Thin-film Field-effect Transistors Research Articles

Improvement of N-type carbon nanotube field effect transistor performance using the combination of yttrium diffusion layer in HfO2 dielectrics and metal contacts.

In this paper, we obtained n-type top-gate carbon nanotube thin film field effect transistors (CNTFET) with source/drain extensions structure through dielectrics optimization strategy, combining the yttrium layer with HfO2 dielectric argon annealing process and metal contacts. The mechanism for enhanced n-type conduction was explained as being due to the vertical diffusion of yttrium to the HfO2 dielectric during argon annealing. This diffusion causes abending of the energy band, which results in more positive fixed charges and a reduction in the electron injection barrier between the low work function source-drain Cr electrode and carbon nanotube. The optimized technology has great prospects for the low cost, large scale and high performance n-type CNTFET to be used in integrated electronic devices.&#xD.

Thin film field-effect transistor with ZnO:Li ferroelectric channel

An n-type channel transparent thin film field-effect transistor (FET) using a top-gate configuration on a sapphire substrate is presented. ZnO:Li film was used as a channel, and MgF2 film as a gate insulator. Measurements showed that ZnO:Li films are ferroelectrics with spontaneous polarization [Formula: see text]–5[Formula: see text][Formula: see text]C/cm2 and coercive field [Formula: see text]–10[Formula: see text]kV/cm. The dependences of drain–source current on drain–source voltage at various gate–source voltages in two antiparallel [Formula: see text] states were measured and the values of field-effect mobility and threshold voltage were determined for two [Formula: see text] states are as follows: (a) [Formula: see text][Formula: see text]cm2/Vs, [Formula: see text][Formula: see text]V; (b) [Formula: see text][Formula: see text]cm2/Vs, [Formula: see text][Formula: see text]V. Thus, [Formula: see text] switching leads to a change in FET channel parameters. Results can be used to create a bistable or, more precisely, digital FET.

Temperature Dependence of Charge Transport Properties of Quasi-2D Chiral Perovskite Thin-Film Field-Effect Transistors.

Chiral halide perovskite materials promise both superior light response and the capability to distinguish circularly polarized emissions, which are especially common in the fluorescence spectra of organic chiral materials. Herein, thin-film field-effect transistors (FETs) based on chiral quasi-two-dimensional perovskites are explored, and the temperature dependence of the charge carrier transport mechanism over the broad temperature range (80-300 K) is revealed. A typical p-type charge transport behavior is observed for both left-handed (S-C6H5(CN2)2NH3)2(CH3NH3)n-1PbnI3n+1 and right-handed (R-C6H5(CN2)2NH3)2(CH3NH3)n-1PbnI3n+1 chiral perovskites, with maximum carrier mobilities of 1.7 × 10-5 cm2 V-1 s-1 and 2.5 × 10-5 cm2 V-1 s-1 at around 280 K, respectively. The shallow traps with smaller activation energy (0.03 eV) hinder the carrier transport over the lower temperature regime (80-180 K), while deep traps with 1 order of magnitude larger activation energy than the shallow traps moderate the charge carrier transport in the temperature range of 180-300 K. From the charge carrier mechanism point of view, impurity scattering is established as the dominant factor from 80 K until around 280 K, while phonon scattering becomes predominant up to room temperature. Responsivities of 0.15 A W-1 and 0.14 A W-1 for left-handed and right-handed chiral perovskite FET devices are obtained.

