Bottom-gate Field-effect Transistors Research Articles

Pixelated large area rGO on silicon based x-ray detector

In this work, the possibility of using reduced Graphene oxide for x-ray detection has been explored. A highly conductive reduced Graphene Oxide (rGO) synthesized using a hybrid method was used to fabricate a pixelated Si/SiO2 bottom gate field effect transistor. The fabricated device is a 3×3 pixelated large area detector and was tested for its response to x-rays at room temperature and low temperatures by irradiating it with x-rays from top. Significant change in resistance of rGO is observed during irradiation which shows its sensitivity to x-rays.

Comparing solution-gate and bottom-gate nanowire field-effect transistors on pH sensing with different salt concentrations and surface modifications

Field-effect transistors (FETs) have been developed as pH sensors by using various device structures, fabrication technologies, and sensing film materials. Different transistor structures, like extended-gate (EG) FETs, floating-gate FET sensors, and dual-gate (DG) FETs, can enhance the sensor performance. In this article, we report the effects of using solution-gate and bottom-gate FET configurations on pH sensing and investigate the influence of different ionic concentrations of buffers in the measured signals. The surface charge of hafnium dioxide (HfO2) affected by the buffer pH, with/without the modification of polyethylene glycol (PEG) terminated with hydroxyl groups, and the location of applied gate voltage are vital factors to the sensor performance in pH sensing. Based on the results, the solution-gate FET exhibits good pH sensitivity even in the high ionic strength solutions of bis-tris propane (BTP), and these values of pH sensitivity are close to the Nernst limit (59.2 mV/pH). In general, silane-PEG-OH modification can reduce the deviations of measured signals in pH sensing. The performance of bottom-gate FET is inferior in the BTP buffers with high ionic solutions but suitable to be operated in low ionic concentrations, such as 0.1, 1, and 10 mM BTP buffers. The size of the ions was also studied and discussed. The solution-gate FET demonstrates excellent performance under high ionic strengths, meaning a more significant potential for detecting biological molecules under physiological conditions.

Three-Dimensional Surface Treatment of MoS2 Using BCl3 Plasma-Derived Radicals.

The realization of next-generation gate-all-around field-effect transistors (FETs) using two-dimensional transition metal dichalcogenide (TMDC) semiconductors necessitates the exploration of a three-dimensional (3D) and damage-free surface treatment method to achieve uniform atomic layer-deposition (ALD) of a high-k dielectric film on the inert surface of a TMDC channel. This study developed a BCl3 plasma-derived radical treatment for MoS2 to functionalize MoS2 surfaces for the subsequent ALD of an ultrathin Al2O3 film. Microstructural verification demonstrated a complete coverage of an approximately 2 nm-thick Al2O3 film on a planar MoS2 surface, and the applicability of the technique to 3D structures was confirmed using a suspended MoS2 channel floating from the substrate. Density functional theory calculations supported by optical emission spectroscopy and X-ray photoelectron spectroscopy measurements revealed that BCl radicals, predominantly generated by the BCl3 plasma, adsorbed on MoS2 and facilitated the uniform nucleation of ultrathin ALD-Al2O3 films. Raman and photoluminescence measurements of monolayer MoS2 and electrical measurements of a bottom-gated FET confirmed negligible damage caused by the BCl3 plasma-derived radical treatment. Finally, the successful operation of a top-gated FET with an ultrathin ALD-Al2O3 (∼5 nm) gate dielectric film was demonstrated, indicating the effectiveness of the pretreatment.

Defect Engineering in Thickness-Controlled Bi2O2Se-Based Transistors by Argon Plasma Treatment.

