Properties Of Field-effect Transistors Research Articles

Intrinsically stretchable naphthalenediimide–bithiophene conjugated statistical terpolymers using branched conjugation break spacers for field–effect transistors

A series of naphthalene−diimide based conjugated polymers was synthesized through statistical terpolymerization with branched conjugation break spacers to enhance their mobility−stretchability properties in field-effect transistors.

Resistive switching behavior and mechanism of multilayer MoS<sub>2</sub> memtransistor under control of back gate bias and light illumination

Memtransistor is a new multi-terminal device which combines the properties of memristor and field effect transistor and simultaneously realizes information storage and processing. In this paper, the multilayer MoS<sub>2</sub> is prepared by micromechanical exploration method, then the back gate MoS<sub>2</sub> memtransistor with field effect transistor structure is fabricated, and the resistive switching characteristics and mechanism of the device under electric field, light field and their synergistic regulation are systematically studied. The experimental results show that the multilayer MoS<sub>2</sub> memtransistor has excellent bipolar resistance behavior and good cycle durability. Under the control of gate voltage, the switching ratio of the device can be tuned in a range of 10<sup>0</sup>－10<sup>5</sup>, up to 1.56 × 10<sup>5</sup>, which indicates that the device has a strong gating effect. Under the control of light illumination, the resistance characteristics of the device are strongly dependent on the incident wavelength. When photoelectric synergistic regulation is performed, the device displays excellent four-terminal control capability, and the switching ratio is enhanced up to 4.8 × 10<sup>4</sup>. The mechanism of resistive switching characteristics can be attributed to the changes of charge capture state and Schottky barrier height at the interface between MoS<sub>2</sub> and metal electrodes, and the continuous photoconductance effect caused by photogenerated carriers in MoS<sub>2</sub> channel.

Посвящается памяти В.Д. Александрова Влияние отжига в инертной атмосфере на электрические свойства кристаллических пленок пентацена

The results of a study of the effect of annealing at 150 °C 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.

Vanadium-Doped Monolayer MoS2 with Tunable Optical Properties for Field-Effect Transistors

Substitutional doping is a promising methodology to tune the optoelectronic properties of transition metal dichalcogenides (TMDs). However, to this date, direct substitution of transition metal ato...

Ultra-Compact Ternary Logic Gates Based on Negative Capacitance Carbon Nanotube FETs

Ternary logic has been studied for several decades as it can offer significant advantages to reduce the number of interconnects and the complexity of operations. However, the excessive transistor count of the existing ternary logic gates can diminish these advantages in practice. In this brief, based on the negative capacitance (NC) feature of the ferroelectric materials and the well-proven electronic properties of the carbon nanotube field-effect transistor (CNTFET), we have proposed ultra-compact ternary logic gates. After developing a NC-CNTFET model, we have designed a 2-transistor ternary inverter, a 4-transistor ternary NAND, and a 4-transistor ternary NOR with the structures and transistor counts similar to the binary complementary metal-oxide-semiconductor (CMOS) logic. The simulation results ascertain the correct and robust functionality of the proposed ternary gates, even in the presence of process variations. Our proposed ternary inverter, NAND, and NOR gates lead to on average 65%, 60%, and 60% improvements in transistor count, 79%, 83%, and 77% improvements in the area, and 34%, 61%, and 54% improvements in energy-delay product (EDP) as compared to the previous state-of-the-art ternary gates. Our approach accentuates that the proposed ternary gates are the potential candidates for demonstrating more complex multi-valued arithmetic-logic units.

Enhanced Gas Sensing Properties of Graphene Transistor by Reduced Doping with Hydrophobic Polymer Brush as a Surface Modification Layer.

