For Field-effect Transistors Research Articles

Solution-Processed Light-Emitting Dielectric Films and Their Applications in Multifunctional Devices

Ionic materials are used as gate dielectric layers for field-effect transistors (FETs) to reduce working voltage. In this paper, we exhibit a newly synthesized ionic iridium complex with interesting multifunction of both insulativity and light emission.1 Based on this material, a multifunctional device which behaves as a normal transistor at low gate voltage and a controllable light emitting device when the voltage is higher has been successfully fabricated. The device achieves area emission and the brightness can be tuned by three electrodes (gate, source and drain electrodes). Other ionic iridium complexes are also investigated. Results demonstrate that when mixed with some polymer, such as polymethyl methacrylate (PMMA), these complexes can form high performance light-emitting dielectric films as well. Through analysis of energy levels, the working mechanism of the device is proposed. Our study verifies that ionic light-emitting complexes have potential to be used in low-voltage multifunctional electronic devices. This work was financially supported by the National Natural Science Foundation of China (No. 61177023, 51173096, and 61474069). Reference: 1 J. Li, G. Dong*, L. Duan, D. Ma, T. Hu, Y. Zhang, L. Wang and Y. Qiu, RSC Adv. 2014, 4, 51294. Figure 1

Liquid crystals for organic thin-film transistors

Crystalline thin films of organic semiconductors are a good candidate for field effect transistor (FET) materials in printed electronics. However, there are currently two main problems, which are associated with inhomogeneity and poor thermal durability of these films. Here we report that liquid crystalline materials exhibiting a highly ordered liquid crystal phase of smectic E (SmE) can solve both these problems. We design a SmE liquid crystalline material, 2-decyl-7-phenyl-[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-10), for FETs and synthesize it. This material provides uniform and molecularly flat polycrystalline thin films reproducibly when SmE precursor thin films are crystallized, and also exhibits high durability of films up to 200 °C. In addition, the mobility of FETs is dramatically enhanced by about one order of magnitude (over 10 cm2 V−1 s−1) after thermal annealing at 120 °C in bottom-gate-bottom-contact FETs. We anticipate the use of SmE liquid crystals in solution-processed FETs may help overcome upcoming difficulties with novel technologies for printed electronics.

General strategy for biodetection in high ionic strength solutions using transistor-based nanoelectronic sensors.

Transistor-based nanoelectronic sensors are capable of label-free real-time chemical and biological detection with high sensitivity and spatial resolution, although the short Debye screening length in high ionic strength solutions has made difficult applications relevant to physiological conditions. Here, we describe a new and general strategy to overcome this challenge for field-effect transistor (FET) sensors that involves incorporating a porous and biomolecule permeable polymer layer on the FET sensor. This polymer layer increases the effective screening length in the region immediately adjacent to the device surface and thereby enables detection of biomolecules in high ionic strength solutions in real-time. Studies of silicon nanowire field-effect transistors with additional polyethylene glycol (PEG) modification show that prostate specific antigen (PSA) can be readily detected in solutions with phosphate buffer (PB) concentrations as high as 150 mM, while similar devices without PEG modification only exhibit detectable signals for concentrations ≤10 mM. Concentration-dependent measurements exhibited real-time detection of PSA with a sensitivity of at least 10 nM in 100 mM PB with linear response up to the highest (1000 nM) PSA concentrations tested. The current work represents an important step toward general application of transistor-based nanoelectronic detectors for biochemical sensing in physiological environments and is expected to open up exciting opportunities for in vitro and in vivo biological sensing relevant to basic biology research through medicine.

Enhancement of photodetection characteristics of MoS2 field effect transistors using surface treatment with copper phthalocyanine.

Recently, two-dimensional materials such as molybdenum disulfide (MoS2) have been extensively studied as channel materials for field effect transistors (FETs) because MoS2 has outstanding electrical properties such as a low subthreshold swing value, a high on/off ratio, and good carrier mobility. In this study, we characterized the electrical and photo-responsive properties of MoS2 FET when stacking a p-type organic copper phthalocyanine (CuPc) layer on the MoS2 surface. We observed that the threshold voltage of MoS2 FET could be controlled by stacking the CuPc layers due to a charge transfer phenomenon at the interface. Particularly, we demonstrated that CuPc/MoS2 hybrid devices exhibited high performance as a photodetector compared with the pristine MoS2 FETs, caused by more electron-hole pairs separation at the p-n interface. Furthermore, we found the optimized CuPc thickness (∼2 nm) on the MoS2 surface for the best performance as a photodetector with a photoresponsivity of ∼1.98 A W(-1), a detectivity of ∼6.11 × 10(10) Jones, and an external quantum efficiency of ∼12.57%. Our study suggests that the MoS2 vertical hybrid structure with organic material can be promising as efficient photodetecting devices and optoelectronic circuits.