We present a simple, effective, and controllable method to uniformly thin down the thickness of as-exfoliated two-dimensional Bi2O2Se nanoflakes using Ar+ plasma treatment. Atomic force microscopy (AFM) images and Raman spectra indicate that the surface morphology and crystalline quality of etched Bi2O2Se nanoflakes remain almost unaffected. X-ray photoelectron spectra (XPS) indicate that the O and Se vacancies created during Ar+ plasma etching on the top surface of Bi2O2Se nanoflakes are passivated by forming an ultrathin oxide layer with UV O3 treatment. Moreover, a bottom-gate Bi2O2Se-based field-effect transistor (FET) was constructed to research the effect of thicknesses and defects on electronic properties. The on-current/off-current (Ion/Ioff) ratio of the Bi2O2Se FET increases with decreasing Bi2O2Se thickness and is further improved by UV O3 treatment. Eventually, the thickness-controlled Bi2O2Se FET achieves a high Ion/Ioff ratio of 6.0 × 104 and a high field-effect mobility of 5.7 cm2 V-1 s-1. Specifically, the variation trend of the Ion/Ioff ratio and the electronic transport properties for the bottom-gate Bi2O2Se-based FET are well described by a parallel resistor model (including bulk, channel, and defect resistance). Furthermore, the Ids-Vgs hysteresis and its inversion with UV irradiation were observed. The pulsed gate and drain voltage measurements were used to extract trap time constants and analyze the formation mechanism of different hysteresis. Before UV irradiation, the origin of clockwise hysteresis is attributed to the charge trapping/detrapping of defects at the Bi2O2Se/SiO2 interface and in the Bi2O2Se bulk. After UV irradiation, the large anticlockwise hysteresis is mainly due to the tunneling between deep-level oxygen defects in SiO2 and p++-Si gate, which implies the potential in nonvolatile memory.

Damage-Free Charge Transfer Doping of 2D Transition Metal Dichalcogenide Channels by van der Waals Stamping of MoO3 and LiF.

To dope 2D semiconductor channels, charge-transfer doping has generally been done by thermal deposition of inorganic or organic thin-film layers on top of the 2D channel in bottom-gate field-effect transistors (FETs). The doping effects are reproducible in most cases. However, such thermal deposition will damage the surface of 2D channels due to the kinetic energy of depositing atoms, causing hysteresis or certain degradation. Here, a more desirable charge-transfer doping process is suggested. A damage-free charge-transfer doping is conducted for 2D MoTe2 (or MoS2 ) channels using a polydimethylsiloxane stamp. MoO3 or LiF is initially deposited on the stamp as a doping medium. Hysteresis-minimized transfer characteristics are achieved from stamp-doped FETs, while other devices with direct thermal deposition-doped channels show large hysteresis. The stamping method seems to induce a van der Waals-like damage-free interface between the channel and doping media. The stamp-induced doping is also well applied for a MoTe2 -based complementary inverter because MoO3 - and LiF-doping by separate stamps effectively modifies two ambipolar MoTe2 channels to p- and n-type, respectively.

Extreme Orientational Uniformity in Large-Area Floating Films of Semiconducting Polymers for Their Application in Flexible Electronics.

Layer-by-layer fabrication of uniformly oriented thin films over large areas by cost-effective solution-based approaches can open new horizons for the realization of high-performance organic circuits in various applications. In this work, fabrication of a large-area ≈40 cm2 film with uniform orientation is reported for poly(3,3‴-dialkylquaterthiophene) (PQT) using a unidirectional floating film transfer method (UFTM). Orientation characteristics and charge transport anisotropy were analyzed using polarized UV-vis spectral mapping and fabrication of bottom-gated organic field-effect transistors (OFETs) from different regions. Films were found to be highly oriented with an optical dichroic ratio of ca. 15. Orientation characteristics reveal that films were highly oriented along the width of the film, covering >70% of the area, and angle-dependent field-effect mobilities are in good agreement with the orientation of the polymer backbones. These highly oriented films resulted in charge transport anisotropy of 8.9. An array of bottom-gated OFETs fabricated along the length of single large-area (≈15 × 2.5 cm2) thin film demonstrated the average field-effect mobility of 0.0262 cm2/(V s) with a very narrow standard deviation of 12.6%. We also demonstrated that film thickness can be easily tuned from 5.6 to 45 nm by increasing the PQT concentration, and field-effect mobility is highly reproducible even when the film thickness is 10 nm. Microstructural characterization of the thus-prepared large-area thin films revealed the edge-on stacked polymer backbones and surface roughness of <1 nm as probed by grazing incidence X-ray diffraction and atomic force microscopy, respectively. Flexible OFETs with bottom-gate top-contact geometry were also fabricated, having average field-effect mobility of 0.0181 cm2/(V s). There was no considerable change in mobility after bending the flexible devices at different radii.

