Mobility Transistors Research Articles

Wafer-Scale Epitaxial Growth of an Atomically Thin Single-Crystal Insulator as a Substrate of Two-Dimensional Material Field-Effect Transistors.

As the electron mobility of two-dimensional (2D) materials is dependent on an insulating substrate, the nonuniform surface charge and morphology of silicon dioxide (SiO2) layers degrade the electron mobility of 2D materials. Here, we demonstrate that an atomically thin single-crystal insulating layer of silicon oxynitride (SiON) can be grown epitaxially on a SiC wafer at a wafer scale and find that the electron mobility of graphene field-effect transistors on the SiON layer is 1.5 times higher than that of graphene field-effect transistors on typical SiO2 films. Microscale and nanoscale void defects caused by heterostructure growth were eliminated for the wafer-scale growth of the single-crystal SiON layer. The single-crystal SiON layer can be grown on a SiC wafer with a single thermal process. This simple fabrication process, compatible with commercial semiconductor fabrication processes, makes the layer an excellent replacement for the SiO2/Si wafer.

Fluoro substitution effect on 2,1,3-benzothiadiazole-based hybridized local and charge transfer state fluorophores for high-performance organic light-emitting diodes

Fluoro substitution of organic semiconductors is an effective way to improve some related performance of the materials, such as the carrier mobility in organic photovoltaics (OPVs) and organic field-effect transistors (OFETs) applications. However, the influence of fluoro substitution for fluorophores in organic light-emitting diodes (OLEDs) applications is rarely reported. Here, with fluorinated electron-deficient 2,1,3-benzothiadiazole (BT) unit as the acceptor and triphenylamine (TPA) as the donor, we construct two new D-A-type fluorophores DTPA-fBT and DTPA-ffBT. Theoretical calculations and experimental results reveal that the lowest excited states (S1) of the materials are characterized by hybridized local and charge-transfer (HLCT) properties, in which the fluorine substitution will increase the proportion of the charge-transfer (CT) component. Due to the introduction of fluoro substitution, multiple fluorine hydrogen bonds are formed, which endow the molecules with more rigid conformation, leading to lower non-radiative rates and higher photoluminescence quantum yield (PLQY) in solution and doped film, as well as improved thermal stability in the solid state. As a result, the two fluorinated DTPA-ffBT demonstrate significantly improved performance in OLEDs with a maximum external quantum efficiency (EQE) of 7.25% (5.90% for fluorine-free counterpart) and very low-efficiency roll-off (roll-off <7% at 10,000 cd m−2).

Interfacial Interaction Enables Enhanced Mobility in Hybrid Perovskite-Conjugated Polymer Transistors with High-k Fluorinated Polymer Dielectrics.

The charge carrier mobility of organic field-effect transistors (OFETs) has been remarkably improved through several engineering approaches and techniques by targeting pivotal parts. Herein, an ultrathin perovskite channel layer that boosts the field-effect mobility of conjugated polymer OFETs by forming perovskite-conjugated polymer hybrid semiconducting channel is introduced. The optimized lead-iodide-based perovskite-conjugated polymer hybrid channel transistors show enhanced hole mobility of over 4cm2 V-1 s-1 (average=2.10cm2 V-1 s-1 ) with high reproducibility using a benchmark poly(3-hexylthiophene) (P3HT) polymer and employing high-k fluorinated polymer dielectrics. A significant hole carrier mobility enhancement of ≈200-400% in benzo[1,2-b:4,5:b']dithiophene (BDT)-based conjugated polymers is also demonstrated by exploring certain interactive groups with perovskite. This significant enhancement in the transistor performance is attributed to the increased charge carrier density in the hybrid semiconducting channel and the perovskite-polymer interactions. The findings of this paper demonstrate an exceptional engineering approach for carrier mobility enhancement in hybrid perovskite-conjugated-polymer-based electronic devices.

Recent Research Progress of Organic Small-Molecule Semiconductors with High Electron Mobilities.

Organic electronics has made great progress in the past decades, which is inseparable from the innovative development of organic electronic devices and the diversity of organic semiconductor materials. It is worth mentioning that both of these great advances are inextricably linked to the development of organic high-performance semiconductor materials, especially the representative n-type organic small-molecule semiconductor materials with high electron mobilities. The n-type organic small molecules have the advantages of simple synthesis process, strong intermolecular stacking, tunable molecular structure, and easy to functionalize structures. Furthermore, the n-type semiconductor is a remarkable and important component for constructing complementary logic circuits and p-n heterojunction structures. Therefore, n-type organic semiconductors play an extremely important role in the field of organic electronic materials and are the basis for the industrialization of organic electronic functional devices. This review focuses on the modification strategies of organic small molecules with high electron mobility at molecular level, and discusses in detail the applications of n-type small-molecule semiconductor materials with high mobility in organic field-effect transistors, organic light-emitting transistors, organic photodetectors, and gas sensors.

