Fabrication Of Field-effect Transistors Research Articles

Optoelectrical anisotropy in graphene oxide supported polythiophene thin films fabricated by floating film transfer

A facile method for the fabrication of highly oriented hybrid thin films of conjugated polymer (CP) with graphene oxide (GO) as 2D nano-filler over the high surface tension liquid substrate by floating film transfer is being reported. Formation of the self-assembled oriented hybrid thin film has been validated using multiple characterization techniques. Hybrid thin films thus prepared have been used for the fabrication of organic field effect transistors (OFETs) followed by investigation of anisotropic charge carrier transport. OFETs made with hybrid films oriented in channel direction exhibited the superior device performance with >6 fold augmentation in field effect mobility (μ) and one order improvement in on/off ratio as compared to that of devices based on pristine CP. However, pristine polymer shows 2.2 fold enhancement in mobility and one order augmentation in on/off ratio in parallel orientation as compared to the corresponding perpendicularly oriented films. Thus, our study has elucidated a path to obtain oriented, aligned, and self - assembled hybrid films for further improvement in the device performance aiming towards the applications in organic electronics.

Monolithic Integration of GaN Nanowire Light-Emitting Diode With Field Effect Transistor

Gallium nitride (GaN)-based light-emitting diodes (LEDs) are being investigated for the next generation display technology. The persistent issue, however, has been the lack of ability to integrate transistors with LEDs for control. Here, a novel vertical integration scheme is utilized to fabricate nanowire LEDs with nanowire field effect transistors (FETs) for the first time. This approach utilizes the unintentionally doped GaN template layer which is common to LED growth for the fabrication of nanowire FETs. The demonstrated voltage-controlled light-emitting unit provides area savings, scaling, and easier fabrication due to the vertical integration. For these initial nanowire devices, light modulation is demonstrated with LED turn OFF at −10 V. Due to the nanowire approach, these devices show over two times improvement in the $I_{ \mathrm{\scriptscriptstyle ON}}$ to $I_{ \mathrm{\scriptscriptstyle OFF}}$ ratio compared with the alternative integration schemes.

Monolayer MoS2 Nanoribbon Transistors Fabricated by Scanning Probe Lithography.

Monolayer MoS2 is a promising material for nanoelectronics; however, the lack of nanofabrication tools and processes has made it very challenging to realize nanometer-scale electronic devices from monolayer MoS2. Here, we demonstrate the fabrication of monolayer MoS2 nanoribbon field-effect transistors as narrow as 30 nm using scanning probe lithography (SPL). The SPL process uses a heated nanometer-scale tip to deposit narrow nanoribbon polymer structures onto monolayer MoS2. The polymer serves as an etch mask during a XeF2 vapor etch, which defines the channel of a field-effect transistor (FET). We fabricated seven devices with a channel width ranging from 30 to 370 nm, and the fabrication process was carefully studied by electronic measurements made at each process step. The nanoribbon devices have a current on/off ratio > 104 and an extrinsic field-effect mobility up to 8.53 cm2/(V s). By comparing a 30 nm wide device with a 60 nm wide device that was fabricated on the same MoS2 flake, we found the narrower device had a smaller mobility, a lower on/off ratio, and a larger subthreshold swing. To our knowledge, this is the first published work that describes a working transistor device from monolayer MoS2 with a channel width smaller than 100 nm.

Isolation and Characterization of the Unexpected 1- n-Octyloxyperopyrene: A Solution-Processable p-Type Organic Semiconductor.

The synthesis and optical, electrochemical, thermal, and electrical characterization of a new and unexpected 1- n-octyloxyperopyrene is reported. The structure of 1- n-octyloxyperopyrene has been unambiguously established by single-crystal X-ray diffraction. The solubility of this polycyclic aromatic hydrocarbon, endowed by the alkoxy substituent, allows the fabrication of thin-film field-effect transistors by liquid deposition methods. These devices show hole mobilities up to 1.61 × 10-3 cm2 V-1 s-1.

Efficient doping modulation of monolayer WS2 for optoelectronic applications**Project supported by the National Natural Science Foundation of China (Grant No. 21405109) and Seed Foundation of State Key Laboratory of Precision Measurement Technology and Instruments, China (Grant No. 1710).