Technique for Fabricating Ferromagnetic/Silicon Active Devices and Their Transport Properties

Semiconductor nanowires are unique materials for studying nanoscale phenomena; the possibility of forming silicon nanowires on bulk silicon-on-insulator substrates in a top-down process ensures complete incorporation of this technology into integrated electronic systems. In addition, the use of ferromagnetic contacts in combination with the high quality of ferromagnetic–semiconductor interfaces open up prospects for the use of such structures in spintronics devices, in particular, spin transistors. A simple approach is proposed to create semiconductor nanowire-based active devices, specifically, bottom-gate Schottky-barrier field-effect transistors with a metal (Fe) source and drain synthesized on a silicon-on-insulator substrate and the transport characteristics of the designed transistors are investigated.

Flexible Bottom-Gated Organic Field-Effect Transistors Utilizing Stamped Polymer Layers from the Surface of Water.

The facile sequential deposition of functional organic thin films by solution processes is critical for the development of a variety of high-performance organic devices without restriction in terms of materials and processes. Herein, we propose a simple fabrication process that entails stacking multiple layers of functional polymers to fabricate organic field-effect transistors (OFETs). The process involves stamping organic semiconducting layers formed on the surface of water onto a commonly used polymeric dielectric layer. Our scheme makes it possible to independently optimize organic semiconductor films by controlling the solvent evaporation time during the process of film formation on the surface of water. This approach eliminates the need to be concerned about any interference with adjacent layers. Utilizing this process, the fabrication of high-performance bottom-gated OFETs is demonstrated on a glass and a flexible substrate. The OFETs consist of a vertically stacked diketopyrrolopyrrole-based polymer semiconducting layer on the poly(methyl methacrylate) film with a maximum hole mobility of 0.85 cm2/V s.

Gallium dopant-induced tunable electrical properties of reduced graphene oxide using metal organic chemical vapor deposition

In this study, we develop a gallium-doped reduced graphene oxide (Ga-rGO) material with desirable electrical properties by a metal-organic chemical vapor deposition system using the pulsed mode technique. The effect of doping level on Ga bonding in rGO layer controlled via the pulsed mode of doping conditions, such as dopant source injection and interruption durations, is investigated using the X-ray photoelectron spectroscopy (XPS). The XPS data indicated that the pulsed mode of doping conditions resulted in a higher Ga composition and a larger O–Ga bonding portion in the Ga-rGO layer. Additionally, the Dirac point of Ga-rGO layer with the pulsed mode was observed to shift toward positive voltage in the bottom-gate field effect transistor device, leading to a p-type behavior. The higher work function of Ga-rGO affected the Schottky contact between the Ga-rGO layer and n-type Si leading to lower fermi-energy level in the energy-band structure. Consequently, this work proposes a simplistic approach of Ga doping in graphene materials that can modulate the tunable electrical properties.

Modeling and Simulation Study of Reduced Self-Heating in Bottom-Gate β-Ga2O3 MISFETs with a h-BN Gate Insulator

Beta-gallium oxide (β-Ga2O3) is an emerging ultra-wide bandgap semiconductor material for high-power devices. However, one of the major drawbacks is the low thermal conductivity resulting in poor heat dissipation, and the so-called self-heating effect reduces carrier mobility and drain current degradation, and even causes a device reliability issue. Here, we propose a bottom-gate β-Ga2O3 field-effect transistor with a hexagonal boron-nitride (h-BN) gate-insulator and investigate the self-heating effect in comparison with an aluminum oxide (Al2O3) insulator using physics-based TCAD simulations. The h-BN with high thermal conductivity reduces the lattice temperature of the β-Ga2O3 channel and decreases drain current degradation. Furthermore, as the thickness of the insulator decreases below 50 nm and the channel length is scaled down to 5 μm, the reduced self-heating effect becomes more prominent. The results imply that the highly thermal-conductive h-BN insulator is promising for achieving high performance β-Ga2O3 metal insulator semiconductor field-effect transistor (MISFET) with the bottom-gate configuration.

Low Bandgap Donor-Acceptor π-Conjugated Polymers From Diarylcyclopentadienone-Fused Naphthalimides.