Flexible Solution-Gated Graphene Field Effect Transistor for Electrophysiological Recording

Graphene represents a high-performance material for field effect transistor (FET) manufacture. As a monolayer of carbon atoms, graphene offers flexibility, high carrier mobility, and transparency. Solution-gated graphene FETs based on flexible substrates have been developed for various applications. However, none of these FETs offer both flexibility and good insulation from the electrolyte simultaneously. This would restrict their applicability in certain areas, such as implantation. To solve this problem, we used commercially available graphene and polyimide to fabricate a graphene FET. Photosensitive polyimide was used as both substrate and insulator for our device. Two Au electrodes were used separately as source and drain, and the graphene was exposed to an electrolytic solution to form a solution-gated structure. In this paper, we have studied the resistance variation between source and drain during specific fabrication steps to optimize the fabrication process. By analyzing the FET performance after repeated bending tests, our device's performance reliability was also confirmed. When compared with similar published flexible devices, our FET showed higher transconductance and lower noise levels. Also, neural activities were recorded from cortex neurons cultured on graphene FET for the first time, which proved our device's biocompatibility and demonstrated its potential for electrophysiological applications.

Proton intercalated two-dimensional WO3 nano-flakes with enhanced charge-carrier mobility at room temperature.

Quasi two-dimensional (Q2D) semiconducting metal oxides with enhanced charge carrier mobility hold tremendous promise for nano-electronics, photonics, catalysis, nano-sensors and electrochromic applications. In addition to graphene and metal dichalcogenides MX2 (M = Mo, W; X = S, Se, Te), 2D sub-stoichiometric WO(3-x) is gaining importance as a promising semiconductor material for field-effect-transistor (FET) based devices. A combination of high permittivity, suppression of the Coulomb effects, and their stratified structure enhances the carrier mobility in such a material. Additionally, the sub-stoichiometry of this semiconductor oxide allows the reduction of the bandgap and increase of the free charge carriers at the same time. Here, we report for the first time H(+) intercalated WO(3) FETs, made of Q2D nano-flakes, with enhanced charge-carrier mobility exceeding 319 cm(2) V(-1) s(-1) comparable with the charge-carrier mobility of Q2D dichalcogenides MoS(2) and WSe(2). Analyses indicate that the enhanced electrical properties of the sub-stoichiometric WO(3-x) depend on the oxygen vacancies in the intercalated nano-flakes. These findings confirmed that Q2D sub-stoichiometric WO(3-x) is a promising material for various functional FET devices.

Enhanced piezoelectric output voltage and Ohmic behavior in Cr-doped ZnO nanorods

Highly crystalline Cr-doped ZnO nanorods (NRs) were synthesized by solution technique. The size distribution was analyzed by high resolution tunneling electron microscope (HRTEM) and particle size analyzer. In atomic force microscope (AFM) studies, peak to peak 8mV output voltage was obtained on the application of constant normal force of 25nN. It showed high dielectric constant (980) with phase transition at 69°C. Polarization vs. electric field (P–E) loops with remnant polarization (6.18μC/cm2) and coercive field (0.96kV/cm) were obtained. In I–V studies, Cr-doping was found to reduce the rectifying behavior in the Ag/ZnO Schottky contact which is useful for field effect transistor (FET) and solar cell applications. With these excellent properties, Cr-doped ZnO NRs can be used in nanopiezoelectronics, charge storage and ferroelectric applications.

