Properties Of Field-effect Transistors Research Articles

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.

Numerical modeling of potassium‐doped polypyrrole/carbon nanotube graphene‐based cholesterol enzyme field effect transistor

AbstractPotassium‐doped polypyrrole/carbon nanotube (K/PPy/CNT)–based graphene enzyme field‐effect transistor has been modeled for cholesterol detection. The device consists of boron‐doped p‐type graphene as substrate, nitrogen‐doped n‐type graphene as source and drain regions, high‐κ dielectric ZrO2 as gate insulator, and K/PPy/CNT composite as sensing membrane on the top of ZrO2 layer. The modeling is done using the enzymatic reactions of the enzyme cholesterol oxidase on cholesterol substrate, diffusion phenomena of the main substrate (ie, cholesterol) in phosphate buffer saline, acid/base reactions of the product (H2O2) in the phosphate buffer saline solution (pH = 7), and the pH detection properties of ion sensitive field effect transistor. The thickness of the enzyme layer here is in nanometer range, which is very less compared to other enzyme field‐effect transistors whose thickness is in micrometer range. The use of K‐doped CNT as sensing membrane has increased the sensitivity of the device at normal temperature and pH. As a result of this increased sensitivity, variations of the substrate and product concentrations are noticed even at nanometer range of the enzyme layer, which results in change in pH of the ion sensitive field effect transistor. A good fit is obtained between the modeling and experimental results.

Correction to Synthesis, Structural Characterization, and Field-Effect Transistor Properties of n-Channel Semiconducting Polymers Containing Five-Membered Heterocyclic Acceptors: Superiority of Thiadiazole Compared with Oxadiazole.

Correction to Synthesis, Structural Characterization, and Field-Effect Transistor Properties of n-Channel Semiconducting Polymers Containing Five-Membered Heterocyclic Acceptors: Superiority of Thiadiazole Compared with Oxadiazole.

Transport properties of graphene field effect transistors

Nowadays, integrated circuit design and manufacturing level is one of important symbols to evaluate the industrial development level of a country. Silicon-based metal oxide semiconductor field effect transistor has an important application in large-scale integrated circuit, and its special size has reached 14nm. However, further miniaturization in reaching the silicon physical limits of 10nm channel length. Graphene is one of the most promising materials to replace silicon. Due to its outstanding performance such as ultrafast carrier mobility, excellent mechanical strength and high transparency invisible light region, graphene is regarded as one of the vital next generation electronic materials. In recent years, the rapid development of the preparation technology of large area and high quality graphene has further promoted the research of new electronic devices based on graphene, which has attracted wide attention of researchers in the field of integrated circuit. Graphene field effect transistor has fast response rate and high cut-off frequency, whose feature size can continue to narrow. Therefore, the study of graphene field effect transistors has very important significance for the continuation of Moore’s law. Currently, SiO2 is generally used to the gate insulating layer of graphene field effect transistors. The dielectric constant value of SiO2 is lower, which leads to higher power consumption of the device. When the thickness of the SiO2 gate insulating layer is reduced to a certain degree, the device is easy to breakdown. However, when the semiconductor materials with high dielectric constant (Ta2O5, TiO2, Al2O3, etc.) as the gate insulating layer, the device can obtain higher mobility in the operating voltage as low as possible. This is an effective way to solve the high power consumption of the device. Although graphene field effect transistors have made some progress in the preparation and performance, the existing field effect transistors based on graphene have no obvious advantage in carrier mobility and current on/off ratio. Accordingly, the research of graphene field effect transistor will be the main direction of the field of integrated circuit in the future. In this paper, the graphene field effect transistors use ITO as the gate electrode, Ta2O5 as the gate insulating layer, graphene as the active layer, Ti/Au double-layer metal as the source/drain electrodes. The results of electrical properties measurements and analyses show that the graphene strip is in good ohm contact with source/drain electrodes. At room temperature, graphene field effect transistors exhibit an unique bipolar characteristic. The mobility for hole carrier is about 2272cm2/(Vs) and the current on/off ratio is about 6.2. The hysteresis phenomenon in the transfer characteristic curve is observed, which becomes more obvious with the increase of gate voltage. Meanwhile, the effect of the temperature on the characteristic of the graphene field effect transistor is studied. With the increase of temperature, the Dirac point voltage gradually shifts to zero point, and the hysteresis phenomenon becomes more and more obvious. The mobility for hole carrier and the current on/off ratio achieve optimum when the temperature is 75°C. The content and conclusions of above may provide a reference for further research of graphene field effect transistors.