Transition metal dichalcogenides (TMDCs) belong to a subgroup of two-dimensional (2D) materials which usually possess thickness-dependent band structures and semiconducting properties. Therefore, for TMDCs to be widely used in electronic and optoelectronic applications, two critical issues need to be addressed, which are thickness-controllable fabrication and doping modulation of TMDCs. In this work, we successfully obtained monolayer WS2 and achieved its efficient doping by chemical vapor deposition and chemical doping, respectively. The n- and p-type dopings of the monolayer WS2 were achieved by drop coating electron donor and acceptor solutions of triphenylphosphine (PPh3) and gold chloride (AuCl3), respectively, on the surface, which donates and captures electrons to/from the WS2 surface through charge transfer, respectively. Both doping effects were investigated in terms of the electrical properties of the fabricated field effect transistors. After chemical doping, the calculated mobility and density of electrons/holes are around 74.6/39.5 cm2·V−1·s−1 and 1.0 × 1012/4.2 × 1011 cm−2, respectively. Moreover, we fabricated a lateral WS2 p–n homojunction consisting of non-doped n-type and p-doped p-type regions, which showed great potential for photodetection with a response time of 1.5 s and responsivity of 5.8 A/W at VG = 0 V and VD = 1 V under 532 nm light illumination.

Improved ferroelectric properties of P(VDF-TrFE) and P(VDF-HFP) blends for organic memory FETs

The electronic device industry has recently focused its attention towards the development of organic memory devices. This research highlights the development of organic memory devices by utilizing pentacene as a semiconductor and ferroelectric polymers, such as poly(vinylidene fluoride-hexafluoropropylene) [P(VDF-HFP)] and poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)], as a gate dielectric. 10 wt% solutions of P(VDF-HFP) and P(VDF-TrFE) were prepared individually and blended (50:50). The resultant solutions were coated on the gate electrode and subsequently annealed for 2 h at 90 °C. The fabricated field-effect transistors (FETs) were studied in terms of their hysteresis characteristics with particular emphasis on the memory window. The results show that the FETs fabricated using the P(VDF-HFP)/P(VDF-TrFE) blend have significantly improved memory characteristics. The enhancement in the memory characteristics is explained through the interface properties between the pentacene semiconductor and dielectric layers. This work provides a design of organic FETs with excellent memory characteristics using low-cost technology.

Environmentally-friendly fabrication of organic field-effect transistors based on small molecule/polymer blend prepared by electrostatic spray deposition

Large-area organic crystalline films were prepared by electrostatic spray deposition of small molecule/polymer blend solutions prepared with environmentally-friendly solvents such as n-butyl acetate. 6,13-bis(triisopropylsilylethynyl) pentacene was used as a small molecule semiconductor, and poly(methyl methacrylate) as a polymer insulator. Due to rapid phase-separation in the blend film prepared by using the non-aromatic and halogen-free solvents, organic field-effect transistors (OFETs) with the well-defined interface exhibited superior charge-carrier transport properties. This study will contribute to green manufacturing of high-performance OFETs by a facile one-step printing process.

Modeling and Parameter Extraction of OFET Compact Models Using Metaheuristics-Based Approach

Research on the fabrication of organic field effect transistors (OFETs) has been dramatically increased in the last decade, considering their lightweight and flexible structure as well as their practical and low-cost production. In the next step of fabrication, building compact models and developing parameter extraction methods have critical importance in designing electronic circuits using fabricated OFETs. In this paper, we propose a parameter extraction approach which benefits from the power of metaheuristics. Although direct extraction tools offer analytical solutions in successive steps to extract model parameters, metaheuristics-based global optimization methods can find all parameters at once by exploring a wide range of parameters. Direct extraction tools are cumbersome and need human expertise. On the other hand, global optimization methods are very flexible, adaptive and can be automated as parameter extraction tools of any compact model. In this study, we introduce three different global optimization algorithms, namely, a genetic algorithm (GA), a hybrid artificial bee colony (h-ABC) algorithm, and bacterial foraging optimization (BFO) algorithm, for the parameter extraction. To the best of our knowledge, h-ABC and BFO algorithms have been used for the first time in extracting parameters of OFET compact models. We use two OFET compact models developed by Estrada et al. and Marinov et al. for two different datasets of OFET transistors, both having pentacene as organic semiconductor. While one of the dataset of transistor (T1) is available in literature, the dataset of the other transistor fabricated in our laboratory (T2) is generated as a new dataset. In order to tune control parameters of the developed algorithms, Taguchi's orthogonal experimental design (OED) method is used. Experimental results show that the proposed metaheuristics-based approach can extract model parameters successfully and can perform better than direct extraction methods. The studied OFET compact models fit to the experimental data with these parameters and predict similar output characteristic curves. The algorithms show a good agreement with the experimental data of T1 and T2, having normalized root-mean-square error values under 3.70%, and 8.74% for the models of Estrada et al. and Marinov et al., respectively. It is shown that h-ABC and BFO algorithms perform better than GA on average. It is also observed that the compact model by Estrada et al. performs better for both T1 and T2 compared to the model of Marinov et al.