Two novel aromatic imides, diarylcyclopentadienone-fused naphthalimides (BCPONI-2Br and TCPONI-2Br), are designed and synthesized by condensation coupling cyclopentadienone derivatives at the lateral position of naphthalimide skeleton. It has been found that BCPONI-2Br and TCPONI-2Br are highly electron-withdrawing acceptor moieties, which possess broad absorption bands and very low-lying LUMO energy levels, as low as −4.02 eV. On the basis of both building blocks, six low bandgap D-A copolymers (P1–P6) are prepared via Suzuki or Stille coupling reactions. The optical and electrochemical properties of the polymers are fine-tuned by the variations of donors (carbazole, benzodithiophene, and dithienopyrrole) and π-conjugation linkers (thiophene and benzene). All polymers exhibit several attractive photophysical and electrochemical properties, i.e., broad near-infrared (NIR) absorption, deep-lying LUMO levels (between −3.88 and −3.76 eV), and a very small optical bandgap as low as 0.81 eV, which represents the first aromatic diimide-based polymer with an of <1.0 eV. An investigation of charge carrier transport properties shows that P5 exhibits a moderately high hole mobility of 0.02 cm2 V−1 s−1 in bottom-gate field-effect transistors (FETs) and a typical ambipolar transport behavior in top-gate FETs. The findings suggest that BCPONI-2Br, TCPONI-2Br, and the other similar acceptor units are promising building blocks for novel organic semiconductors with outstanding NIR activity, high electron affinity, and low bandgap, which can be extended to various next-generation optoelectronic devices.

Channel-length-dependent performance of photosensitive organic field-effect transistors

Pentacene is a small molecule organic semiconductor and has high absorption in the UV and visible region. In this work, we report the channel-length-dependent performance of bottom-gate top-contact photosensitive organic field-effect transistors based on pentacene with different channel lengths ranging from 25 to 150μm. The results show that, for a given drain voltage, the photoresponsivity, external quantum efficiency, and specific detectivity decrease with increasing channel length. For a given channel length and gate voltage, the photoresponsivity, external quantum efficiency, and specific detectivity increase with drain voltages, and then tend to saturate at high drain voltages.

Solution-processed organic field-effect transistors with cross-linked poly(4-vinylphenol)/polyvinyl alcohol bilayer dielectrics

In bottom-gated organic field-effect transistors (OFETs) carrier transport and device performances show strong dependencies on the dielectric and surface properties of gate insulating materials. A single-layer gate dielectric cannot meet the requirements of large permittivity (k) and low surface polarity for fabricating low-operating-voltage, high-performance OFETs. Here, we have designed a bilayer gate dielectric consisting of a low-k cross-linked poly(4-vinylphenol) (CL-PVP) in contact with an active layer to provide a low polar surface and an underneath high-k polyvinyl alcohol (PVA) layer to ensure a high capacitance. The results show that the CL-PVP/PVA bilayer not only offers a smooth and uniform surface with low polarity and low surface energy as well as good solvent resistance for efficient carrier transport, but also possesses a relatively high capacitance and low leakage current for low threshold and operating voltages. We have utilized this bilayer dielectric to fabricate OFETs based on a conventional semiconducting polymer poly(3-hexylthiophene) (P3HT). High overall device performances have been achieved in the devices with a mobility of 0.1 cm2 V−1 s−1. The maximum density of trap states at the interface between the P3HT film and the CL-PVP/PVA bilayer is estimated to be 7.10 × 1012 cm−2 eV−1, further proving that this bilayer dielectric provides a high quality interface for P3HT. The CL-PVP/PVA bilayer is thus a promising candidate as a gate dielectric for fabricating high-performance organic or even organic-inorganic hybrid perovskite transistors by a solution processing technique.