Carrier transport properties of MoS2 field-effect transistors produced by multi-step chemical vapor deposition method

We report the transistor properties of MoS2 thin films formed with a multi-step chemical vapor deposition (CVD) method. The established multi-step CVD technique has four steps: MoO3 thermal evaporation, annealing for MoO3 crystallization, sulfurization, and post-annealing. We found that the MoS2 transistor properties were greatly affected by the post-annealing temperature (TPA). The films worked as ambipolar transistors below TPA = 1000 °C. Meanwhile, the transistor operation transited from ambipolar to n-type transport at a TPA of 1000 °C. X-ray photoelectron spectroscopy measurements revealed that the films annealed below 1000 °C had sulfur-rich compositions (S/Mo &amp;gt; 2). The excess S atoms were reduced by elevating the annealing temperature to produce an almost stoichiometric composition (S/Mo = 2) at 1000 °C. These results indicate that excess sulfurs are responsible for the ambipolar operation by acting as acceptors that generate holes. Moreover, the high-temperature annealing at 1000 °C had another distinct effect, i.e., it improved the crystallinity of the MoS2 films. The electron mobility consequently reached 0.20 ± 0 .12 cm2/V s.

Electrical properties of nanoscale field effect transistor

In this paper structure of a H-passivated silicon nanowire along [111] direction with hexagonal cross section is defined by using Virtual nanolab (VNL), and set up a nanoscale field effect transistor (FET) structure with a wrap-around gate. Then the transmission spectrum and conductance is computed as the gate bias is varied. The device characteristic curve and conductance with respect to gate voltage is studied. Also, the effect of doping of Al atom on characteristic curve and conductance of nanoscale field effect transistor is analysed.

Electrical properties of nanoscale field effect transistor

In this paper structure of a H-passivated silicon nanowire along [111] direction with hexagonal cross section is defined by using Virtual nanolab (VNL), and set up a nanoscale field effect transistor (FET) structure with a wrap-around gate. Then the transmission spectrum and conductance is computed as the gate bias is varied. The device characteristic curve and conductance with respect to gate voltage is studied. Also, the effect of doping of Al atom on characteristic curve and conductance of nanoscale field effect transistor is analysed.

Vacuum Annealing and Passivation of HfS&lt;sub&gt;2&lt;/sub&gt; FET for Mitigation of Atmospheric Degradation

The performance of devices based on two dimensional (2D) materials is significantly affected upon prolonged exposure to atmosphere. We analyzed time based environmental degradation of electrical properties of HfS2 field effect transistors. Atmospheric entities like oxygen and moisture adversely affect the device surface and reduction in drain current is observed over period of 48 hours. Two corrective measures, namely, PMMA passivation and vacuum annealing, have been studied to address the diminution of current by contaminants. PMMA passivation prevents the device from environment and reduces the effect of Coulomb scattering. Improvement in current characteristics signifies the importance of dielectric passivation for 2D materials. On the other hand, vacuum annealing is useful in removing contaminants from the affected surface. In order to figure out optimum process conditions, properties have been studied at various annealing temperatures. The improvement in drain current level was observed upon vacuum annealing within optimum range of annealing temperature.

Highly efficient and stable MoS2 FETs with reversible n-doping using a dehydrated poly(vinyl-alcohol) coating.

Despite rapid progress in 2D molybdenum disulfide (MoS2) research in recent years, MoS2 field-effect transistors (FETs) still suffer from a high metal-to-MoS2 contact resistance and low intrinsic mobility, which are major hindrances to their future application. We report an efficient technique to dope thin-film MoS2 FETs using a poly(vinyl-alcohol) (PVA) polymeric coating. This results in a reduction of the contact resistance by up to 30% as well as a reduction in the channel resistance to 20 kΩ sq-1. Using a dehydration process, we were able to effectively control the surface interactions between MoS2 and the more electropositive hydroxyl groups (-OH) of PVA, which provided a controllable and yet reversible increase in the charge carrier density to a value of 8.0 × 1012 cm-2. The non-covalent, thus non-destructive, PVA doping of MoS2 increases the carrier concentration without degrading the mobility, which shows a monotonic increase while enhancing the doping effect. The PVA doping technique is then exploited to create heavily doped access regions to the intrinsic MoS2 channel, which yields 200% increase of the ON-state source-drain current. This establishes PVA doping as an effective approach to enhance the transport properties of MoS2 FETs for a variety of applications.