Surface modification layer of a silicon substrate has been used to enhance the performance of graphene field-effect transistors (FETs). In this report, ultrathin and chemically robust polymer brush was used as a surface modification to enhance the gas sensing properties of graphene FETs. The insertion of the polymer brush decreased substrate-induced doping of graphene. This leads to a huge increase in field-effect mobility as well as a minimum shift of the Dirac point voltage. The use of the polymer brush enables fast detection of target gas molecules because graphene sensing modality can be maximized at the undoped state of graphene. The increase of source-drain current, as well as the abrupt decrease of electron mobility upon NO2 exposure, was utilized for the instantaneous detection, and a limit of detection of 4.8 ppb was achieved with graphene FETs on PS brush. We also showed excellent cross-sensitivity of graphene gas sensors to NH3, CO2, and relative humidity condition; the source-drain current decreases upon NH3 exposure, while response to CO2 or relative humidity condition is extremely low. Our results prove that reducing the substrate-induced doping of graphene with a polymer brush is a direct method for boosting the gas sensing properties of graphene FETs.

In situ study of sensor behavior of MoS2 field effect transistor for methyl orange molecule in ultra high vacuum condition

We investigate the sensor behavior of the MoS2 field effect transistor (FET) device with the deposition of methyl orange (MO) molecule which is widely used as a chemical probe. The channel of the FET is made of the single layer of MoS2 which makes it highly sensitive to the molecule adsorption, but at the same time the behavior depends much on the surface conditions of the MoS2 channel. In order to make the channel-surface conditions more defined, we prepare an in situ experimental system in which the molecule deposition and the surface- and electrical-characterization of the MoS2 FET are executed in a single ultra-high vacuum chamber. This system makes it possible to examine the change of the FET properties with precise control of the molecule coverage in the sub-monolayer region without the effect of the atmosphere. We detected the shift of the I d–V g curve of the MoS2-FET device with the increase of the molecule coverage (θ) of the MO molecule, which is quantitatively analyzed by plotting the threshold voltage (V th) of the I d–V g curve as a function of θ. The V th shifts towards the negative direction and the initial change with θ can be expressed with an exponential function of θ, which can be accounted for with the Langmuir type adsorption of the molecule for the first layer and the charge transfer from the molecule to the substrate. The V th versus θ curve shows a kink at a certain θ, which is conserved as the starting of the second layer growth. We detected the adsorption of MO far less than monolayer and the phase change from the first layer to the second layer growth, which is realized by the benefit of the in situ UHV experimental condition.

Role played by the nanowire/nanowire junctions in the electrical properties of semiconductor percolating silicon nanowire networks

In this paper, we highlight the key role played by Si nanowire/nanowire junctions in the electrical performance of field-effect transistors (FETs) based on percolating Si nanowire (SiNW) networks, also called nanonets. Using our original technological process to fabricate simultaneously numerous silicon nanonet-based transistors with a variable geometry, we thoroughly investigated in this paper their electrical properties for various channel lengths from 5 to 100 μm. In particular, we evidenced a clear transition in the FET performance, when NW/NW junctions are involved in the conduction path. On the one hand, for channel length shorter than that of nanowires (NWs), the devices are called multiple-parallel-channel FETs as NWs directly bridge the channel, and they exhibit poor electrical performance in terms of OFF current, ON-to-OFF ratio, and subthreshold slope. On the other hand, for channel length longer than that of nanowires (NWs), the devices are called nanonet-FETs (NN-FETs) as the current flows through percolating paths made of NWs and NW/NW junctions and the performance are considerably enhanced. By combining our structural knowledge of the material with experimental observations of electrical properties and modeling results, by relying on the percolation theory, the model for electrical transport in polycrystalline silicon and the principle of potential barrier lowering by the field effect, we propose the junction driven percolation mechanism. It explains the role played by the nanowire/nanowire junctions in the electrical properties of semiconductor percolating nanowire networks and how they drastically enhanced the electrical properties of nanowire-based field-effect transistors despite the disorder introduced by the increasing number of nanowires. As a consequence, by controlling properly the junction interfaces and device geometry for a given NW density, thanks to NW/NW junctions, the NN-based devices are a valuable and promising alternative to improve the electrical performances of NW-based devices for future developments.