N-Type Polycrystalline Germanium Layers Formed by Impurity-Doped Solid-Phase Growth

The carrier mobility of polycrystalline Ge thin-film transistors has significantly improved in recent years, raising hopes for the realization of next-generation electronic devices. Here, we adapted advanced solid-phase crystallization, which achieved the highest hole mobility of the polycrystalline semiconductor layer, to Ge layers doped with n-type impurities (P, As, and Sb). The type and amount of dopants had marked effects on the growth morphology and electrical properties of the Ge layers because they altered the activation energies in crystal growth, dopant activation rates, and grain boundary properties. In particular, P doping was effective in increasing the grain size (25 μm) and lowering the grain boundary barrier height (20 meV), which improved the electron concentration (8.0 × 1018 cm–3) and electron mobility (380 cm2 V–1 s–1) in n-type polycrystalline Ge layers. The electron mobility is greater than that of most semiconductor layers synthesized at low temperatures (≤500 °C) on insulators, and this will pave the way for advanced electronic devices, such as multifunctional displays and three-dimensional large-scale integrated circuits.

Synthesis of component-controllable monolayer MoxW(1-x)S2ySe2(1-y) alloys with continuously tunable band gap and carrier type.

Alloying can effectively modify electronic and optical properties of two-dimensional (2D) transition metal dichalcogenides (TMDs). However, efficient and simple methods to synthesize atomically thin TMD alloys need to be further developed. In this study, we synthesized 25 monolayer MoxW(1-x)S2ySe2(1-y) alloys by using a new liquid phase edge epitaxy (LPEE) growth method with high controllability. This straightforward approach can be used to obtain monolayer materials and operates on a self-limiting growth mechanism. The process allows the liquid solution to come into contact with the two-dimensional grains only at their edges, resulting in epitaxy confined only along the in-plane direction, which produces exclusively monolayer epitaxy. By controlling the weight ratio of MoS2/WSe2 (MoSe2/WS2), 25 monolayer MoxW(1-x)S2ySe2(1-y) alloys with different atomic ratios can be obtained on sapphire substrates, with band gap ranging from WS2 (1.55 eV) to MoSe2 (1.99 eV) and a continuously broad spectrum ranging from 623 nm to 800 nm. By adjusting the alloy composition, the carrier type and carrier mobility of alloy-based field-effect transistors can be modulated. In particular, the adjustable conductivity of MoxW(1-x)S2ySe2(1-y) alloys from n-type to bipolar type is achieved for the first time. This general synthetic strategy provides a foundation for the development of monolayer TMD alloys with multiple components and various 2D materials.

Structural control of charge transport in polymer monolayer transistors by a thermodynamically assisted dip-coating strategy

A thermodynamically assisted strategy is developed to precisely control the growth rate of polymer aggregates in solution. The resulting polymer monolayer exhibits improved molecular order and doubled mobility in field-effect transistors.

CVD-deposited Cu2O thin films with a record Hall hole mobility of 263 cm2 V−1 s−1 and field-effect mobility of 0.99 cm2 V−1 s−1

Using a combination of chemical vapor deposition, controlled nucleation, and surface energy optimization, we demonstrate record hole mobility in Cu2O thin-films and thin-film transistors.

Mobility and quasi-ballistic charge carrier transport in graphene field-effect transistors

The optimization of graphene field-effect transistors (GFETs) for high-frequency applications requires further understanding of the physical mechanisms concerning charge carrier transport at short channel lengths. Here, we study the charge carrier transport in GFETs with gate lengths ranging from 2 μm down to 0.2 μm by applying a quasi-ballistic transport model. It is found that the carrier mobility, evaluated via the drain–source resistance model, including the geometrical magnetoresistance effect, is more than halved with decreasing the gate length in the studied range. This decrease in mobility is explained by the impact of ballistic charge carrier transport. The analysis allows for evaluation of the characteristic length, a parameter of the order of the mean-free path, which is found to be in the range of 359–374 nm. The mobility term associated with scattering mechanisms is found to be up to 4456 cm2/Vs. Transmission formalism treating the electrons as purely classical particles allows for the estimation of the probability of charge carrier transport without scattering events. It is shown that at the gate length of 2 μm, approximately 20% of the charge carriers are moving without scattering, while at the gate length of 0.2 μm, this number increases to above 60%.

