Properties Of Field-effect Transistors Research Articles

Unraveling the Solution-State Supramolecular Structures of Donor-Acceptor Polymers and their Influence on Solid-State Morphology and Charge-Transport Properties.

Polymer self-assembly in solution prior to film fabrication makes solution-state structures critical for their solid-state packing and optoelectronic properties. However, unraveling the solution-state supramolecular structures is challenging, not to mention establishing a clear relationship between the solution-state structure and the charge-transport properties in field-effect transistors. Here, for the first time, it is revealed that the thin-film morphology of a conjugated polymer inherits the features of its solution-state supramolecular structures. A "solution-state supramolecular structure control" strategy is proposed to increase the electron mobility of a benzodifurandione-based oligo(p-phenylene vinylene) (BDOPV)-based polymer. It is shown that the solution-state structures of the BDOPV-based conjugated polymer can be tuned such that it forms a 1D rod-like structure in good solvent and a 2D lamellar structure in poor solvent. By tuning the solution-state structure, films with high crystallinity and good interdomain connectivity are obtained. The electron mobility significantly increases from the original value of 1.8 to 3.2 cm2 V-1 s-1 . This work demonstrates that "solution-state supramolecular structure" control is critical for understanding and optimization of the thin-film morphology and charge-transport properties of conjugated polymers.

Cyclometalated Platinum-Containing Diketopyrrolopyrrole Complexes and Polymers: Photophysics and Photovoltaic Applications

A series of organometallic complexes and polymers has been synthesized with an objective of studying their fundamental photophysical properties together with their organic photovoltaic and organic field-effect transistor properties. The metal chromophores consist of a diketopyrrolopyrrole (DPP) core, end functionalized with cyclometalated platinum “auxochrome”. The photophysical properties of the metal complex and polymers are compared with the unmetalated chromophore DPP-C8-Th-Py. The polymers Poly-DPP-Th-Pt and Poly-DPP-Ph-Pt differ structurally in their cyclometallating ligands, where they consist of 2-thienylpyridine and 2-phenylpyridine, respectively. Efficient solar spectrum coverage was observed for all chromophores; specifically, the polymer Poly-DPP-Th-Pt has an onset of absorption at ∼900 nm with an optical band gap of 1.4 eV. The triplet excited state was detected for all chromophores and probed by both nanosecond and picosecond transient absorption spectroscopy. Both polymers were employed as ...

Short-channel field-effect transistors with 9-atom and 13-atom wide graphene nanoribbons

Bottom-up synthesized graphene nanoribbons and graphene nanoribbon heterostructures have promising electronic properties for high-performance field-effect transistors and ultra-low power devices such as tunneling field-effect transistors. However, the short length and wide band gap of these graphene nanoribbons have prevented the fabrication of devices with the desired performance and switching behavior. Here, by fabricating short channel (Lch ~ 20 nm) devices with a thin, high-κ gate dielectric and a 9-atom wide (0.95 nm) armchair graphene nanoribbon as the channel material, we demonstrate field-effect transistors with high on-current (Ion > 1 μA at Vd = −1 V) and high Ion/Ioff ~ 105 at room temperature. We find that the performance of these devices is limited by tunneling through the Schottky barrier at the contacts and we observe an increase in the transparency of the barrier by increasing the gate field near the contacts. Our results thus demonstrate successful fabrication of high-performance short-channel field-effect transistors with bottom-up synthesized armchair graphene nanoribbons.

B-doped diamond field-effect transistor with ferroelectric vinylidene fluoride–trifluoroethylene gate insulator

A B-doped diamond field-effect transistor (FET) with a ferroelectric vinylidene fluoride–trifluoroethylene (VDF–TrFE) copolymer gate insulator was fabricated. The VDF–TrFE film deposited on the B-doped diamond showed good insulating and ferroelectric properties. Also, a Pt/VDF–TrFE/B-doped diamond layered structure showed ideal behavior as a metal–ferroelectric–semiconductor (MFS) capacitor, and the memory window width was 11 V, when the gate voltage was swept from 20 to −20 V. The fabricated MFS-type FET structure showed the typical properties of a depletion-type p-channel FET and a maximum drain current density of 0.87 mA/mm at room temperature. The drain current versus gate voltage curves of the proposed FET showed a clockwise hysteresis loop owing to the ferroelectricity of the VDF–TrFE gate insulator. In addition, we demonstrated the logic inverter with the MFS-type diamond FET coupled with a load resistor, and obtained the inversion behavior of the input signal and a maximum gain of 18.4 for the present circuit.