In Situ Study of Molecular Doping of Chlorine on MoS2 Field Effect Transistor Device in Ultrahigh Vacuum Conditions.

We report a precise measurement of the sensor behavior of the field effect transistor (FET) formed with the MoS2 channel when the channel part is exposed to Cl2 gas. The gas exposure and the electrical measurement of the MoS2 FET were executed with in situ ultrahigh-vacuum (UHV) conditions in which the surface analysis techniques were equipped. This makes it possible to detect how much sensitivity the MoS2 FET can provide and understand the surface properties. With the Cl2 gas exposure to the channel, the plot of the drain current versus the gate voltage (Id–Vg curve) shifts monotonically toward the positive direction of Vg, suggesting that the adsorbate acts as an electron acceptor. The Id–Vg shifts are numerically estimated by measuring the onset of Id (threshold voltage, Vth) and the mobility as a function of the dosing amounts of the Cl2 gas. The behaviors of both the Vth shift and the mobility with the Cl2 dosing amount can be fitted with the Langmuir adsorption kinetics, which is typically seen in the uptake curve of molecule adsorption onto well-defined surfaces. This can be accounted for by a model where an impinging molecule occupies an empty site with a certain probability, and each adsorbate receives a certain amount of negative charge from the MoS2 surface up to the monolayer coverage. The charge transfer makes the Vth shifts. In addition, the mobility is reduced by the enhancement of the Coulomb scattering for the electron flow in the MoS2 channel by the accumulated charge. From the thermal desorption spectroscopy (TDS) measurement and density functional theory (DFT) calculations, we concluded that the adsorbate that is responsible for the change of the FET property is the Cl atom that is dissociated from the Cl2 molecule. The monotonic shift of Vth with the coverage suggests that the MoS2 device sensor has a good sensitivity to detect 10–3 monolayers (ML) of adsorption corresponding to the ppb level sensor with an activation time of 1 s.

2D Ruddlesden–Popper Perovskite Single Crystal Field‐Effect Transistors

Abstract2D Ruddlesden–Popper perovskites (2D PVKs) have attracted huge interest because of their excellent optoelectronic properties, yet the understanding of their electrical properties is inadequate due to the difficulties in obtaining 2D PVK field‐effect transistors (FETs) with decent performance. Herein, the fabrication and characterization of 2D PVK ((BA)2(MA)n−1PbnI3n+1) single crystal FETs are reported, which exhibit reliable field effect electrical characteristics at low temperatures. Kelvin probe force microscopy (KPFM) results reveal that both ion migration and contact resistance seriously degrade device performance. While ion migration can be suppressed at low temperatures, contact resistance seems to fundamentally determine device performance. On one hand, Schottky contacts are observed to form at the metal/2D PVK interface because of Fermi level pinning, resulting in significant charge injection resistance, although this can be remarkably improved by replacing Au electrodes with Ca. On the other hand, the out‐of‐plane mobility is found to be three orders of magnitude lower than the in‐plane mobility in 2D PVKs, causing large interlayer transport resistance. Thus, a low work‐function metal and a thin crystal are important for achieving high device performance. This work provides important experimental insights into fabrication and electrical properties of 2D PVK FETs.

Development, Processing and Applications of a UV-Curable Polymer with Surface Active Thiol Groups.

We present here a novel resist formulation with active thiol groups at the surface. The material is UV curable, and can be patterned at the micro- and nanoscale by UV nanoimprint lithography. The resist formulation development, its processing, patterning and surface characterization are presented here. In addition, a possible application, including its use to modify the electrical properties of graphene devices is shown. The cured material is highly transparent, intrinsically hydrophilic and can be made more hydrophilic following a UV-ozone or an O2 plasma activation. We evaluated the hydrophilicity of the polymer for different polymer formulations and curing conditions. In addition, a protocol for patterning of the polymer in the micro and nanoscale by nanoimprinting is given and preliminary etching rates together with the polymer selectivity are measured. The main characteristic and unique advantage of the polymer is that it has thiol functional groups at the surface and in the bulk after curing. These groups allow for direct surface modifications with thiol-based chemistry e.g., thiol-ene reactions. We prove the presence of the thiol groups by Raman spectroscopy and perform a thiol-ene reaction to show the potential of the easy “click chemistry”. This opens the way for very straightforward surface chemistry on nanoimprinted polymer samples. Furthermore, we show how the polymer improves the electrical properties of a graphene field effect transistor, allowing for optimal performance at ambient conditions.