Low-frequency noise characterization of positive bias stress effect on the spatial distribution of trap in β-Ga2O3 FinFET

The reliability of a β-Ga2O3 thin-film field-effect transistor is investigated under positive-bias stress (PBS). The transistor has a tri-gate structure with a gate dielectric of Al2O3. By characterizing low-frequency noise (LFN), the spatial distribution of trap in the gate dielectric was quantitatively extracted. The measured power spectral density (PSD) followed a 1/f-shape due to trapping and de-trapping of the channel carriers to and from the gate dielectric. Notably, the vertical distribution of the traps perpendicular to the interface of β-Ga2O3 and Al2O3 was mapped

Tungsten oxide thin film field-effect transistor based real-time sensing system for non-invasive oral cancer biomarker detection

We have presented real-time and label-free CYFRA 21–1 detection in artificial saliva using a tungsten oxide (WO3) thin film transistor with wafer scale batch fabrication. In this work, we developed a miniaturized (FET) biosensing platform based on antibody-antigen interaction at the channel surface for real-time CYFRA 21–1 detection in artificial saliva. The fabricated FET device showed rapid and high response towards the CYFRA 21–1 antigen in artificial saliva, with the highest response of 122.88 at 1 pg/mL and a limit of detection (LOD) of 1.26 pg/mL. This device demonstrated the detection capability of different CYFRA 21–1 concentrations in artificial saliva with high specificity towards the targeted antigen to detect oral cancer biomarker and a response in real-time. Unlike other methods for oral cancer detection, this study demonstrates an innovative biosensing device platform for directly detecting oral cancer biomarkers in saliva without labeling or pre-processing the sample. This method shows the highest sensitivity with a minimum sample volume requirement (2 µL) for early-stage oral cancer diagnosis. Moreover, this work showed the prospect of batch fabrication and uniformity among batch-fabricated devices.

Enhancing Oxygen Sensing Precision Through Gate-Controlled TiO2 Field Effect Transistors Under Ultraviolet Excitation

We report on the fabrication and characterization of a novel oxygen sensor based on a TiO2 thin film field effect transistor (FET) deposited on a silicon substrate with an oxide layer by magnetron sputtering. TiO2 is a n-type semiconductor with a wide band gap, which allows the formation of oxygen vacancies or adsorbed oxygen species on its surface under ambient conditions. These oxygen-related defects act as electron traps that modulate the electrical conductivity of the TiO2 film. Under ultraviolet (UV) irradiation, at 310 nm wavelength, the photogenerated carriers in the TiO2 film are captured by the oxygen defects, resulting in a decrease of the film resistance that depends on the oxygen concentration. We demonstrate that the sensitivity and resolution of the oxygen sensor can be enhanced by applying a positive gate voltage to the FET device. The photocurrent variation per unit of oxygen concentration (ΔIphoto/ΔCPO) increases from 1.08 at VG =0 V to 2.5 at VG= 20 V in the range of 5%–20% oxygen concentration. The gate voltage also extends the controllable range of oxygen defects and photocurrent. Our study provides a new insight into the design and optimization of gas sensors based on TiO2 thin film FETs.

Improving the mobility of D-A polymer PTB7-Th thin film field-effect transistors through organic salt doping

Organic salt doping is an effective method to control the electrical properties of organic semiconductor materials, while its underlying mechanism remains to be studied. In this work, we report the improvement of organic field-effect transistor (OFET) performance by doping the D-A polymer semiconductor PTB7-Th with organic salts. The underlying mechanism of PTB7-Th's organic salt doping in OFETs has been explored to fill the existing research gap. The doping of organic salts can improve the hole mobility of the device and change the threshold voltage. In addition, at low doping ratios, the on-off ratio of the device is not sacrificed but remains unchanged. Finally, we have made a series of characterizations of the properties of doped polymers through absorption, EPR, and UPS, which provide specific guidelines for modifying D-A polymer devices.