Investigation on Metal–Oxide Graphene Field-Effect Transistors With Clamped Geometries

In this work, we report on the design, fabrication and characterization of Metal-Oxide Graphene Field-effect Transistors (MOGFETs) exploiting novel clamped gate geometries aimed at enhancing the device transconductance. The fabricated devices employ clamped metal contacts also for source and drain, as well as an optimized graphene meandered pattern for source contacting, in order to reduce parasitic resistance. Our experimental results demonstrate that MOGFETs with the proposed structure show improved high frequency performance, in terms of maximum available gain and transition frequency values, as a consequence of the higher equivalent transconductance obtained.

Design and synthesis of stable indigo polymer semiconductors for organic field-effect transistors with high fluoride sensitivity and selectivity.

We report the design and synthesis of two novel indigo donor–acceptor (D–A) polymers, PIDG-T-C20 and PIDG-BT-C20, comprising an indigo moiety that has intramolecular hydrogen-bonds as the acceptor building block and thiophene (T) and bithiophene (BT) as the donor building block, respectively. PIDG-T-C20 and PIDG-BT-C20 exhibited characteristic p-type semiconductor performance, achieving hole mobilities of up to 0.016 and 0.028 cm2 V−1 s−1, respectively, which are highest values reported for indigo-based polymers. The better performing PIDG-BT-C20 was used for the fabrication of water-gated organic field-effect transistors (WGOFETs), which showed excellent stability at ambient conditions. The PIDG-BT-C20-based WGOFETs exhibited rapid response when fluoride ions were introduced to the water gate dielectric, achieving a limit of detection (LOD) of 0.40 mM. On the other hand, the devices showed much lower sensitivities towards other halide ions with the order of relative response: F− ≫ Cl− > Br− > I−. The high sensitivity and selectivity of PIDG-BT-C20 to fluoride over other halides is considered to be realized through the strong interaction of the hydrogen atoms of the N–H groups in the indigo unit with fluoride ions, which alters the intramolecular hydrogen-bonding arrangement, the electronic structures, and thus the charge transport properties of the polymer.

Fabrication of flexible high-performance organic field-effect transistors using phenacene molecules and their application toward flexible CMOS inverters

The transport properties of 3,10-ditetradecylpicene ((C14H29)2-picene) and [6]phenacene thin-film field-effect transistors (FETs) on Si and plastic substrates are reported, in which SiO2 and parylene are used as gate dielectrics, respectively.

Stability of diamond/Si bonding interface during device fabrication process

Diamond/Si bonding interface with an entire contact area and high thermal stability is achieved by surface activated bonding method. The fabrication of diamond field-effect transistors (FETs) on the diamond bonded to Si is demonstrated. The FET exhibits clear saturation and pinch-off characteristics. A 5 nm thick SixCx-1 layer was formed at the interface with annealing at 1000 °C. The layer was formed by the inter-diffusion of carbon and Si atoms near the bonding interface, which plays a role of residual stress relaxation between diamond and Si. These results suggest that diamond/Si heterostructures are applicable for combining diamond devices with Si LSI.