Visible light emission in graphene field effect transistors

We present an experimental study of the light-emitting properties of graphene field effect transistors in the visible and near infrared spectral range. Using spectroscopic and imaging techniques, the effects of source–drain and gate voltages on the spectrum and location of the light emission were investigated. Raman spectroscopy, electronic measurements, and scanning electron microscopy combined with energy dispersive x-ray spectroscopy were used to characterize the devices. Results show that the spectral features (peak spectral intensity and wavelength) were controllable via applied source–drain or gate voltages, while the physical location of the light emission was strongly affected by scattering sites, including defects, nanoparticles, and edges. A possible explanation of the observed light emission is the outcoupling of surface plasmons excited by hot carriers in graphene according to the quantum Čerenkov effect. Hence, this work suggests the feasibility of all-electrical graphene devices for applications in light emission and plasmonics.

Cover Picture: Photoreduction Dependent p‐ and n‐Type Semiconducting Field‐Effect Transistor Properties in Undoped Reduced Graphene Oxide (ChemistrySelect 24/2017)

Cover Picture: Photoreduction Dependent p‐ and n‐Type Semiconducting Field‐Effect Transistor Properties in Undoped Reduced Graphene Oxide (ChemistrySelect 24/2017)

Solution-processed field-effect transistors based on polyfluorene –cesium lead halide nanocrystals composite films with small hysteresis of output and transfer characteristics

We have designed and investigated electrical and optical properties of solution-processed organic field-effect transistors (OFETs) based on conjugated polymer PFO and perovskite –cesium lead halide nanocrystals (CsPbI3) composite films. It was shown that OFETs based on PFO:CsPbI3 films exhibit current-voltage (I-V) characteristics of OFETs with dominant hole transport and saturation current behavior at temperatures 200–300 K. It was found that PFO:CsPbI3 OFETs have a negligible hysteresis of output and transfer characteristics especially at temperatures below 250 K. The values of the hole mobility estimated from I-Vs of PFO:CsPbI3 OFETs were found to be ∼2.4 10−1 cm2/Vs and ∼1.9 10−1 cm2/Vs in saturation and low fields regimes respectively at 300 K; the hole mobility dropped down to ∼6 10−3 cm2/Vs and 2.8 10−3 cm2/Vs respectively at 200 K, and then down to 5.5 10−5 cm2/Vs at 100 K (in low field regime), which is characteristic of hopping conduction. The effect of sensitivity to light and light-emitting effect were found under application of negative source-drain and gate pulse voltages to PFO:CsPbI3 OFETs at 300 K. The mechanism of charge carrier transport in OFETs based on PFO:CsPbI3 hybrid films is discussed.

Photoreduction Dependent p‐ and n‐Type Semiconducting Field‐Effect Transistor Properties in Undoped Reduced Graphene Oxide

AbstractBoth p‐ and n‐type field effect transistors (FET) properties have been observed in undoped reduced graphene oxide (rGO). Short and long time exposure of GO during photo reduction results in the formation of respective p‐ and n‐type rGO semiconductor. Achieving duel behavior of this type in an undoped material is exceedingly unusual. Herein we report the presence of such behavior in the reduced from of graphene oxide (rGO) for the first time.

Polymer-dielectric molecular interactions in defect-free poly(3-hexylthiophene): dependence and consequences of regioregularity on transistor charge transport properties