Realizing metallicity in Sr2IrO4 thin films by high-pressure oxygen annealing

Perovskite iridates are a promising material platform for hosting unconventional superconductivity. Transport measurements of Sr2IrO4 thin-film field-effect transistors are expected to provide irrefutable evidence for the existence of superconductivity. However, these experiments have revealed a remarkably robust insulating state over wide electron and hole doping ranges; this finding is in contrast to the case of the bulk material, in which metallicity appears upon moderate electron doping by substituting cations in place of Sr. The nature of this robust insulating state and whether any metallic state can be realized in the Sr2IrO4 thin film are two remaining challenges that preclude further progress in the search for superconductivity in this system. Here, we show that this insulating state is enhanced in Sr2IrO4 thin films by thermal annealing under vacuum conditions, while it can be destroyed upon annealing in an oxygen atmosphere within restricted ranges of oxygen pressure, annealing temperature and ion substitution levels. The resulting films exhibit metallic transport behavior near room temperature and a metal–insulator crossover at ~200 K. Our results point to the potentially important roles of the oxygen vacancies at different atomic sites in the formation of the robust insulating state and the new metallic state and to their interplay in the Sr2IrO4 thin film. This finding opens new possibilities in the search for unconventional superconductivity by further tailoring the as-found metallic state in properly oxygen-annealed Sr2IrO4 thin films.

Interfacial molecular screening of polyimide dielectric towards high-performance organic field-effect transistors

The compatibility of the gate dielectrics with semiconductors is vital for constructing efficient conducting channel for high charge transport. However, it is still a highly challenging mission to clearly clarify the relationship between the dielectric layers and the chemical structure of semiconductors, especially vacuum-deposited small molecules. Here, interfacial molecular screening of polyimide (Kapton) dielectric in organic field-effect transistors (OFETs) is comprehensively studied. It is found that the semiconducting small molecules with alkyl side chains prefer to form a high-quality charge transport layer on polyimide (PI) dielectrics compared with the molecules without alkyl side chains. On this basis, the fabricated transistors could reach the mobility of 1.2 cm2 V−1 s−1 the molecule with alkyl side chains on bare PI dielectric. What is more, the compatible semiconductor and dielectric would further produce a low activation energy (EA) of 3.01 meV towards efficient charge transport even at low temperature (e.g., 100 K, 0.9 cm2 V−1 s−1). Our research provides a guiding scheme for the construction of high-performance thin-film field-effect transistors based on PI dielectric layer at room and low temperatures.

Metal Halide Perovskite Photo-Field-Effect Transistors with Chiral Selectivity.

Organic-inorganic (hybrid) metal halide perovskites (MHPs) incorporating chiral organic ligand molecules are naturally sensitive to left- and right-handed circular polarized light, potentially enabling selective circular polarized photodetection. Here, the photoresponses in chiral MHP polycrystalline thin films made of ((S)-(-)-α-methyl benzylamine)2PbI4 and ((R)-(+)-α-methyl benzylamine)2PbI4, denoted as (S-MBA)2 PbI4 and (R-MBA)2PbI4, respectively, are investigated by employing a thin-film field-effect transistor (FET) configuration. The left-hand-sensitive films made of (S-MBA)2PbI4 perovskite show higher photocurrent under left-handed circularly polarized (LCP) light than under right-handed circularly polarized (RCP) illumination under otherwise identical conditions. Conversely, the right-hand-sensitive films made of (R-MBA)2PbI4 are more sensitive to RCP than LCP illumination over a wide temperature range of 77-300 K. Furthermore, based on FET measurements, we found evidence of two different carrier transport mechanisms with two distinct activation energies in the 77-260 and 280-300 K temperature ranges, respectively. In the former temperature range, shallow traps are dominant in the perovskite film, which are filled by thermally activated carriers with increasing temperature; in the latter temperature range, deep traps with one order of magnitude larger activation energy dominate. Both types of chiral MHPs show intrinsic p-type carrier transport behavior regardless of the handedness (S or R) of these materials. The optimal carrier mobility for both handedness of material is around (2.7 ± 0.2) × 10-7 cm2 V-1 s-1 at 270-280 K, which is two magnitudes larger than those reported in nonchiral perovskite MAPbI3 polycrystalline thin films. These findings suggest that chiral MHPs can be an excellent candidate for selective circular polarized photodetection applications, without additional polarizing optical components, enabling simplified construction of detection systems.