Graphene-based field effect transistor with ion-gel film gate

Graphene is a kind of two-dimensional material with high light transmittance, high mechanical properties and high carrier mobility. The energy band of graphene can be turned by doping and electric field. Researches on the application of graphene to electronic devices focused on field effect transistors. For improving the performance, one generally improves the fabrication process and device structure, but many researchers chose to change the material or structure of dielectric layer. Ion-gel is a kind of mixture of organic polymer mesh structure with good thermal stability and high dielectric value, prepared by macromolecule organic polymer and ionic salt electrolyte material. With the effect of electric field, cations and anions in ion-gel diffuse to form a double charge layer distribution with a charge layer on the surface of material. This capacitance characteristic is similar to that of traditional capacitor. In this paper, ion-gel (PVDF-[EMIM]TF2N) film is used as a dielectric layer material to prepare the bottom-gate graphene-based field effect transistor (GFET), which is compared with the GFET with SiO&lt;sub&gt;2&lt;/sub&gt; bottom-gate, according to electrical characteristic curves. The effect of the ion-gel film on the transconductance, switching ratio and Dirac voltage of the GFET are analyzed. The effect of the vacuum environment and temperature on the GFET performance with ion-gel film gate are also investigated. The results show that in the room-temperature environment, the switching ratio and transconductance of the ion-gel film gate GFET device increase to 6.95 and 3.68 × 10&lt;sup&gt;–2&lt;/sup&gt; mS, respectively, compared with those of the SiO&lt;sub&gt;2&lt;/sub&gt; gate GFET, while the Dirac voltage decreases to 1.3 V. The increase in transconductance and switching ratio of ion-gel film gate GFETs are mainly due to the high capacitance of ion-gel film compared with those of conventional SiO&lt;sub&gt;2&lt;/sub&gt; gate dielectrics. There will be more carriers inside the graphene while in the carrier accumulation region of GFET transfer characteristic curve, which makes graphene more conductive. The Dirac voltage of ion-gel film gate GFET can be reduced to 0.4 V in the vacuum environment; as the temperature increases, the transconductance of GFET can increase up to 6.11×10&lt;sup&gt;–2&lt;/sup&gt; mS. The results indicate that the ion-gel film-based graphene field effect transistor shows good electrical properties in serving as high dielectric constant organic dielectric materials.

Uniform Vapor-Pressure-Based Chemical Vapor Deposition Growth of MoS2 Using MoO3 Thin Film as a Precursor for Coevaporation.

Chemical vapor deposition (CVD) is a powerful method employed for high-quality monolayer crystal growth of 2D transition metal dichalcogenides with much effort invested toward improving the growth process. Here, we report a novel method for CVD-based growth of monolayer molybdenum disulfide (MoS2) by using thermally evaporated thin films of molybdenum trioxide (MoO3) as the molybdenum (Mo) source for coevaporation. Uniform evaporation rate of MoO3 thin films provides uniform Mo vapors which promote highly reproducible single-crystal growth of MoS2 throughout the substrate. These high-quality crystals are as large as 95 μm and are characterized by scanning electron microscopy, Raman spectroscopy, photoluminescence spectroscopy, atomic force microscopy, and transmission electron microscopy. The bottom-gated field-effect transistors fabricated using the as-grown single crystals show n-type transistor behavior with a good on/off ratio of 106 under ambient conditions. Our results presented here address the precursor vapor control during the CVD process and is a major step forward toward reproducible growth of MoS2 for future semiconductor device applications.

Temperature Dependent Electrical Characteristics of Nanostructured WO3 Based Ambipolar Bottom Gate FET

A detailed experimental investigation on nanostructured tungsten trioxide (WO3) based bottom gate field effect transistor (FET) is presented. These were fabricated using scalable process and thoroughly characterized for morphology and device performance. It was also observed that thickness of WO3 and ambient during annealing played a key role in tuning the morphology. The transistors bearing nanostructured morphology of WO3 were tested for device performance such as input-output and transfer characteristics at low, room, and high temperatures. Of all the devices tested, best on-off ratio obtained was 105 and sub-threshold slope obtained was 150 mV/decade inferring an efficient performance of the FET. A tradeoff between the on-off ratio ( I ON/ I OFF) and sub-threshold slope was noticed at higher temperatures. This was attributed to the ambipolar nature of the WO3 films. At a given temperature and bias, either the holes or electrons were present for transport and conduction. Such ambipolar films find great potential in transistors that can be used in switching applications.

Facile Synthesis of Molybdenum Diselenide Layers for High-Performance Hydrogen Evolution Electrocatalysts.