We investigate the molecular interaction of poly(3-hexylthiophene-2,5-diyl) (P3HT) molecules with polar functional groups of the dielectric surface, and its dependence on the regioregularity of P3HT. With this aim, we consider thickness-dependent molecular order of 100% regioregular defect-free P3HT (DF-P3HT) and 93% regioregular P3HT (LT-P3HT), deposited on top of cross-linked poly(vinyl alcohol) (cr-PVA) substrates. Intimate contact of P3HT molecules and cr-PVA surface defects affects the molecular order of P3HT differently, depending on the regioregularity. Consequently, these molecular order changes on the charge transport properties of organic field-effect transistors (OFETs) are investigated using four thicknesses (20, 40, 80 and 120 nm) of P3HT. As compared to other thicknesses, μsat for 20 nm DF-P3HT OFETs shows further improvement, while the opposite occurs for 20 nm LT-P3HT OFETs. Depending on the regioregularity (and thus the chain orientation), P3HT molecules exhibit a difference in dipole moments. Consequently, the interaction of edge-on or face-on P3HT molecules with cr-PVA surface dipoles has different contributions towards the electrostatic energetic disorder at cr-PVA/P3HT interface. This subtle difference of behavior helps one to understand the huge spread of characteristics of P3HT based transistors found in literature.

Titania–silica hybrid films derived by a sol–gel process for organic field effect transistors

Titania–silica hybrid films with a thickness of 300 nm are fabricated by combining a sol–gel method with a spin-coating process from the acid-catalyzed organically modified silane solution of γ-glycidoxypropyltrimethoxysilane, methyltriethoxysilane and tetrapropylorthotitanate. The dielectric constants of the titania–silica hybrid films can be easily controlled by adjusting titanium content. Effects of titanium content and heat treatment temperature on the leakage current and surface roughness of the as-fabricated films are also optimized, thus, the hybrid film with smooth surface (R q < 0.3 nm), high dielectric constant (k = 59.44) and low leakage current density (<1 nA cm−2 at 5 V) is obtained and then used as the dielectric layer for the organic field effect transistors. Results indicate that the electrical properties of the organic field effect transistor fabricated by the titania–silica hybrid film as dielectric layer show an obvious improvement as compared with those of the organic field effect transistor fabricated by the thermally grown SiO2 as dielectric layer, especially, in the improvement of the operating voltage (−2 V), field effect mobility (0.53 cm2 V−1 s−1) and sub-threshold swing (~130 mV/dec). It can be concluded that the titania–silica hybrid film shows a potential for the dielectric layer of the organic field effect transistors.

Effect of ultrathin Fe dusting layer on electrical transport properties of few-layer MoS2 field-effect transistors

Transition-metal-doped MoS2 has been predicted to be a potential candidate for a two dimensional dilute magnetic semiconductor, while the effect of transition metal dopants on the electrical properties of MoS2-based devices has received relatively less attention so far. Here, the authors report on a systematic electrical transport study of Fe-dusted few-layer molybdenum disulfide (MoS2) field-effect transistors via repeated in situ Fe deposition (total nominal thickness ≤ 2 nm) and electrical measurements in ultrahigh vacuum. It is found that an ultrathin Fe adsorption layer (≪0.5 nm) n-dopes MoS2 without noticeably affecting the electrical characteristics of the device. In contrast, a thicker Fe layer (0.5–2 nm) results in the loss of the carrier tunability and a nonlinear current-voltage characteristic with the differential conductance approximately linear to drain voltage. The authors show through result analysis and qualitative modeling that Fermi level pinning in MoS2 directly underneath Fe nanoclusters plays an important role in causing these degradations. The pinning effect can be partially removed by in situ oxidizing the Fe nanoclusters for a short duration of 16 min. The mechanism of Fermi level pinning is discussed.

Room temperature ethanol sensing properties of FET sensors based on ZnO nanostructures

Abstract In this work, the ethanol sensing properties of field effect transistor (FET) sensors based on ZnO nanostructures were investigated at the room temperature and compared with those of based on ZnO thin film. ZnO thin film was deposited by RF magnetron sputtering. While ZnO nanostructures were prepared by a thermal oxidation technique. They were completely configured as the bottom gate FET structures. It was found that our devices exhibit the n-channel semiconductor with a drain-source current on/off ratio in order of 105 and 104 for ZnO thin film and ZnO nanostructures, respectively. In addition, the sub-threshold swing (SS) of ZnO nanostructures extremely change with ethanol supply flowed into the system. It can be seen that this SS of FET with ZnO nanostructures in air is found to be 1.5 V/decade. On the other hand, the SS of devices are achieved with the value of 1.1 and 0.7 V/decade for the ethanol concentrations of 50 and 100 ppm, respectively. The sensor response of FET sensor based on ZnO nanostructures is about 6 whereas thin film structure is about 3 at 100 ppm concentration. This suggests the sensor response improvement of FET sensors at the room temperature due to surface to volume ratio effect of nanostructures formation.