Films enriched with semiconducting single-walled carbon nanotubes by aerosol N2O etching

Semiconducting single-walled carbon nanotubes (SWCNTs) are of particular interest to electronics. As most of synthesis methods yield 1/3 of metallic SWCNTs, the route for a purely semiconducting species is a complex multistep procedure. Herein, we report a simple one-step method for selective etching of metallic SWCNTs in the aerosol phase with nitrous oxide. Our approach is based on a tandem of two flow reactors, where the first one is needed to produce individual SWCNTs, and the second one is to eliminate metallic SWCNTs due to their faster oxidization rate. The oxidant (N2O) adsorption on the SWCNT surface was modelled by DFTB taking into account geometric characteristics of the nanotube and its electronic properties, resulting in less N2O adsorption energy for metallic SWCNTs of some chiralities than for semiconducting ones. Thus, we obtained SWCNT films enriched with semiconducting nanotubes up to optical selectivity of 97% by treatment of pristine SWCNT aerosols for 6 s at 600 °C in an atmosphere of 30% N2O. The treated films show a turnaround of the temperature dependence of the resistance. As a proof of the concept, enriched SWCNT films were utilized as a channel material for thin film field effect transistors, which show an average improvement of ON/OFF current ratio from 1- 10 to 10 - 104 for the same open state resistance for the statistical sampling based on more than 7500 devices.

Design and analysis of high-performance double-gate ZnO nano-structured thin-film ISFET for pH sensing applications

This paper reports a high-performance double-gate zinc-oxide thin-film ion-sensitive field-effect transistor (DG-ZnO-ISFET) for pH sensing applications. The sensing mechanism of DG-ZnO-ISFET and a simulation model for pH sensing analysis are demonstrated herein. Various aspects of the device are elucidated through the study of channel conductance, electron density, energy band, and surface-potential profiles. The DG-ZnO-ISFETs enhance minor surface potential variations by capacitive coupling across the dielectrics of the top and bottom gates. Further, it operates by altering the bottom-gate voltage (VBG), obviating the need for a reference electrode system. The device depicts enhanced voltage sensitivity of 205.57 mV/pH, which is substantially 3.48 × greater than the Nernst limit sensitivity. In addition, the DG-ZnO-ISFET renders a maximum current sensitivity of 10.82 mA/mm.pH at VBG=5V. Furthermore, it exhibits high linearity in both voltage and current sensitivity, with the coefficient of determination (R2) values of 0.984 and 0.964, respectively. With their increased sensitivity and linearity, DG-ZnO-ISFETs have the potential device for next-generation biosensors.

Applying ferroelectric materials in transistors for electronic and optoelectronic applications

Thin-film field-effect transistors provide unique benefits in electronics research.

Single-walled carbon nanotubes synthesized by laser ablation from coal for field-effect transistors.

Single-walled carbon nanotubes (SWCNTs) have been attracting extensive attention due to their excellent properties. We have developed a strategy of using coal to synthesize SWCNTs for high performance field-effect transistors (FETs). The high-quality SWCNTs were synthesized by laser ablation using only coal as the carbon source and Co-Ni as the catalyst. We show that coal is a carbon source superior to graphite with higher yield and better selectivity toward SWCNTs with smaller diameters. Without any pre-purification, the as-prepared SWCNTs were directly sorted based on their conductivity and diameter using either aqueous two-phase extraction or organic phase extraction with PCz (poly[9-(1-octylonoyl)-9H-carbazole-2,7-diyl]). The semiconducting SWCNTs sorted by one-step PCz extraction were used to fabricate thin film FETs. The transformation of coal into FETs (and further integrated circuits) demonstrates an efficient way of utilizing natural resources and a marvelous example in green carbon technology. Considering its short steps and high feasibility, it presents great potential in future practical applications not limited to electronics.

Mechanically robust stretchable semiconductor metallization for skin-inspired organic transistors.