Low-Temperature Processed Metal-Semiconductor Field-Effect Transistor with In–Ga–Zn–O Channel Deposited by Ar+O2+H2 Sputtering

We investigated the electrical properties of metal-semiconductor field-effect transistors (MES-FETs) with a stacked channel consisting of hydrogenated In–Ga–Zn–O (IGZO:H) on conventional IGZO (IGZO). The IGZO and IGZO:H films were deposited by Ar+O2 and Ar+O2+H2 sputtering, respectively. The maximum process temperature is 150°C. By applying the IGZO:H/IGZO stacked channel, the on-current of the MES-FET drastically improved compared to the IGZO single-channel, and it exhibited the on-off current ratio of 4.2×108 and subthreshold swing of 155 mV/decade. It was found that diffused hydrogen from IGZO:H to IGZO layer increased electrons up to 1.5×1019 cm-3 near the IGZO:H/IGZO interface. Moreover, the increased electrons confined at the IGZO:H/IGZO interface to form a pseud two-dimensional electron gas because the conduction band offset of 0.18 eV is formed at the IGZO:H/IGZO interface. Thus, the presence of both of the conduction band offset and hydrogen diffusion plays an important role in the MES-FETs performance.

Air Pressure, Gas Exposure and Electron Beam Irradiation of 2D Transition Metal Dichalcogenides

In this study, we investigate the electrical transport properties of back-gated field-effect transistors in which the channel is realized with two-dimensional transition metal dichalcogenide nanosheets, namely palladium diselenide (PdSe2) and molybdenum disulfide (MoS2). The effects of the environment (pressure, gas type, electron beam irradiation) on the electrical properties are the subject of an intense experimental study that evidences how PdSe2-based devices can be reversibly tuned from a predominantly n-type conduction (under high vacuum) to a p-type conduction (at atmospheric pressure) by simply modifying the pressure. Similarly, we report that, in MoS2-based devices, the transport properties are affected by pressure and gas type. In particular, the observed hysteresis in the transfer characteristics is explained in terms of gas absorption on the MoS2 surface due to the presence of a large number of defects. Moreover, we demonstrate the monotonic (increasing) dependence of the width of the hysteresis on decreasing the gas adsorption energy. We also report the effects of electron beam irradiation on the transport properties of two-dimensional field-effect transistors, showing that low fluences of the order of few e-/nm2 are sufficient to cause appreciable modifications to the transport characteristics. Finally, we profit from our experimental setup, realized inside a scanning electron microscope and equipped with piezo-driven nanoprobes, to perform a field emission characterization of PdSe2 and MoS2 nanosheets at cathode–anode separation distances as small as 200 nm.

In Situ Cleaning and Fluorination of Black Phosphorus for Enhanced Performance of Transistors with High Stability.