Energy and charge transfer effects for hybrids of perovskite CsPbBr3 quantum dots on organic semiconducting rubrene nanosheet

Abstract Functionalized perovskite CsPbBr3 quantum dots (CsPbBr3-QDs) were loaded on the surface of a p-type organic rubrene (5,6,11,12-tetraphenyltetracene) nanosheet (NS) for the fabrication of a new hybrid nano-system. Using a high–resolution laser confocal microscope, the nanoscale photoluminescence (PL) characteristics of the CsPbBr3-QDs/rubrene-NS hybrids were investigated. We observed a significant enhancement of the PL intensity of the rubrene-NS after the hybridization with the CsPbBr3-QDs, which originated from the fluorescence resonance energy transfer from the CsPbBr3-QDs to the rubrene-NS. The PL enhancement ratio varied according to the density of the QDs on the NS. To investigate the hybridization effect with CsPbBr3-QDs on the electrical characteristics of the rubrene-NS, we fabricated field-effect transistors (FETs) using the pristine and hybrid samples as an active layer. The currents of both FETs using the pristine rubrene-NS and hybrid QDs/rubrene-NS increased with light irradiation through the contribution of photo-excited charges. After the hybridization with the CsPbBr3-QDs on the rubrene channel, both the electrical current and mobility decreased compared with the pristine rubrene-based FET. This can be explained by the relative decrease of hole concentration in the active channel region near dielectric layer through the interfacial charge transfer between the QDs and the NS, as well as the de-doping effect of the QDs on the rubrene-NS. Thus, we observed the energy and charge transfer effects for the CsPbBr3-QDs/rubrene-NS hybrids, which can be applied to advanced nanoscale photonics and optoelectronics.

Germanium Wafers Possessing Facet-Dependent Electrical Conductivity Properties.

Electrical conductivity properties of Ge {100}, {110}, {111}, and {211} facets have been measured by breaking Ge (100) and (111) wafers to expose {110} and {211} surfaces and contacting the different facets with tungsten probes. Ge {111} and {211} faces are far more conductive than the already conductive Ge {100} and {110} faces, matching with recent density functional theory (DFT) predictions. Asymmetric I-V curves resembling those of p-n junctions have been collected for the {110}/{111} and {110}/{211} facet combinations. The current-rectifying effects stem from different degrees of surface band bending for the highly and less conductive faces and the direction of current flow. This work demonstrates that germanium wafers also possess facet-dependent electrical conductivity responses that can be utilized in the fabrication of novel fin field-effect transistors (finFET).

All CVD Boron Nitride Encapsulated Graphene FETs With CMOS Compatible Metal Edge Contacts

We report on the fabrication and characterization of field effect transistors (FETs) based on chemical vapor deposited (CVD) graphene encapsulated between few layer CVD boron nitride (BN) sheets with complementary metal oxide semiconductor (CMOS) compatible nickel edge contacts. Non-contact Tera-hertz time domain spectroscopy (THz-TDS) of large-area BN/graphene/BN (BN/G/BN) stacks reveals average sheet conductivity >1 mS/sq and average mobility of 2500 cm$^{2}$/Vs. Improved output conductance is observed in direct current (DC) measurements under ambient conditions, indicating potential for radio-frequency (RF) applications. Moreover, we report a maximum voltage gain of 6 dB from a low frequency signal amplifier circuit. RF characterization of the GFETs yields an f$_{T}$ x L$_{g}$ product of 2.64 GHz$\mu$m and an f$_{Max}$ x L$_{g}$ product of 5.88 GHz$\mu$m. This study presents for the first time THz-TDS usage in combination with other characterization methods for device performance assessment on BN/G/BN stacks. The results serve as a step towards scalable, all CVD 2D material-based FETs for CMOS compatible future nanoelectronic circuit architectures.

Sub-5 nm monolayer black phosphorene tunneling transistors

The successful fabrication of sub-5 nm 2D MoS2 field-effect transistors (FETs) announces the approaching post-silicon era. It is possible for tunneling field-effect transistors (TFETs) based on monolayer black phosphorene (ML BP) to work well in the sub-5 nm region because of its moderate direct band gap, anisotropic electronic properties and high carrier mobility. We simulate the device performance limit of the ML BP TFETs at the sub-5 nm scale using ab initio quantum transport calculations. We predict that the on-state currents (Ion) of the sub-5 nm ML BP TFETs will exceed those of the ML WTe2 TFETs, which possess the highest Ion among the transition-metal dichalcogenide family. In particular, the Ion of the ML BP TFETs can fulfill the 2028 requirements of the international technology roadmap for semiconductors (ITRS) for the high-performance (HP) devices until the gate length is scaled down to 4 nm, while the delay times and power dissipations always surpass the 2028 requirements of the ITRS HP devices significantly in the whole sub-5 nm region.