Despite recent remarkable advances in stretchable organic thin-film field-effect transistors (OTFTs), the development of stretchable metallization remains a challenge. Here, we report a highly stretchable and robust metallization on an elastomeric semiconductor film based on metal-elastic semiconductor intermixing. We found that vaporized silver (Ag) atom with higher diffusivity than other noble metals (Au and Cu) forms a continuous intermixing layer during thermal evaporation, enabling highly stretchable metallization. The Ag metallization maintains a high conductivity (>104 S/cm) even under 100% strain and successfully preserves its conductivity without delamination even after 10,000 stretching cycles at 100% strain and several adhesive tape tests. Moreover, a native silver oxide layer formed on the intermixed Ag clusters facilitates efficient hole injection into the elastomeric semiconductor, which transcends previously reported stretchable source and drain electrodes for OTFTs.

Alkyl-Substituted N,S-Embedded Heterocycloarenes with a Planar Aromatic Configuration for Hosting Fullerenes and Organic Field-Effect Transistors.

Cycloarenes and heterocycloarenes display unique physical structures and hold great potential as organic semiconductors. However, the synthesis of (hetero)cycloarenes remains a big challenge, and there are limited reports on their applications. Herein, a series of nitrogen- and sulfur-codoped cycloarenes NS-Octulene-n (n = 2, 3, 4) with branched alkyl substituents containing linear spacer groups from C2 to C4 have been conveniently synthesized. Compared with their isoelectronic analogues Octulene and S-Octulene, both having a saddle-shaped configuration, the coincorporation of two nitrogen atoms and two sulfur atoms leads to a fully coplanar aromatic backbone structure. Each of these three planar heterocycloarenes acts as a supramolecular host for encapsulation of both fullerenes C60 and C70 with a stronger donor-acceptor interaction for the complexation between the heterocycloarene and C70 due to the unique molecular geometry and defined cavity. Meanwhile, the electron-rich nitrogen atoms also slightly increase the energies of both highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) in this series of planar heterocycloarenes, indicating that they can be used as p-type semiconductors. Most importantly, benefitting from the planar π-conjugated backbone structure accompanied by excellent crystallinity and ordered molecular packing, as well as upon the engineering of the alkyl chain branching position, thin-film field-effect transistors of NS-Octulene-3 with moderate alkyl branching point exhibit the maximum hole mobility of 0.86 cm2 V-1 s-1, which is the highest for (hetero)cycloarene-based organic semiconductors. This study will shed new light on designing novel high-performance macrocyclic polycyclic aromatic hydrocarbon (PAH) semiconductors.

Two-Dimensional Layered Simple Aliphatic Monoammonium Tin Perovskite Thin Films and Potential Applications in Field-Effect Transistors.

Two-dimensional (2D) layered organic-inorganic perovskites have great potential for fabricating field-effect transistors due to their unique structure that enables the horizontal transport of charge carriers in metal-halide octahedra, resembling the transport behavior in semiconducting channels. Their electronic band structures are mainly dominated by the metal-halide octahedra, which eventually determine the optical and electrical characteristics, whereas organic cations have no direct contributions but would impact the electronic structures via distorting the octahedra. So far, high performance has been achieved in 2D Sn perovskites compared to their Pb counterparts because the intrinsic differences of Sn promote transport properties. The champion hole mobility has been obtained in single-ring aromatic phenylethylammonium tin iodide perovskite [(PEA)2SnI4]. However, simple aliphatic monoammonium tin perovskites and their device applications have rarely been reported. Herein, 2D layered n-butylammonium tin iodide perovskite [(BA)2SnI4] thin films have been synthesized by a spin-coating approach. A structural phase transition occurs at about 225 K in the films, accompanied by the changes in the photoluminescence peak and exciton binding energy. Longitudinal optical (LO) phonons are found to govern the scattering of charge carriers and excitons via the Fröhlich interactions in the temperature range 77-300 K. The first-principles calculations predict that the perovskite has excellent transport characteristics comparable to those of molybdenum disulfide (MoS2) and methylammonium lead iodide perovskite (MAPbI3). The (BA)2SnI4 thin film field-effect transistors constructed on polymer dielectrics with a maximum hole mobility of 0.03 cm2 V-1 s-1 in ambient conditions have been successfully demonstrated for the first time. Our findings not only offer a deep insight into the physical properties of 2D layered aliphatic monoammonium tin perovskite thin films but also provide important experimental and theoretical guidance for their potential applications in lateral-type flexible optoelectronic devices.

