling the top-contact length has no impact on the contact resistance and electrostatics of three monolayers MoS2 transistors, because edge injection is dominant. Further, we identify that SS degradation occurs at short channel length and can be mitigated by reducing the CET and lowering the Schottky barrier height. Finally, using a power performance area (PPA) analysis, we present a roadmap of material improvements to make 2D devices competitive with silicon gate-all-around devices.

Two-step chemical vapor deposition synthesis of NiTe2-MoS2 vertical junctions with improved MoS2 transistor performance

The primary challenge for the widespread application of two-dimensional (2D) electronics is to achieve satisfactory electrical contacts because, during the traditional metal integration process, difficulties arise due to inevitable physical damage and selective doping. Two-dimensional metal–semiconductor junctions have attracted attention for the potential application to achieve reliable electrical contacts in future atomically thin electronics. Here we demonstrate the van der Waals epitaxial growth of 2D NiTe2-MoS2 metal–semiconductor vertical junctions where the upper NiTe2 selectively nucleates at the edge of the underlying MoS2. Optical microscopy (OM), scanning electron microscopy (SEM), atomic force microscopy (AFM), and scanning transmission electron microscope (STEM) studies confirmed that NiTe2-MoS2 metal–semiconductor vertical junctions had been successfully synthesized. The electrical properties of the NiTe2-contacted MoS2 field-effect transistors (FETs) showed higher field-effect mobilities (μ FE) than those with deposited Cr/Au contacts. This study demonstrates an effective pathway to improved MoS2 transistor performance with metal–semiconductor junctions.

Enhancement of Memory Properties of Pentacene Field-Effect Transistor by the Reconstruction of an Inner Vertical Electric Field with an n-Type Semiconductor Interlayer

Organic field-effect transistors (OFETs) as nonvolatile memory units are essential for lightweight and flexible electronics, yet the practical application remains a great challenge. The positively charged defects in pentacene film at the interface between pentacene and polymer caused by environmental conditions, as revealed by theoretical and experimental research works, result in unacceptable high programming/erasing (P/E) gate voltages in pentacene OFETs with polymer charge-trapping dielectric. Here, we report a pentacene OFET in which an n-type semiconductor layer was intercalated between a polymer and a blocking insulator. In this structure, the hole barrier caused by the defect layer can be adjusted by the thickness and charge-carrier density of the n-type semiconductor interlayer based on the electrostatic induction theory. This idea was implemented in an OFET structure Cu/pentacene/poly(2-vinyl naphthalene) (PVN)/ZnO/SiO2/Si(p+), which shows low P/E gate voltages, large field-effect mobility (0.73 cm2 V-1 s-1), fast P/E speeds (responding to a pulse width of 5 × 10-4 s), and long retention time in air.