Most two-dimensional (2D) semiconductors suffer from intrinsic instability under ambient conditions, especially 2D black phosphorus (BP). Although much effort has been made to study the passivation of 2D materials against corrosion by oxygen and water molecules, facile and effective passivation with long-term stability is still challenging; in particular, selective passivation, which is critical for integration into nanoelectronics, is still lacking. Here, we develop a novel passivation route for BP using a fluorinated self-assembled thin film of PFSA (perfluorosulfonic acid, PFSA), where the surface modifier with high hydrophobicity on BP presents extremely stable characteristics over five months under ambient conditions. Moreover, we report for the first time in situ cleaning and selective fluorination of only BP flakes on a SiO2/Si substrate by a spin-coating process followed by ultrasonication, which was attributed to the formation of P-F covalent bonds on the BP surface. Selectively fluorinated BP shows not only enhanced stability in air but also electrical properties of the BP field-effect transistor (FET), with the on-current of the BP FET increasing and presenting enhanced carrier mobility (125 cm2 V-1 s-1) and on/off ratio (104). This significant finding sheds light on fabricating vertical 2D heterostructures to realize high performance and reliability with versatile 2D materials. This work demonstrates an emerging passivation approach for long-term stability together with superior electrical properties, which paves the way for integrating 2D semiconductors into critical channel materials in FETs that are favorable for next-generation digital logic circuits.

Phenothiazine derivatives, diketopyrrolopyrrole-based conjugated polymers: synthesis, optical and organic field effect transistor properties

Novel donor–acceptor (D-A) conjugated polymers containing phenothiazine and diketopyrrolopyrrole derivatives were successfully synthesized via direct arylation polymerization using the palladium catalyst system. The C-N coupling reaction was performed for the synthesis of 4-(10H-phenothiazin-10-yl)-N,N-diphenylaniline (PDA) and 10-(pyren-1-yl)-10H-phenothiazine (PYP) as donor monomers. The D-A conjugated polymers synthesized by polymerization of PDA/PY with diketopyrrolopyrrole have been characterized via GPC, 1H NMR, FTIR, XRD, DSC, PL and UV-Vis spectroscopy, and used for fabrication of organic field effect transistors.

Impact of Read Operation on the Performance of HfO2-Based Ferroelectric FETs

This letter investigates the impact of read operation on the electrical properties of hafnium oxide-based ferroelectric field-effect transistors (FeFETs). We report that a quasi-static read may induce a series of phenomena, including a severe underestimation of the memory window, undesirable loss of the stored state, i.e. destructive read, increased variability and an apparent steep-slope behavior with a subthreshold slope lower than 7 mV/decade, which are not present under a fast read. We explain the results with the time-voltage dependence for the ferroelectric switching. Based on this, we provide comprehensive yet essential guidelines for disturb-free write and read operations.

Optimization of gate-bias stability and gas-sensing properties of triethylsilylethynyl anthradithiophene micro-strip field-effect transistors by incorporating insulating polymer

Abstract The control of the microstructure and patterning of 5,11-bis(triethylsilylethynyl)anthradithiophene (TES-ADT) film is essential for high-performance TES-ADT field-effect transistors (FETs) as a NO2 gas sensor. In this study, a binary blend film based on TES-ADT and insulating polymer was annealed by solvent vapor with patterned PDMS molds. The solvent-containing PDMS mold led to the TES-ADT crystallization under the phase-separation between TES-ADT and PMMA. This resulted in highly crystalline micro-strip films where TES-ADT and the insulating polymer were phase-separated on top and bottom, respectively. The micro-strip structures can have superior gas sensor performance because they have additional gas diffusion paths compared to non-patterned structures. Initial investigations to clarify morphology and microstructure of the films revealed that lateral phase separation got dominant with increasing the portion of the insulating polymer. The blended films were then used as the active layer in the FET-based gas sensors. The use of blended films instead of non-blended TES-ADT film in FETs is advantageous in reducing both the threshold voltage and subthreshold slope while compromising the field-effect mobility. The sensing response as the gas sensors is slightly lower in the blended TES-ADT FET than in the non-blended TES-ADT FET. However, the utilization of blended films significantly enhanced the bias-stress stability. Accordingly, a uniform baseline of the sensor performance was achieved in TES-ADT: insulating polymer blended FETs. This trend was in contrast to an abrupt decrease in the sensing signals of the non-blended TES-ADT FETs. The enhanced bias stability of the blended FETs was exhibited as a result of covering the silanol groups in SiO2 with a phase-separated insulating polymer. Our results provide a route for to the optimization of bias stability and response in TES-ADT micro-strip gas sensors through the application of a one-step blend technology.