Structures and Electronic Properties of Heavier Congeners of Disk-Like Molecules: (Si, Ge) Sulflower and (Si, Ge) Olympicene

The ground-state structures, HOMO–LUMO gaps, singlet–triplet splitting, and the UV–vis absorption spectra for Si, Ge-substituted analogues of the recently synthesized disk-like π-conjugated molecules, like octathio[8]circulene, popularly termed “sulflower”, and 2H-benzo[cd]pyrene, popularly termed “olympicene”, are studied using density functional theory. Unlike their pure organic counterparts, these molecules are found to be nonplanar with substantial puckering from the high symmetric structures. The origin of puckering is traced to pseudo Jahn–Teller (PJT) distortions due to favorable mixing of the occupied molecular orbitals (OMO) and unoccupied molecular orbitals (UMO). Even though the HOMO–LUMO and singlet–triplet gaps for these molecules are smaller than their organic counterparts, the reorganization energies (both hole, λh, and electron, λe) for the Si, Ge analogues of sulflower and olympicene are much higher. Therefore, these molecules are expected to be rather inefficient for field effect transistor (FET) device fabrication.

Investigations on Work Functions of Gasochromic Color Dyes as Gate Materials for FET Based Gas Sensors

Abstract We present investigations on gasochromic color dyes as gas sensitive gate materials for field-effect transistor (FET) based gas sensors. Therefore the work function of two color dyes N’,N’,N,N-tetramethyl-p-phenylenediamine (TMPD) and bromphenolblue (BPB) were characterized using a Kelvin probe. TMPD is selective to NO2 and shows changes in the output of the Kelvin probe of 50 mV during exposure to 0.5 ppm NO2. BPB is an indicator for ammonia. Even 3 ppm NH3 cause a change of 10 mV. In addition, the gasochromic materials are highly selective to only one gas. The results show, that these dyes are suitable as sensitive gate materials and offer the possibility to build a low-power FET, detecting NO2 and NH3 in air at room temperature.

Mobility engineering and a metal–insulator transition in monolayer MoS2

Two-dimensional (2D) materials are a new class of materials with interesting physical properties and ranging from nanoelectronics to sensing and photonics. In addition to graphene, the most studied 2D material, monolayers of other layered materials such as semiconducting dichalcogenides MoS2 or WSe2 are gaining in importance as promising insulators and channel materials for field-effect transistors (FETs). The presence of a direct band gap in monolayer MoS2 due to quantum mechanical confinement, allows room-temperature field-effect transistors with an on/off ratio exceeding 108. The presence of high-k dielectrics in these devices enhanced their mobility, but the mechanisms are not well understood. Here, we report on electrical transport measurements on MoS2 FETs in different dielectric configurations. Mobility dependence on temperature shows clear evidence of the strong suppression of charge impurity scattering in dual-gate devices with a top-gate dielectric together with phonon scattering that shows a weaker than expected temperature dependence. High levels of doping achieved in dual-gate devices also allow the observation of a metal-insulator transition in monolayer MoS2. Our work opens up the way to further improvements in 2D semiconductor performance and introduces MoS2 as an interesting system for studying correlation effects in mesoscopic systems.

Control of Current Hysteresis of Networked Single‐Walled Carbon Nanotube Transistors by a Ferroelectric Polymer Gate Insulator

AbstractFilms made of 2D networks of single‐walled carbon nanotubes (SWNTs) are one of the most promising active‐channel materials for field‐effect transistors (FETs) and have a variety of flexible electronic applications, ranging from biological and chemical sensors to high‐speed switching devices. Challenges, however, still remain due to the current hysteresis of SWNT‐containing FETs, which has hindered further development. A new and robust method to control the current hysteresis of a SWNT‐network FET is presented, which involves the non‐volatile polarization of a ferroelectric poly(vinylidene fluoride‐trifluoroethylene) (P(VDF‐TrFE)) gate insulator. A top‐gate FET with a solution‐processed SWNT‐network exhibits significant suppression of the hysteresis when the gate‐voltage sweep is greater than the coercive field of the ferroelectric polymer layer (≈50 MV m−1). These near‐hysteresis‐free characteristics are believed to be due to the characteristic hysteresis of the P(VDF‐TrFE), resulting from its non‐volatile polarization, which makes effective compensation for the current hysteresis of the SWNT‐network FETs. The onset voltage for hysteresis‐minimized operation is able to be tuned simply by controlling the thickness of the ferroelectric film, which opens the possibility of operating hysteresis‐free devices with gate voltages down to a few volts.

