Mobility Of Thin-film Transistors Research Articles

Characteristics of MgIn2O4 Thin Film Transistors Enhanced by Introducing an MgO Buffer Layer

In this work, an MgIn2O4 (MIO) thin film transistor (TFT) with a bottom gate structure was fabricated. The MIO channel layer was deposited by RF sputtering using a single MgIn2O4 target. The performance of MIO TFT was highly related to oxygen vacancies. As-deposited MIO TFT showed a low field-effect mobility due to doping of Mg. An MgO buffer layer was introduced to enhance the mobility of MIO TFT due to improvement of the interface with the channel layer. In addition, oxygen vacancies in the MIO channel were suppressed because of oxygen diffusion from the buffer layer. MIO TFT with a 5 nm MgO buffer layer showed an on/off current ratio of 9.68 × 103, a field-effect mobility of 4.81 cm2/V∙s, which was increased more than an order of magnitude compared with the device without a buffer layer, a threshold voltage of 2.01 V, and a subthreshold swing of 0.76 V/decade, which was improved more than 20% compared with the as-deposited one.

Stretching Compensation Pixel Circuit Based on Oxide Thin Film Transistor

Thin film transistors (TFTs) are the essential elements to implement active matrix organic light-emitting diode (AMOLED) display. The TFT on the AMOLED controls the emission of the OLED. Pixel circuits are being studied to improve the display quality. Poly-crystalline silicon TFT and oxide TFT have been manufactured and integrated on glass substrates for AMOLED. Various kinds of display such as foldable, rollable, wearable and stretchable displays are being studied.In stretchable displays, the device and wires manufactured on the substrate are stretched out. The resistances of wire, parasitic capacitances are affected by stretching of the substrate. In addition, when the substrate is stretched, electrical characteristic such as threshold voltage and mobility of TFT would be able to change, which results in the luminance change of the OLED. Therefore, analysis of changes in TFT is required when the substrate is stretched, and compensation circuits are required. In this study, the compensation pixel circuit is proposed for the stretchable display.To make the circuit, IGZO was used as an active layer of the TFT and deposition by RF magnetron sputter. Insulator layer was deposited for the gate insulator using atomic layer deposition (ALD) and inter layer dielectric was formed after doping of the source-drain region in the same way after doping of the source-drain region.

Effects of Tantalum Doping on Electrical Characteristics of Indium-Gallium-Zinc Oxide Thin-Film Transistors

Thin-film transistors (TFTs) that utilize oxide semiconductors as channel materials have been widely used in various electronic device applications, such as displays, sensors, and memory devices. Among the oxide semiconductors, transparent amorphous indium-gallium-zinc oxides (a-IGZO) have superior characteristics, namely, high field-effect mobility (~10cm2/Vs), good uniformity, low processing temperatures, optical transparency, and low-cost fabrication processability compared with conventional amorphous/poly-silicon TFTs. However, oxide TFTs including a-IGZO TFTs have bias stress-induced instabilities, which are still critical issues in launching commercial oxide TFT products. These issues can be attributed to the oxygen vacancy-related defects, which are known to act as a major source of electrons and as carrier traps. To suppress such oxygen vacancy-related defects, many researchers reported various techniques, such as ozone treatment, oxygen annealing on oxide TFTs, passivation, and metal-doped oxide semiconductors.In this study, we investigated the device performance of tantalum (Ta)-doped IGZO TFTs as a function of Ta content. Ta was used as a dopant because of its bond-dissociation-energy (BDE, 805kJ/mol) with oxygen higher than those of In, Ga, and Zn. The Ta-doped IGZO, which was adopted as a channel layer of bottom-gate TFTs, was fabricated using a magnetron co-sputtering method. Heavily doped p-type Si wafer with thermally grown SiO2 (300nm) was used as substrates. The RF power for the a-IGZO (In2O3 :Ga2O3 : ZnO = 1:1:1)target was fixed at 100 W while the power for the Ta target increased from 0 to 50 W. Aluminum (Al) was deposited as source/drain electrodes using an electron beam evaporator. With increasing Ta content in the IGZO films, the field-effect mobility (μFE) of the Ta-doped IGZO TFTs decreased, the threshold voltage (Vth) increased from negative to positive values, and the subthreshold swing (SS) became smaller with higher on/off ratio. Especially, Ta (10W)-doped IGZO TFTs exhibited μFE of 7.11 cm2/Vs, a SS of 0.195V/decade, and an on/off ratio ~1x108. These results show that the Ta-doping improves the switching characteristics of the Ta-IGZO TFTs. Furthermore, to evaluate the bias-stress stability, positive bias stress (PBS), negative bias stress (NBS) tests were conducted.