A cost-effective solution-based synthesis route to grow MoSe2 thin films with vertically aligned atomic layers, thereby maximally exposing the edge sites on the film surface as well as enhancing charge transport to the electrode, is demonstrated for hydrogen evolution reaction. The surface morphologies of thin films are investigated by scanning electron microscopy and atomic force microscopy, and transmission electron microscopy analyses confirm the formation of the vertically aligned layered structure of MoSe2 in those films, with supporting evidences obtained by Raman. Additionally, their optical and compositional properties are investigated by photoluminescence and X-ray photoelectron spectroscopy, and their electrical properties are evaluated using bottom-gate field-effect transistors. The resultant pristine MoSe2 thin film exhibited low overpotential of 88 mV (at 10 mA·cm–2) and a noticeably high exchange current density of 0.845 mA·cm–2 with excellent stability, which is superior to most of other reported MoS2 or MoSe2-based catalysts, even without any other strategies such as doping, phase transformation, and integration with other materials.

Hybrid PN Diode and CMOS Inverters Composed of MoTe2 Nanosheet-Amorphous in-Ga-Zn-O Thin Film

We report a hybrid material system which couples a dry-transfer-processed p-type MoTe2 nanosheet and a conventional photolithography-patterned n-type amorphous InGaZnO (IGZO) thin-film. The PN junction diode shows a good ideality factor of 1.57 and a high ON/OFF rectification ratio of ∼3 × 104. Under illumination, our PN diode shows somewhat stable only responding to high-energy photons of blue and ultraviolet. Our CMOS inverter composed of p-MoTe2 bottom-gate field-effect transistor and n-IGZO top-gate field-effect transistors. Our 2D nanosheet – oxide thin film CMOS inverter shows high voltage gains as high as ~40 at 5 V, low power consumption less than ~ a few nW at 1 V, and ~200 μs switching speed. Both hybrid PN junction diode and complementary (CMOS) inverters are realized by such novel hybrid material system that now opens any further possibilities toward practical applications of 2D nanosheets combined with existing technologies. Figure 1

A Delamination Strategy for Thinly Layered Defect-Free High-Mobility Black Phosphorus Flakes.

Extraordinary electronic and photonic features render black phosphorus (BP) an important material for the development of novel electronics and optoelectronics. Despite recent progress in the preparation of thinly layered BP flakes, scalable synthesis of large-size, pristine BP flakes remains a major challenge. An electrochemical delamination strategy is demonstrated that involves intercalation of diverse cations in non-aqueous electrolytes, thereby peeling off bulk BP crystals into defect-free flakes comprising only a few layers. The interplay between tetra-n-butylammonium cations and bisulfate anions promotes a high exfoliation yield up to 78 % and large BP flakes up to 20.6 μm. Bottom-gate and bottom-contact field-effect transistors, comprising single BP flakes only a few layers thick, exhibit a high hole mobility of 252±18 cm2 V-1 s-1 and a remarkable on/off ratio of (1.2±0.15)×105 at 143 K under vacuum. This efficient and scalable delamination method holds great promise for development of BP-based composites and optoelectronic devices.

Graphene exfoliation in the presence of semiconducting polymers for improved film homogeneity and electrical performances

We report on the production of hybrid graphene/semiconducting polymer films in one step procedure by making use of ultrasound-assisted liquid-phase exfoliation of graphite powder in the presence of π-conjugated polymers, i.e. poly(3-hexylthiophene) (P3HT) or poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b']dithiophen-2-yl)-alt-[1,2,5]thiadiazolo-[3,4-c]pyridine] (PCDTPT). The polymers were chosen in view of their different propensity to form crystalline structures, their decoration with alkyl chains that are known to possess high affinity for the basal plane of graphene, the energy levels of their frontier orbitals which are extremely similar to the work function of graphene, and their high electrical performance when integrated in field-effect transistors (FETs). The polymers act as a dispersion-stabilizing agent and prevent the re-aggregation of the exfoliated graphene flakes, ultimately enabling the production of homogeneous bi-component dispersions. The electrical characterization of few-layer graphene/PCDTPT hybrids, when integrated as active layer in bottom-contact bottom-gate FETs, revealed an increase of the field-effect mobility compared to the π-conjugated-based pristine devices, a result which can be attributed to the joint effect of the few-layer graphene sheets and semiconducting polymers improving the charge-transport in the channel of the field-effect transistor. In particular, few-layer graphene/PCDTPT films displayed a 30-fold increase of PCDTPT's mobility if compared to pristine polymer samples. Such findings represent a step forward towards the optimization of graphene exfoliation and processing into electronic devices, as well as towards improved electrical performance in organic-based field-effect transistors.