Characteristics of [6]phenacene thin film field-effect transistor

Transistor characteristics are studied for field-effect transistors (FETs) with thin films of [6]phenacene, which has six benzene rings and W-shape structure. The molecular alignment preferable for FET transport is found to be formed in [6]phenacene thin films. The transistor shows clear p-channel FET characteristics with field-effect mobility μ as high as 3.7 cm2 V−1 s−1. The similar O2 sensing properties to picene FET are observed in [6]phenacene thin film FET. The bias stress properties are observed in [6]phenacene thin film FET. The pulse-voltage application suppresses the bias-stress effect and it enables a continuous O2 sensing in [6]phenacene FET.

ELECTRICAL CHARACTERIZATION OF PLANAR SILICON NANOWIRE FIELD-EFFECT TRANSISTORS

Silicon nanowires (Si NWs) are promising candidates for field-effect transistor (FET) conduction channel. Planar configuration using a back gate is an easy way to study these devices. We demonstrate the possibility to build high performance FET using a simple silicidation process leading to high effective holes' mobility between 130 cm2⋅V-1⋅s-1 and 200 cm2⋅V-1⋅s-1 and good ION/IOFF ratio up to 105. Moreover we investigated the possibility to passivate the NWs using either a high-k dielectric layer or a thermal oxide shell around the NWs. This leads to a reduction of the hysteretic behavior during the gate voltage sweep from 30 V to 1 V depending on the material and the gate configuration.

Site‐controlled SiGe islands on patterned Si(001): Morphology, composition profiles, and devices

AbstractDeterministic control of position on a substrate, uniform size and shape are prerequisite for most of the envisioned applications of SiGe islands in electronic and optoelectronic devices. As an example of electronic application, tensile strained Si layers on top of coherent SiGe islands may be used as channels for field effect transistors (FETs) with enhanced electron mobility. For such a kind of application, site‐controlled islands are required to allow for their external addressability. In this feature article we investigate the morphological and compositional evolution of site‐controlled SiGe islands on pit‐patterned Si(001) substrates. We then report on the first demonstrated n‐channel FET with enhanced electron mobility based on SiGe islands. Finally, a new approach for further increase of the tensile strain is presented.

Time-of-Flight Technique Limits of Applicability for Thin-Films of Π-Conjugated Polymers

ABSTRACTTime of flight (ToF) is the most straightforward technique to determine polymeric semiconductor mobility for electronic applications. We demonstrate ToF limits of applicability to amorphous PPV derivatives, such as poly[2-methoxy-5-(3’,7’-dimethylloctyloxy)-1-4-phenylene vinylene] (MDMO-PPV) and poly[2-methoxy-5-(2’-ethylhexyloxy)-1-4-phenylene vinylene] (MEH-PPV), and polycrystalline poly(3-hexylthiophene) (P3HT). Hole and electron mobility (μ) in submicrometric films (200 – 500 nm) is overestimated compared to casted layers, due to reduced absorption capability, which is confirmed by Charge Extraction by Linearly Increasing Voltage (CELIV) measurements. Charge transport properties in nanometric films, such as for Field-Effect Transistors (FET), can not be studied by current-mode ToF. Hole mobility of ca. 10-5 cm2/Vs with Poole-Frenkel behavior for PPV derivatives and 10-3 cm2/Vs for P3HT is at least one order of magnitude higher than ToF results.

Multifunctional Biosensor Based on Zno Nanostructures

The present invention provides the multifunctional biological and biochemical sensor technology based on ZnO nanostructures. The ZnO nanotips serve as strong DNA or protein molecule binding sites to enhance the immobilization. Patterned ZnO nanotips are used to provide conductivity-based biosensors. Patterned ZnO nanotips are also used as the gate for field-effect transistor (FET) type sensors. Patterned ZnO nanotips are integrated with SAW or BAW based biosensors. These ZnO nanotip based devices operate in multimodal operation combining electrical, acoustic and optical sensing mechanisms. The multifunctional biosensors can be arrayed and combined into one biochip, which will enhance the sensitivity and accuracy of biological and biochemical detection due to strong immobilization and multimodal operation capability. Such biological and biochemical sensor technology are useful in detection of RNA-DNA, DNA-DNA, protein-protein, protein-DNA and protein-small molecules interaction. It can be further applied for drug discovery, and for environmental monitoring and protection.

Vertically integrated silicon-germanium nanowire field-effect transistor

We demonstrate in this paper the possibility to vertically integrate SiGe nanowires in order to use them as vertical channel for field-effect transistors (FETs). We report a threshold voltage close to 3.9 V, an ION/IOFF ratio of 104. The subthreshold slope was estimated to be around 0.9 V/decade and explained by a high traps density at the nanowire core/oxide shell interface with an estimated density of interface traps Dit ∼ 1.2 × 1013 cm−2 eV−1. Comparisons are made with both vertical Si and horizontal SiGe FETs performances.