Extremely Stable, High Performance Gd and Li Alloyed ZnO Thin Film Transistor by Spray Pyrolysis

AbstractThe simultaneous doping effect of Gadolinium (Gd) and Lithium (Li) on zinc oxide (ZnO) thin‐film transistor (TFT) by spray pyrolysis using a ZrOx gate insulator is reported. Li doping in ZnO increases mobility significantly, whereas the presence of Gd improves the stability of the device. The Gd ratio in ZnO is varied from 0% to 20% and the Li ratio from 0% to 10%. The optimized ZnO TFT with codoping of 5% Li and 10% Gd exhibits the linear mobility of 25.87 cm2 V−1 s−1, the subthreshold swing of 204 mV dec−1, on/off current ratio of ≈108, and zero hysteresis voltage. The enhancement of both mobility and stability is due to an increase in grain size by Li incorporation and decrease of defect states by Gd doping. The negligible threshold voltage shift (∆VTH) under gate bias and zero hysteresis are due to the reduced defects in an oxide semiconductor and decreased traps at the LiGdZnO/ZrOx interface. Li doping can balance the reduction of the carrier concentration by Gd doping, which improves the mobility and stability of the ZnO TFT. Therefore, LiGdZnO TFT shows excellent electrical performance with high stability.

Macromodel for AC and Transient Simulations of Organic Thin-Film Transistor Circuits Including Nonquasistatic Effects

A charge-based macromodel for small-signal ac and transient analyses of organic thin-film transistors (TFTs) is presented. Due to the comparatively small charge-carrier mobility in organic TFTs, the dynamic behavior of the gate-field-induced carrier channel is greatly affected by the frequency of the applied gate–source and drain–source voltages. The model presented here is, therefore, based on the transmission-line model and shown to reproduce the results of frequency-dependent admittance measurements and numerical simulations of the transient switching behavior of organic TFTs fabricated in the staggered architecture with good accuracy. The model has been implemented in the hardware description language Verilog-A.

Mobility and threshold voltages comparison of zinc nitride-based thin-film transistor fabricated on Si and glass

The present work reports the fabrication and characterization of high mobility thin-film transistors, where zinc nitride is used as the active layer (∼100 nm thick). For the TFT, the active layer was deposited at room temperature on different substrates (Si-p type and glass) by RF magnetron sputtering method and annealed at 350 °C post-fabrication and HfO2 was used as the gate insulation layer (∼50 nm thick). The obtained value of field-effect mobility was greater than 5 cm2 Vs−1, with optical bandgap ∼3.07 eV. The two MIS (metal insulator semiconductor) structures were analyzed using I–V and C–V measurements. It is demonstrated that Zinc Nitride is a potential candidate to be used as an active layer in TFT fabrication. The threshold voltages of the device built on Si and glass substrates were obtained as 0.8 volts and 2.6 volts respectively.

Substrate-free, stretchable electrolyte gated transistors

Abstract Improving device deformability under mechanical stress is an essential requirement for the realization of next-generation wearable electronic applications. Herein, we report a new concept of stretchable thin-film transistors (TFTs) that can be fabricated without substrate components. Instead of using plastic substrates, we employ an elastic (and also free-standing) solid-state electrolyte as a gate insulator. The strategy not only realizes stretchable TFTs, but also enhances the device performance (e.g. mobility, operation voltage) by the formation of high-capacitance electric double layers. As source-drain electrodes, semi-circular shaped gold (Au) thin-film was employed to enhance the device stretchability. By the combination of a solid electrolyte (gate insulator), metal film (electrodes), and polymer semiconductor (channel layer), the device is stretchable up to around 10%, and shows typical p-channel switching performance with low operation voltage of 1 V and TFT mobility of about 0.81 cm2/V s. The devices were processed by a simple solution process and thermal evaporation under low temperature (

P‐2: Hydrogenated SnO for p‐channel oxide Thin Film Transistor

Subgap defect termination for p‐type Tin monoxide (SnO) is developed with thermal annealing in hydrogen ambient for p‐channel oxide thin‐film transistor (TFT). High‐performance p‐channel oxide TFT with the TFT mobility of ~1.4‐1.8 cm2/Vs and large on‐to‐off current as large as 105 was demonstrated. Hydrogen‐containing annealing also maintains SnO phase stable up to 390 °C and effectively reduces the subgap defect, which offer the improvement in TFT characteristics.