The effect of residual palladium on the performance of organic electrochemical transistors

Organic electrochemical transistors are a promising technology for bioelectronic devices, with applications in neuromorphic computing and healthcare. The active component enabling an organic electrochemical transistor is the organic mixed ionic-electronic conductor whose optimization is critical for realizing high-performing devices. In this study, the influence of purity and molecular weight is examined for a p-type polythiophene and an n-type naphthalene diimide-based polymer in improving the performance and safety of organic electrochemical transistors. Our preparative GPC purification reduced the Pd content in the polymers and improved their organic electrochemical transistor mobility by ~60% and 80% for the p- and n-type materials, respectively. These findings demonstrate the paramount importance of removing residual Pd, which was concluded to be more critical than optimization of a polymer’s molecular weight, to improve organic electrochemical transistor performance and that there is readily available improvement in performance and stability of many of the reported organic mixed ionic-electronic conductors.

Strong Magnetic Field Annealing for Improved Phthalocyanine Organic Thin-Film Transistors.

Thin-film microstructure, morphology, and polymorphism can be controlled and optimized to improve the performance of carbon-based electronics. Thermal or solvent vapor annealing are common post-deposition processing techniques; however, it can be difficult to control or destructive to the active layer or substrates. Here, the use of a static, strong magnetic field (SMF) as a non-destructive process for the improvement of phthalocyanine (Pc) thin-film microstructure, increasing organic thin-film transistor (OTFTs) mobility by twofold, is demonstrated. Grazing incident wide-angle X-ray scattering (GIWAXS), X-ray diffraction (XRD), and atomic force microscopy (AFM) elucidate the effect of SMF on both para- and diamagnetic Pc thin-films when subjected to a magnetic field. ASMF is found to increase the concentration of oxygen-induced radical species within the Pc thin-film, lending a paramagnetic character to ordinarily diamagnetic metal-free Pc and resulting in magnetic field induced changes to its thin-film microstructures. In a nitrogen environment, without competing degradation effects of molecular oxygen, SMF processing is found to favorably improve charge transport characteristics and increase OTFT mobility. Thus, post-deposition thin-film annealing with a magnetic field is presented as an alternative and promising technique for future thin-film engineering applications.

Hydrogen-Doping-Enabled Boosting of the Carrier Mobility and Stability in Amorphous IGZTO Transistors.

This study investigated the effect of hydrogen (H) on the performance of amorphous In-Ga-Zn-Sn oxide (a-In0.29Ga0.35Zn0.11Sn0.25O) thin-film transistors (TFTs). Ample H in plasma-enhanced atomic layer deposition (PEALD)-derived SiO2 can diffuse into the underlying a-IGZTO film during the postdeposition annealing (PDA) process, which affects the electrical properties of the resulting TFTs due to its donor behavior in the a-IGZTO. The a-In0.29Ga0.35Zn0.11Sn0.25O TFTs at the PDA temperature of 400 °C exhibited a remarkably higher field-effect mobility (μFE) of 85.9 cm2/Vs, a subthreshold gate swing (SS) of 0.33 V/decade, a threshold voltage (VTH) of -0.49 V, and an ION/OFF ratio of ∼108; these values are superior compared to those of unpassivated a-In0.29Ga0.35Zn0.11Sn0.25O TFTs (μFE = 23.3 cm2/Vs, SS = 0.36 V/decade, and VTH = -3.33 V). In addition, the passivated a-In0.29Ga0.35Zn0.11Sn0.25O TFTs had good stability against the external gate bias duration. This performance change can be attributed to the substitutional H doping into oxygen sites (HO) leading to a boost in ne and μFE. In contrast, the beneficial HO effect was barely observed for amorphous indium gallium zinc oxide (a-IGZO) TFTs, suggesting that the hydrogen-doping-enabled boosting of a-IGZTO TFTs is strongly related to the existence of Sn cations. Electronic calculations of VO and HO using density functional theory (DFT) were performed to explain this disparity. The introduction of SnO2 in a-IGZO is predicted to cause a conversion from shallow VO to deep VO due to the lower formation energy of deep VO, which is effectively created around Sn cations. The formation of HO by H doping in the IGZTO facilitates the efficient connection of atomic states forming the conduction band more smoothly. This reduces the effective mass and enhances the carrier mobility.

Solution-Processed Isoindigo- and Thienoisoindigo-Based Donor-Acceptor-Donor π-Conjugated Small Molecules: Synthesis, Morphology, Molecular Packing, and Field-Effect Transistor Characterization.