Coplanar Donor-Acceptor Semiconducting Copolymers to Achieve Better Conjugated Structures: Side-Chain Engineering

It is reported on the allocation effects of branched alkyl chains, when used as solubility and ordering enhancers of the conjugated donor–accepter (D–A) copolymer backbones, on the ordering and π–π overlapping of the copolymers, that drastically affect the electrical properties of organic field-effect transistors (OFETs). Triisopropylsilylethynyl-benzo[1,2-b:4,5-b′]dithiophene (TIPSBDT) and diketopyrrolopyrrole (DPP)-based copolymers, which have two linear alkyl spacers (methylene (C1) or butylene (C4)) between the DPP and side-substituent (C10H21)CH(C8H17), are synthesized by Suzuki cross-coupling. These copolymer films are spun cast onto a polymer-treated SiO2 dielectric surface, and some are further thermally annealed. The longer spacer, C4, is found to efficiently enhance the coplanarity and conjugation of the D–A backbone, while the C1 does not. The resulting C4-bridged TIPSBDT-DPP-based copolymer readily develops a superior π-extended layer on the dielectric surface; the edge-on chains with randomly oriented side chains can be closely packed with a short π-planar distance (d(010)) of 3.57 A. Its properties are superior to those of the short spacer C1 system with d(010) ≈3.93 A. The C4-bridged TIPSBDT-DPP copolymer films yield a field-effect mobility up to 1.2 cm2 V−1 s−1 in OFETs, 12 times as higher than that of the C1 spacer system.

A solution-processed and low threshold voltage p-type small molecule based on indolocarbazole- and benzothiophene-fused rings

A novel planar heteroacene, 8,17-dihexadecyl-8,17-dihydrobenzo[4′,5′]thieno[2′,3′:5,6] indolo[3,2-b]benzo[4,5]thieno[2,3-h]carbazole (ICzDBT), was designed and synthesized. The thermal, optical, electrochemical and field-effect transistor properties were investigated systematically. The highest occupied molecular orbital energy levels (EHOMO) measured by cyclic voltammetry and photoelectron yield spectra were −5.15 and −5.29 eV, respectively, which are close to the work function (WF) of Au (∼5.2 eV). This suitable energy level alignment between the EHOMO of ICzDBT and WF of Au is favorable for hole carrier injection from the Au electrode to the ICzDBT layer. The ICzDBT showed a hole mobility as high as 0.17 cm2 V−1 s−1 and a current on/off ratio of 1.2 × 106, with a very low threshold voltage of −0.8 V. Moreover, the device displayed excellent stability with little roll-off of hole mobility in air. Hence, this kind of molecule is a promising candidate for p-type organic field-effect transistors (OFETs) with high mobility and air stability.

Effects of high-k dielectric environment on the full ballistic transport properties of monolayer MoS2 FETs

High-k dielectric materials are indispensable as gate layers for micro- and nano-electronic devices. Using first-principles calculations and non-equilibrium Green's function simulations, we studied the electrical transport characteristics of p-type and n-type monolayer MoS2 field effect transistors (FETs) under various gate dielectric environments. We found that the intrinsic dielectric property of the gate insulator played an important role in the transport performance of nanodevices. For both types of MoS2 FETs, a high-k gate insulator enhances the current on/off ratio and reduces the subthreshold swing by properly shifting the valence (p-type) or conduction (n-type) bands around the bias energy window, which has benefits for the design of MoS2-based short-channel nanodevices in the future.

Synthesis, characterization and printing application of alkylated indolo[3,2-b]carbazoles

The synthesis, characterization, and field-effect transistor (FET) properties of new alkylated indolo[3,2-b]carbazole (ICz) based materials are reported. Dialkyl substitution at the 2,8-positions of the ICz core structure do not affect strong NH⋯ π interactions, thus favoring a herringbone packing motif. OFET devices fabricated by compounds of this substitution pattern displayed a charge carrier mobility of ∼10−2cm2Vs−1. These interactions are suppressed when introducing substituents at the 5,11-positions, resulting in greatly enhanced solubility. A solution processable ICz derivative was finally used in a newly developed pneumatic printing setup to yield a crystalline film with aligned crystals along the printing direction.