Enhanced performance of In2O3 nanowire field effect transistors with controllable surface functionalization of Ag nanoparticles

Indium oxide (In2O3) nanowire field effect transistors (FETs) have great potential in electronic and sensor applications owing to their suitable band width and high electron mobility. However, the In2O3 nanowire FETs reported previously were operated in a depletion-mode, not suitable to the integrated circuits result of the high-power consumption. Therefore, tuning the electrical properties of In2O3 nanowire FETs into enhancement-mode is critical for the successful application in the fields of high-performance electronics, optoelectronics and detectors. In the work, a simple but effective strategy was carried out by preparing Ag nanoparticle functionalized In2O3 NWs to regulate the threshold voltage (Vth) of In2O3 NW FETs, successfully achieving enhanced-mode devices. The threshold voltage can be regulated from −6.9 V to +7 V by controlling Ag density via deposition time. In addition, the devices exhibited high performance: huge Ion/Ioff ratio > 108, large maximum saturation current ≈ 800 mA and excellent carrier mobility ≈ 129 cm2 Vċs−1. The enhanced performance is attributed to the surface passivation by Ag nanoparticles to reduce the density of traps and the charge transfer between traps and the nanowires to regulate the Vth. The result indicates the application of metal nanoparticles significantly improve oxide NW for low-power FETs.

Ohmic contacts of monolayer Tl2O field-effect transistors

Monolayer (ML) Tl2O, as a newly discovered metal-shrouded two-dimensional semiconductor with an appropriate bandgap and high carrier mobility, is a promising candidate as the channel materials for the next-generation field-effect transistors (FETs). Using ab initio electronic structure calculation and quantum transport simulation, the contact properties of ML Tl2O–metal interfaces based on FET are comprehensively investigated with Au, Sc, Tl, Ni, graphene, Ti2C, Ti2CF2, and Ti2C(OH)2 electrodes. ML Tl2O undergoes metallization with Au, Sc, Tl, Ni, and Ti2C, while it forms van der Waals-type contact with graphene, Ti2CF2, and Ti2C(OH)2. An n-type lateral Schottky contact is formed with Ni, Au, Sc, and Ti2C electrodes with the electron Schottky barrier height (SBH) of 0.25, 0.27, 0.27, and 0.36 eV, respectively, while a p-type lateral Schottky contact is formed with graphene electrode with the hole SBH of 0.10 eV. Surprisingly, a desired n-type Ohmic contact arises with Tl and Ti2C(OH)2 electrodes and a desired p-type Ohmic contact arises with Ti2CF2 electrode. The study not only provides a deep understanding of the interfacial properties of the ML Tl2O FETs but also reveals a versatile approach to realize both n- and p-type Ohmic contact for the ML Tl2O FETs.

Photodetectors based on controllable growth of large-area graphene films

In this work, plasma-enhanced chemical vapor deposition method has realized large-area controllable growth of graphene. Thegraphene field effect transistor photodetectorhas been designed and its electrical and photoelectric properties were analyzed. The results show that graphene exhibits good electrical properties in field effect transistors with carrier mobility of 1649 cm−2·V−1•S−1 and conductivity of 7.4 × 104 S/m. The device also shows broad photodetection spectral range from 400 nm to 700 nm. The maximum photo-generated current of the device can reach to mA grade, and the photoresponsivity is 250 A•W−1, 75 A•W−1 and 58.3 A•W−1 at the wavelength of 405 nm, 515 nm, and 650nm, respectively. Meanwhile, the maximum photoconductive gain and detectivity are 1.6 × 1011, 1.3 × 1011 Jones obtained at the wavelength of 405 nm respectively. The results of this work demonstrate that the graphene can be a promising candidate for efficient optoelectronic devices, such as transparent electrodes, opticalmodulator, polarizer, surface plasmonic devices andphotodetectors.