Comproportionation Reaction Synthesis to Realize High‐Performance Water‐Induced Metal‐Oxide Thin‐Film Transistors

AbstractSolution‐processed metal‐oxide thin films have been widely studied in low‐power and flexible electronics. However, the high temperature required to form a condensed and uniform film limits their applications in flexible and low‐cost electronics. Here, a novel and environmental‐friendly comproportionation reaction synthesis (CRS) is presented to obtain amorphous aluminum oxide (AlOx) thin films for solution‐processed thin‐film transistors (TFTs) employing water as the precursor solvent. The thermal decomposition of CRS‐AlOx precursor is completed at ≈300 °C, which is 100 °C lower than that of the conventional water‐induced AlOx. The morphological, optical, compositional, and electrical properties of CRS‐AlOx dielectric films are studied systematically. Meanwhile, TFTs based on water‐induced In2O3 metal oxide semiconductor layers deposited on these dielectrics at low temperatures are formed and characterized. Compared with TFTs based on conventional AlOx showing low mobility and low clockwise hysteresis, In2O3 TFTs based on CRS‐AlOx exhibit improved electrical performance and counterclockwise hysteresis in the transfer curves. Water‐induced TFTs fabricated on CRS‐AlOx formed at a low temperature of 250 °C have average mobility of 98 cm2 V−1 s−1. Through chemical composition characterization and electrical characterization, the high mobilities of TFTs based on CRS‐AlOx dielectrics are correlated to trap states, which resulted in counterclockwise hysteresis in the transfer curves.

Floating Ni Capping for High-Mobility p-Channel SnO Thin-Film Transistors.

We utilized Ni as a floating capping layer in p-channel SnO thin-film transistors (TFTs) to improve their electrical performances. By utilizing the Ni as a floating capping layer, the p-channel SnO TFT showed enhanced mobility as high as 10.5 cm2·V−1·s−1. The increase in mobility was more significant as the length of Ni capping layer increased and the thickness of SnO active layer decreased. The observed phenomenon was possibly attributed to the changed vertical electric field distribution and increased hole concentration in the SnO channel by the floating Ni capping layer. Our experimental results demonstrate that incorporating the floating Ni capping layer on the channel layer is an effective method for increasing the field-effect mobility in p-channel SnO TFTs.

Significant improvement of spray pyrolyzed ZnO thin film by precursor optimization for high mobility thin film transistors

Metal-oxide thin-film transistors (TFT) fabricated by spray pyrolysis are of increasing interest because of its simple process and scalability. A bottleneck issue is to get a bubble-free and dense material. We studied the effect of ammonium acetate (AA) addition in the oxide precursor solution on the performance of spray-coated ZnO TFTs. AA acts as a stabilizer, which increases the solubility of the solution and enhances the film quality by reducing the defects. With AA addition in ZnO precursor, the films are coffee ring free with high mass density and better grain orientation. The ZnO TFT with AA exhibit a remarkable improvement of its device performance such as saturation mobility increasing from 5.12 to 41.53 cm2V−1s−1, the subthreshold swing decreasing from 340 to 162 mV/dec and on/off current ratio increasing from ~105 to 108. Additionally, the TFTs show excellent stability with a low threshold voltage shift of 0.1 V under gate bias stress. Therefore, the addition of AA is a promising approach to achieve high-performance ZnO TFTs for low-cost manufacturing of displays.

The Effect of Anodization Parameters on the Aluminum Oxide Dielectric Layer of Thin-Film Transistors.

Aluminum-oxide (Al2O3) is a low cost, easily processable and high dielectric constant insulating material that is particularly appropriate for use as the dielectric layer of thin-film transistors (TFTs). Growth of aluminum-oxide layers from anodization of metallic aluminum films is greatly advantageous when compared to sophisticated processes such as atomic layer deposition (ALD) or deposition methods that demand relatively high temperatures (above 300 °C) such as aqueous combustion or spray-pyrolysis. However, the electrical properties of the transistors are highly dependent on the presence of defects and localized states at the semiconductor/dielectric interface, which are strongly affected by the manufacturing parameters of the anodized dielectric layer. To determine how several fabrication parameters influence the device performance without performing all possible combination of factors, we used a reduced factorial analysis based on a Plackett-Burman design of experiments (DOE). The choice of this DOE permits the use of only 12 experimental runs of combinations of factors (instead of all 256 possibilities) to obtain the optimized device performance. The ranking of the factors by the effect on device responses such as the TFT mobility is possible by applying analysis of variance (ANOVA) to the obtained results.