Molecular design and precise control of thin-film morphology and crystallinity of solution-processed small molecules are important for enhancing charge transport mobility of organic field-effect transistors and gaining more insight into the structure-property relationship. Here, two donor-acceptor-donor (D-A-D) architecture small molecules TRA-IID-TRA and TRA-TIID-TRA comprising an electron-donating triarylamine (TRA) and two different electron-withdrawing cores, isoindigo (IID) and thienoisoindigo (TIID), respectively, were synthesized and characterized. Replacing the phenylene rings of central IID A with thiophene gives a TIID core, which reduces the optical band gap and upshifts the energy levels of frontier molecular orbitals. The single-crystal structures and grazing-incidence wide-angle X-ray scattering (GIWAXS) analysis revealed that TRA-TIID-TRA exhibits the relatively tighter π-π stacking packing with preferential edge-on orientation, larger coherence length, and higher crystallinity due to the noncovalent S···O/S···π intermolecular interactions. The distinctly oriented and connected ribbon-like TRA-TIID-TRA crystalline film by the solution-shearing process achieved a superior hole mobility of 0.89 cm2 V-1 s-1 in the organic field-effect transistor (OFET) device, which is at least five times higher than that (0.17 cm2 V-1 s-1) of TRA-IID-TRA with clear cracks. Eventually, rational modulation of fused core in the π-conjugated D-A-D small molecule provides a new understanding of structural design for enhancing the performance of solution-processed organic semiconductors.

Study of ambipolar properties of organic-inorganic CH3NH3PbI3 perovskite for vertical field effect transistor

A vertical field effect transistor (VFET) with short channel length of 120 nm, fabricated using organic-inorganic perovskite (CH3NH3PbI3) material shows the ambipolar properties and switches from P mode to N mode or vice versa, just by changing the polarity of applied bias. The operating voltage is as low as 0.5 V, which allows it to be integrated with solar cell devices and hence having the possibility of battery less mobile gadgets. To ensure the quality of VFET with CH3NH3PbI3 as a channel length, different parameters have been extracted. The transistor efficiency (TE), mobility (μ p), and transconductance (gm) in P mode, have been estimated as 14 V−1, 1.5 cm2 V−1 s−1 and 500 ± 150 μS, respectively. For N mode, these parameters have been found to be as 4 V−1, 0.97 cm2V−1 s−1 and 60 ± 10 μS respectively. The hole and electron densities are found to be as ∼1016 cm−3 and ∼1014 cm−3 respectively which confirm the ambipolar nature of CH3NH3PbI3.

Failure Physics and Reliability of GaN‐Based HEMTs for Microwave and Millimeter‐Wave Applications: A Review of Consolidated Data and Recent Results

Herein, the results are reviewed concerning reliability of high‐electron mobility transistors (HEMTs) based on GaN, which currently represent the technology of choice for high‐efficiency microwave and millimeter‐wave power amplifiers. Several failure mechanisms of these devices are extensively studied, including converse piezoelectric effects, formation of conductive percolation paths at the edge of gate toward the drain, surface oxidation of GaN, time‐dependent breakdown of GaN buffer, and of field‐plate dielectric. For GaN HEMTs with scaled gate length, the simultaneous control of short‐channel effects, deep‐level dispersion, and hot‐electron‐induced degradation requires a careful optimization of epitaxial material quality and device design.

Enhanced stability and mobility of solution-processed oxide thin-film transistors with bilayer terbium-incorporated indium oxide channel

The trade-off between mobility and stability in oxide thin-film transistors (TFTs) hinders further advances of an active-matrix flat panel display. Herein, a solution-processed bilayer active channel is designed to improve the stability and mobility simultaneously. The optical bandgap and work function of Tb:In2O3 films are modulated by tuning the film thickness and Tb concentration of Tb-doped indium oxide (Tb:In2O3) films. Large conduction band offset is achieved in a Tb:In2O3 bilayer channel, which induces accumulation of abundant electrons at the interface. The mobility is significantly improved to 38.2 cm2/V s, and the photoinduced stability of bilayer Tb:In2O3 TFTs is improved with low threshold voltage shift of 0.26 and −0.38 V under negative-bias illumination stress and negative-bias temperature illumination stress, respectively.