Silicon nanowire pH sensors fabricated with CMOS compatible sidewall mask technology

Silicon nanowire (SiNW) field-effect transistors (FETs) have been developed as various bio/chemical sensors due to their high sensitivity as a result of the huge surface-to-volume ratios. This paper presents a CMOS compatible sidewall mask technology (SMT) for top-down fabrication of SiNWs and a highly sensitive pH sensor based on SiNW FETs. The essence of SMT is to use stand-alone vertical sidewalls of a conformal thin film, fabricated by anisotropically etching away the planar part of the thin film deposited on a thin mesa, as hard masks for SiNW etching. As the feature size of the sidewalls, depending on the film thickness rather than lithography, can be controlled readily to tens to hundreds of nanometers by tailoring the film thickness, SMT allows high output and low cost fabrication of SiNWs without need of advanced lithography facilities. Complete SMT processes have been developed for fabrication of SiNW FETs, and pH sensors using SiNW FETs have been realized. Measurement results show that the pH sensors have good static characteristics and transient performance, and the sensitivity reaches 54.5 mV/pH for a wide range of pH values from 1 to 12, approaching the theoretical Nernstian limit.

Spray coating of electrochemically exfoliated graphene/conducting polymer hybrid electrode for organic field effect transistor

We report the fabrication of organic field-effect transistors (OFETs) via spray coating of electrochemically exfoliated graphene (EEG) and conducting polymer hybrid as electrodes. To reduce the roughness and sheet resistance of the EEG electrodes, subsequent coating of conducting polymer (poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS)) and acid treatment was performed. After that, active channel layer was developed by spin coating of semiconducting poly(3-hexylthiophene) on the hybrid electrodes to define the bottom gate bottom contact configuration. The OFET devices with the EEG/PEDOT:PSS hybrid electrodes showed a reasonable electrical performances (field effect mobility=0.15cm2V−1s−1, on/off current ratio=102, and threshold voltage=−1.57V). Furthermore, the flexible OFET devices based on the Polydimethlsiloxane (PDMS) substrate and ion gel dielectric layer exhibited higher electrical performances (field effect mobility=6.32cm2V−1s−1, on/off current ratio=103, and threshold voltage=−1.06V) and excellent electrical stability until 1000 cycles of bending test, which means that the hybrid electrode is applicable to various organic electronic devices, such as flexible OFETs, supercapacitors, organic sensors, and actuators.

Polar Organic Gate Dielectrics for Graphene Field-Effect Transistor-Based Sensor Technology.

We have pioneered the use of liquid polar organic molecules as alternatives to rigid gate-dielectrics for the fabrication of graphene field-effect transistors. The unique high net dipole moment of various polar organic molecules allows for easy manipulation of graphene’s conductivity due to the formation of an electrical double layer with a high-capacitance at the liquid and graphene interface. Here, we compare the performances of dimethyl sulfoxide (DMSO), acetonitrile, propionamide, and valeramide as polar organic liquid dielectrics in graphene field-effect transistors (GFETs). We demonstrate improved performance for a GFET with a liquid dielectric comprised of DMSO with high electron and hole mobilities of 154.0 cm2/Vs and 154.6 cm2/Vs, respectively, and a Dirac voltage <5 V.

Three-Dimensional Atomistic Tomography of W-Based Alloyed Two-Dimensional Transition Metal Dichalcogenides.

Increased interest in two-dimensional (2D) materials and heterostructures for use as components of electrical devices has led to the use of an atomically mixed phase between semiconducting and metallic transition metal dichalcogenides that exhibited enhanced interfacial characteristics. To understand the lattice structure and properties of 2D materials on the atomic scale, diverse characterization methods such as Raman spectroscopy, high-resolution transmission electron microscopy (HR-TEM), and X-ray photoemission spectroscopy (XPS) have been applied. However, determination of the exact chemical distribution, which is a critical factor for the interfacial layer, was hindered by limitations of these typical methods. In this work, atom-probe tomography (APT) was introduced for the first time to analyze the three-dimensional atomic distribution and composition variation of the atomic-scale multilayered alloy structure W xNb(1- x)Se2. Composition profiles and theoretical calculations for each atom demonstrated the reaction kinetics and stoichiometric inhomogeneity of the W xNb(1- x)Se2 layer. The role of the intermediate layer was investigated by fabrication of a WSe2-based field-effect transistor. Introduction of W xNb(1- x)Se2 between metallic NbSe2 and semiconducting WSe2 layers resulted in improved charge transport with lowering of the contact barrier.