Effects of interfacial dielectric layers on the charge transport characteristics in sol-gel based amorphous metal-oxide thin-film transistors

We report the effects of interfacial dielectric layers on the field-effect mobility of thin-film transistors (TFTs) with sol-gel based amorphous indium-gallium-zinc oxide (IGZO) films on thermally-grown silicon dioxide (SiO2) and sol-gel based zirconium dioxide (ZrO2) dielectrics. When the field-effect mobility as a function of the channel carrier concentration was extracted, the SiO2 based IGZO-TFT exhibited higher mobility than the ZrO2 based IGZO-TFT at a low carrier concentration. However, the field-effect mobility of the ZrO2 based IGZO-TFT increased as the gate charges increasing due to the gate bias applied. We also investigate the effects of interfacial dielectric layers on the charge transport characteristics in IGZO-TFTs by conducting temperature-dependent field-effect mobility measurements in the temperature range of 120 – 300 K. By analyzing an Arrhenius plot of the results, we confirm that the charge transport of IGZO-TFTs is dominant by the multiple-trapping-and-release model and discuss the effective mobility, which is significantly affected by the interfacial trap level and channel carrier concentration. Finally, we investigate the figure of merit and the activation energy of IGZO-TFTs with ZrO2 dielectrics fabricated by the optimized solution-process developed in this study through comparisons with other sol-gel based amorphous metal-oxide TFTs reported in the literature.

Solution processed amorphous gallium-incorporated tin oxide thin-film transistors

In this paper, we prepared amorphous gallium-incorporated tin oxide (a-SnGaO) thin-film transistors (TFTs) with respect to various Ga contents and annealing temperatures. All TFTs exhibit excellent switching characteristics. As the gallium content increases, the off-state current of the a-SnGaO TFTs decreases, the threshold voltage (Vth) shifts towards positive voltage, and the subthreshold swing (S.S.) improves. The optimal gallium content is 20 at%, the corresponding saturation mobility of a-SnGaO TFTs is 5.2 cm2 V−1 s−1, the threshold voltage is −2.3 V, and the S.S. is 0.7 V dec.−1. The switching ratio is 1.5 × 106. Furthermore, we investigated the effect of annealing temperature on the electrical properties of a-SnGaO TFTs. It was found that as the annealing temperature increases, the mobility of a-SnGaO TFTs increases and the S.S. improves, which is greatly due to the increase of free electrons and reduction of defect states in the channel layer.

Evidence for Pseudocapacitance and Faradaic Charge Transfer in High-Mobility Thin-Film Transistors with Solution-Processed Oxide Dielectrics.

In developing low-power electronics, low-voltage transistors have been intensively investigated. One of the most important findings is that some high-k oxide gate dielectrics can lead to remarkable enhancement of apparent mobility in thin-film transistors (TFTs), which is not clearly understood. Here, we investigate InOx TFTs with solution-processed AlOx dielectrics. At very low frequencies (<1 Hz), the AlOx films feature strong voltage-dependent capacitance. Also, cyclic voltammograms show clear features of surface-controlled Faradaic charge transfer. The two independent experiments both point to the formation of pseudocapacitance, which is similar to the mechanism behind some supercapacitors. A physical model including charge transfer is established to describe ion distribution. The charge transfer is probably related to residual hydrogens, as revealed by secondary-ion mass spectroscopy. The results provide direct evidence of the formation of pseudocapacitance in TFTs with high apparent mobilities and advance the understanding of mechanisms, measurements, and applications of such TFTs for low-power electronics.

Solution processed In-Si-O thin film transistors on hydrophilic and hydrophobic substrates

Solution processed silicon-doped indium oxide, In-Si-O (ISO), thin-film transistors (TFTs) were fabricated on hydrophilic and hydrophobic substrates. A precursor solution fabricated via dissolving indium chloride and tetraethyl orthosilicate into solvents of acetonitrile and ethylene glycol was spin coated on the hydrophilic substrate. Annealing conditions were optimized, and the highest mobility of the ISO TFT increased to 8.1 cm2/Vs. We used another precursor solution by dissolving indium(III) isopropoxide and tetraethyl orthosilicate into a solvent of 2-(diethylamino)ethanol, and we fabricated ISO TFTs on the hydrophobic substrates. The highest mobility corresponded to 3.4 × 10−1 cm2/Vs.