Analysis of Assembling ZnO Nanoparticles Into Nanogap Electrodes for Nanoscale Electronic Device Applications

We report the assembly of semiconducting ZnO nanoparticles into nanogap electrodes using ac dielectrophoresis (DEP) process. ZnO nanoparticles were grown using sol-gel method. As grown ZnO nanoparticles (diameter approximately 4 nm) were in a quasi-crystalline state and with increase in annealing temperature (in air) upto 400 degrees C the crystallinity was found to increase as revealed by X-ray diffraction and transmission electron microscopy. Absorption spectroscopy revealed that absorption edge of the as grown and annealed samples exhibited a blue-shift and could be attributed to the size quantization. A strong band tailing in the absorption spectra indicates the presence of shallow defects in the nanoparticles. The annealed ZnO nanoparticles (diameter approximately 9 nm) were assembled into nanogap electrodes (60 nm) using DEP process. DEP parameters such as frequency, applied voltage, and time were optimized to assemble ZnO nanoparticles into nanogap electrodes. Frequency variation study revealed that positive DEP is active at frequencies less than 500 kHz, and the negative DEP starts dominating at 1 MHz. It was observed that when DEP force overcomes the Brownian motion, the nanoparticles start chaining together in the region between the electrodes tip due to particle-field and particle-particle interactions. Under the optimized DEP parameters such as frequency of 150 kHz, peak-to-peak voltage of 3 V, and trapping time of 30 s, a minimum number of ZnO nanoparticles were assembled into nanogap electrodes. The ZnO nanoparticles based device was studied for field effect transistor (FET) characteristics and the typical current-voltage (I(D)-V(DS)) characteristic curves as a function of gate voltage (V(G)) were obtained. The curves indicate that ZnO nanoparticles based FET exhibits a significant gate effect as the conductance of the ZnO nanoparticles increases with gate voltage suggesting n-type semiconducting behavior of ZnO nanoparticles. Results show the myriad potential of dielectrophoretically assembled ZnO nanoparticles into nanogap electrodes for designing nanoscale devices for electronic and optoelectronic applications.

A new approach for small-signal modelling of the field effect transistor based on cepstral coefficients and discrete transforms

In this article, a new technique for field effect transistor (FET) small-signal modelling using neural networks is proposed. This technique is based on the combination of the Mel-frequency cepstral coefficients (MFCCs) with different discrete transforms such as the discrete cosine transform (DCT), the discrete sine transform (DST) and the discrete wavelet transform (DWT) of the inputs to the neural networks. The input datasets to traditional neural systems for FET small-signal modelling are the scattering parameters and the corresponding frequencies in a certain band, and the output datasets are the circuit elements. In the proposed approach, these datasets are considered to form random signals. The MFCCs of the random signals are used to generate a small number of features characterising the signals. In addition, other vectors are calculated from the DCT, DST or DWT of the random signals and appended to the MFCC vectors calculated from the signals. The new feature vectors are used to train the neural networks. The objective of using these new vectors is to characterise the random input sequences with more features, to encourage robustness against measurement errors. There are two benefits to these approaches: (1) a reduction in the number of neural network inputs, and hence a faster convergence of the neural training algorithm and (2) robustness against measurement errors in the testing phase. Experimental results show that the techniques based on the discrete transforms are less sensitive to measurement errors than using the traditional and MFCC methods.

Active Switching Devices in a Tunable EBG Structure: Placement Strategies and Modelling

Different placement strategies for Field-Effect-Transistor (FET) active switching devices in an electromagnetic band gap (EBG) periodic structure are investigated. Two possible placements for the switches are considered: (a) switches on top of a substrate and (b) switches at the ground-plane level. Their advantages and drawbacks are analyzed and discussed in relation to fabrication, extension to 2-D and ground plane design. The transmission results and band gaps of the EBG structure are presented for both switch placements. When determining the transmission of the structure, the FET switches are modeled in two different ways, first as an ideal conductor and then using a more accurate model consisting of embedded scattering parameters. Significant differences have been observed between the results predicted using simple and accurate models. Experimental characterization of the switch reveals technological issues related to their biasing.