Effects of solution-shearing process parameters on charge carrier mobility in green solvent-processed organic field-effect transistors

In this work, we explored the effect of solution-shearing processing parameters, coating speed and substrate temperature, on the electrical properties of solution-processed organic field-effect transistors (OFETs) based on 2-decyl-7-phenyl[1]benzothieno[3,2-b][1]benzothiophene (Ph-BTBT-C10) when green solvents are used. Organic semiconductor thin films were solution-sheared with various coating speeds and substrate temperatures and film morphologies/microstructures were investigated by atomic force microscopy (AFM) and X-ray diffraction (XRD) analysis. Our data demonstrate that crystal growth of Ph-BTBT-C10 molecule is highly affected by the two solution shearing processing parameters and, especially, the OFET devices fabricated with cyclohexanone as green solvent exhibited the charge carrier mobility as high as 5.1 cm2/Vs with optimized processing condition. This work could further provide a guideline for process optimization for fabricating high-performance green solvent-processed OFETs.

The continuous fiber networks with a balanced bimodal orientation of P(NDI2OD-T2) by controlling solution nucleation and face-on and edge-on crystallization rates

The conjugated semiconducting polymer film morphology with long fiber and optimizing molecular packing is strongly correlated with charge transport properties. Herein, we report a method to obtain continuous fiber networks with a balanced bimodal orientation of poly{[N,N′-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} [P(NDI2OD-T2)] thin films by controlling solution nucleation and edge-on and face-on crystallization rates. This is enabled by adding ethylene glycol (EG) (the optimal content is 3 vol%) to P(NDI2OD-T2) in chloroform solution which improves the polarization of the NDI2OD units along the backbone direction and solution pre-aggregation. Therefore, the crystal nuclei are formed in solution state during the three-phase line receding as characterized by in situ polarized optical microscopy. Moreover, the peak intensity of the aggregated state increases with solvent evaporation, illustrating that the nucleation and crystal growth proceed simultaneously in the drying process as revealed by in situ ultraviolet–visible absorption spectra. The film morphology of continuous fibrous networks with an average fiber length of 531 nm is formed. In addition, the slopes of (100) peak area vs time in the out-of-plane and in-plane directions are similar, indicating that the edge-on and face-on crystallization rates are almost equal as obtained from in situ grazing incidence wide-angle X-ray scattering. Thus, the bimodal orientation with an edge-on and face-on mixing ratio is approximately 1. The average electron mobility of the field effect transistors of P(NDI2OD-T2) films with this continuous fiber network reaches to 0.2 cm2 V−1 s−1, an almost 3-fold improvement compared with devices without EG addition films.

Pulsed gate voltage measurement for charge mobility extraction of organic transistors

The field-effect mobility of organic semiconductors is an important parameter that describes a physical characteristic of charge carriers and is extracted from the current–voltage characteristics of field-effect transistors. For the parameter to be consistent with its physical definition, charge transport in organic semiconductors should be in a steady state. However, charge transport in organic field-effect transistors is often in a non-steady state because of the occurrence of charge-carrier trapping and the Joule heating effect; therefore, careful attention is required for the extraction of the field-effect mobility in such cases. In this study, the effects of gate pulses on the transport characteristics of organic field-effect transistors are investigated. Consequently, an ideal sequence of gate pulses to minimize the effects of intermediate trap states and Joule-heating during extraction of the field-effect mobility of organic transistors is proposed. Finally, an equation that relates the field-effect mobility to the sequence of gate pulses is derived and experimentally validated. • Systematic study of the effects of pulsed-gate measurements on the device characteristics of organic field-effect transistors. • Analytical model to describe the relationship between the effective mobility and gate pulse characteristics. • Ideal sequence of gate pulses for the extraction of the field-effect mobility of organic semiconductors.

Highly-sensitive organic field effect transistor sensors for dual detection of humidity and NO2

Sensors based on organic field effect transistors have attracted extensive interests, but most sensors are applied to a single analyte detection. Herein, a facile strategy was proposed to optimize the growth of active layers by regulating the thickness of vanadyl-phthalocyanine (VOPc) films and carrying out the thermal annealing. Thanks to the improved film morphology and crystal properties, the carrier mobility of transistors reached 0.15 cm2V−1s−1, humidity and nitrogen dioxide (NO2) induced multi-parameter response of transistors could be considered as an efficient sensing approach. The sensitivity of threshold voltage reached 0.59 V/% RH ranging from 42%−82% RH. In addition, the gas sensing performance was systematically studied. Sensors based on 10 nm VOPc films achieved a 4710% response to 10 ppm NO2 gas, with a theoretical limit of detection down to 125.2 ppb, and the response current was improved by two orders of magnitude under the amplification effect of transistor. The obtained gas sensors also exhibited high repeatability after being stored at 90% ambient humidity. It was concluded that different charge carrier transfer mechanism in moisture and NO2 gas provided a simple idea for tunable applications of phthalocyanine-based transistors.