Air-Stable Humidity Sensor Using Few-Layer Black Phosphorus.

As a new family member of two-dimensional layered materials, black phosphorus (BP) has attracted significant attention for chemical sensing applications due to its exceptional electrical, mechanical, and surface properties. However, producing air-stable BP sensors is extremely challenging because BP atomic layers degrade rapidly in ambient conditions. In this study, we explored the humidity sensing properties of BP field-effect transistors fully encapsulated by a 6 nm-thick Al2O3 encapsulation layer deposited by atomic layer deposition. The encapsulated BP sensors exhibited superior ambient stability with no noticeable degradation in sensing response after being stored in air for more than a week. Compared with the bare BP devices, the encapsulated ones offered long-term stability with a trade-off in slightly reduced sensitivity. Capacitance-voltage measurement results further reveal that instead of direct charge transfer, the electrostatic gating effect on BP flakes arising from the dipole moment of adsorbed water molecules is the basic mechanism governing the humidity sensing behavior of both bare and encapsulated BP sensors. This work demonstrates a viable approach for achieving air-stable BP-based humidity or chemical sensors for practical applications.

Chalcogen bridged pyrene derivatives: Synthesis, crystal−packing structures and field effect transistors properties

To explore how the different intermolecular non-covalent interactions synergistically affect the molecular packing and properties when they existed at the same molecules. A series of new pyrene derivatives with chalcogen atoms as bridges were designed and synthesized. The single crystal structures of the compounds revealed the significant effect of different intermolecular non-covalent interactions on the molecular packing. The thin film transistor devices were fabricated and exhibit high mobility over 0.04 cm2 V−1 s−1. The distinct difference of the OFETs performance suggests that the intermolecular non-covalent interactions have very important effect on gearing the molecular packing and properties.

The doping mechanism and electrical performance of polyethylenimine‐doped MoS­2 transistor

Seongin Hong et al. (article no. 1600262) have investigated the polyethylenimine (PEI) doping mechanism and its effect on the electrical and optical properties of multilayer MoS2 field effect transistors (FETs). Density functional theory (DFT) calculation and X-ray photoelectron spectroscopy (XPS) measurement confirm that the PEI molecules were successfully doped and formed Mo–N bonds on the MoS2 channel, generating new energy states near the valence band. The strong n-doping changed the threshold voltage as well as the Schottky barrier width attributed to the induced interfacial dipoles. Therefore, the ON-current of the doped MoS2 FETs was improved in comparison with the pristine FETs. Furthermore, the PEI doping also enhanced the photoresponsivity of the MoS2 FETs from 0.14 A/W to 4.41 A/W. This study suggests that PEI molecular doping could be widely applicable to two-dimensional materials in order to improve the electrical and optical properties of respective devices.

Engineering Chemically Active Defects in Monolayer MoS2 Transistors via Ion‐Beam Irradiation and Their Healing via Vapor Deposition of Alkanethiols

Irradiation of 2D sheets of transition metal dichalcogenides with ion beams has emerged as an effective approach to engineer chemically active defects in 2D materials. In this context, argon-ion bombardment has been utilized to introduce sulfur vacancies in monolayer molybdenum disulfide (MoS2 ). However, a detailed understanding of the effects of generated defects on the functional properties of 2D MoS2 is still lacking. In this work, the correlation between critical electronic device parameters and the density of sulfur vacancies is systematically investigated through the fabrication and characterization of back-gated monolayer MoS2 field-effect transistors (FETs) exposed to a variable fluence of low-energy argon ions. The electrical properties of pristine and ion-irradiated FETs can be largely improved/recovered by exposing the devices to vapors of short linear thiolated molecules. Such a solvent-free chemical treatment-carried out strictly under inert atmosphere-rules out secondary healing effects induced by oxygen or oxygen-containing molecules. The results provide a guideline to design monolayer MoS2 optoelectronic devices with a controlled density of sulfur vacancies, which can be further exploited to introduce ad hoc molecular functionalities by means of thiol chemistry approaches.