Transparent Flexible High Mobility TFTs Based on ZnON Semiconductor With Dual Gate Structure

High mobility thin film transistors (TFTs) based on zinc oxynitride (ZnON) semiconductor were fabricated onto polyethylene-2.6-naphthalate (PEN) substrates. The application of a dual gate structure enhanced the field-effect mobility from 65.8 to 147 cm2/ $\text{V}\cdot \text{s}$ , which is generally attributed to the bulk accumulation effect. Dual gate driving also results in improved device stability with respect to bias and illumination stress. This is most likely due to the fact that the bulk channel confines the charge carriers away from the semiconductor-dielectric interfaces, where charge trapping is anticipated. The electrical performance of dual gate devices did not change significantly after 5,000 bending cycles, while the single gate transistors exhibited clear degradation.

Enhanced piezoelectric tactile sensing behaviors of high-density and low-damage CF4-plasma-treated IGZO thin-film transistors coated by P(VDF-TrFE) copolymers

Piezoelectric pressure sensing behaviors of high-density and low-damage CF4-plasma-treated indium gallium zinc oxide (IGZO) thin-film transistors (TFTs) coated by poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) films have been investigated. The CF4 plasma treatment was performed on the IGZO channel by using an inductively coupled plasma (ICP) system with a quartz filter. Prior to the fabrication of devices, X-ray diffractometer (XRD) and atomic force microscopy (AFM) were applied to identify the enhancement in the crystallinity of P(VDF-TrFE) films on the plasma-fluorinated IGZO films. With the analyses of X-ray photoelectron spectroscopy (XPS), it is proved that the fluorine radicals reacted with the metal-oxygen bonds will increase the oxygen vacancies in IGZO films, contributing to an enhancement in field effect mobility (μFE) and drain current (IDS) of IGZO TFTs. Furthermore, the fluorine atoms diffused from the IGZO channel into the bottom SiO2 layer were confirmed by secondary ion-mass spectroscopy (SIMS), reducing the interfacial and oxide charges of the devices for a negative shift in threshold voltage (Vt). Under a 0.5-kg applied force press/release cyclic test, a 4.8-fold increase in drain current response of 1-min CF4-plasma-treated piezoelectric pressure sensors was optimized, because of the enhanced electrical behaviors of IGZO TFTs and an increase in aligned dipole moments of P(VDF-TrFE) copolymers. The promising results make the piezoelectric pressure sensors with high-density and low-damage CF4-plasma-treated IGZO TFTs coated by P(VDF-TrFE) copolymer suitable for future high-performance tactile sensing applications.

Communication—Wide Bandgap Tin Oxide Thin Film Transistor by Doping Rare Earth Element Europium

–Amorphous europium-doped tin oxide (a-SnEuO) thin film transistors (TFTs) with a wide bandgap of 3.87 eV were prepared by sol-gel method. The results indicate that as Eu content increases, the off-state current of the a-SnEuO TFTs decreases, the threshold voltage (Vth) shifts in the positive direction and the subthreshold swing (S.S.) is also improved. When Eu content is 30 at%, the mobility of a-SnEuO TFTs is 4.5 cm2V−1 s−1, with an S.S. of 0.8 V dec−1. and a switching ratio of 9.5 × 106. Our study indicates Eu-doped SnO2 TFT is expected to be the next generation of display technology.

Generalized Gated Four-Probe Method for Intrinsic Mobility Extraction With Van Der Pauw Structure

Measuring the mobility of thin-film transistors fabricated using newly developed semiconductors is difficult because of various factors, such as contact resistance. In this work, the generalized gated four-probe and van der Pauw structure were combined to precisely measure the carrier mobility of semiconductors. The method is geometry-independent, suitable for gate-dependent mobility in all operation regimes, and the test devices could be easily fabricated. Experimental verifications were performed on specially designed InGaZnO devices with different electrodes and at various temperatures. The developed approach enables simple and precise investigation of the semiconductor's properties via easily fabricated devices. In particular, the technique could be used for newly developed semiconductors that can only be patterned via low-resolution techniques, such as shadow mask patterning.