Poly(2,6-azuleneethynylene)s: Design, Synthesis, and Property Studies

Two poly(2,6-azuleneethynylene)s (PAzE-1 and PAzE-2) were designed and synthesized. The 2,6-azulene units are head-to-tail-arranged in PAzE-1, while there are three types of orientations of 2,6-azulene units in PAzE-2: head-to-tail, head-to-head, and tail-to-tail segments, of which the head-to-tail one accounts for about 27% (determined from 1H NMR spectra). Such a structural distinction endows the two polymers with different absorption spectra in solution and aggregation states as well as different thin-film morphologies, microstructures, and field-effect transistor (FET) performances, suggesting that the dipole orientation of azulene units in a polymer backbone may be a critical issue that deserves careful consideration during the molecular design and synthesis. PAzE-1 with an ordered dipole orientation has stronger aggregation even in a very dilute solution (10–6 M). PAzE-2 films have a higher in-plane microstructural order and lower surface roughness than PAzE-1 films; hence, the PAzE-2-based transistor devices exhibit 1–2 orders higher hole and electron mobilities. Unlike typical p-type alkyl-substituted poly(p-phenyleneethynylene)s (PPEs), PAzE-1 and PAzE-2 are ambipolar semiconductors.

Optoelectronic Modulation of Silver Antimony Sulfide Thin Films for Photodetection.

Silver antimony sulfide, as a ternary chalcogenide, has attracted great attention in the field of optoelectronics in recent years. In particular, it has appealing properties, such as excellent stability, solution processability, and versatile composition tunability. Benefiting from the recent development of processing techniques, AgSbS2 has emerged as a promising candidate for next-generation, thin-film photovoltaics. On the contrary, AgSbS2-based photodetectors have been barely reported. In this work, we systematically investigated the composition engineering of silver antimony sulfide compounds with a precursor route. Their optoelectronic properties were fully characterized, and the composition was optimized for photodetection. High-performance phototransistors were first reported based on field-effect thin film transistors with interfacial modification. The obtained AgSbS2 phototransistors exhibited relatively high photosensitivity, low dark current and noise, superior device stability, and excellent detectivity covering the whole range from ultraviolet to near-infrared, highlighting the great potential for next-generation photodetection.

Enhanced performance of p-type SnO x thin film transistors through defect compensation

Due to the unique outermost orbitals of Sn, hole carriers in tin monoxide (SnO) possess small effective mass and high mobility among oxide semiconductors, making it a promising p-channel material for thin film field-effect transistors (TFTs). However, the Sn vacancy induced field-effect mobility deterioration and threshold voltage (V th) shift in experiments greatly limit its application in complementary metal-oxide-semiconductor (CMOS) transistors. In this study, the internal mechanism of vacancy defect compensation by aluminum (Al) doping in SnO x film is studied combining experiments with the density functional theory (DFT). The doping is achieved by an argon (Ar) plasma treatment of Al2O3 deposited onto the SnO x film, in which the Al2O3 provides both the surface passivation and Al doping source. Experimental results show a wide V th modulation range (6.08 to −19.77 V) and notable mobility enhancement (11.56 cm2V−1s−1) in the SnO x TFTs after the Al doping by Ar plasma. DFT results reveal that the most possible positions of Al in SnO and SnO2 segments are the compensation to Sn vacancy and interstitial. The compensation will create an n-type doping effect and improve the hole carrier transport by reducing the hole effective mass (m h*), which is responsible for the device performance variation, while the interstitial in the SnO2 segment can hardly affect the valence transport of the film. The defect compensation is suitable for the electronic property modulation of SnO towards the high-performance CMOS application.