Characteristics Of Thin-film Transistors Research Articles

Static and Radio-Frequency Characteristics of Green-Nanoseconds Laser-Crystallized Poly-Si Thin-Film Transistors

In this study, we investigate the static and radio-frequency characteristics of poly-Si thin-film transistors (TFTs) with the channels treated by using either green-nanoseconds laser crystallization (GLC) or solid phase crystallization (SPC) processes. The influences of the crystallization schemes on the granular structures of the resulted poly-Si as well as the device characteristics were studied and compared. Scanning Electron Microscope observations indicate that mean grain size of the 50 nm-thick poly-Si films is approximate 70 nm following the SPC process, whereas GLC process enlarges the mean grain size to 0.5 μm. As a consequence, GLC poly-Si TFTs exhibit greatly improved electron mobility and transconductance as compared to the SPC ones. Moreover, superior high-frequency characteristics with cut-off frequency (f T ) and maximum oscillation frequency (f max) of 21.1 GHz and 24.1 GHz, respectively, are recorded in GLC devices with channel length of 0.26 μm.

Enhancement of Photodetective Properties on Multilayered MoS2 Thin Film Transistors via Self-Assembled Poly-L-Lysine Treatment and Their Potential Application in Optical Sensors.

Photodetectors and display backplane transistors based on molybdenum disulfide (MoS2) have been regarded as promising topics. However, most studies have focused on the improvement in the performances of the MoS2 photodetector itself or emerging applications. In this study, to suggest a better insight into the photodetector performances of MoS2 thin film transistors (TFTs), as photosensors for possible integrated system, we performed a comparative study on the photoresponse of MoS2 and hydrogenated amorphous silicon (a-Si:H) TFTs. As a result, in the various wavelengths and optical power ranges, MoS2 TFTs exhibit 2~4 orders larger photo responsivities and detectivities. The overall quantitative comparison of photoresponse in single device and inverters confirms a much better performance by the MoS2 photodetectors. Furthermore, as a strategy to improve the field effect mobility and photoresponse of the MoS2 TFTs, molecular doping via poly-L-lysine (PLL) treatment was applied to the MoS2 TFTs. Transfer and output characteristics of the MoS2 TFTs clearly show improved photocurrent generation under a wide range of illuminations (740~365 nm). These results provide useful insights for considering MoS2 as a next-generation photodetector in flat panel displays and makes it more attractive due to the fact of its potential as a high-performance photodetector enabled by a novel doping technique.

Implementation of High‐Performance Solution‐Processed Amorphous Indium–Gallium–Zinc–Oxide Thin‐Film Transistors with Low Charge Traps by Microwave Heat Treatment of Low Thermal Budget

Herein, high‐performance solution‐processed amorphous indium–gallium–zinc–oxide (a‐IGZO) thin‐film transistors (TFTs) with low trap densities due to post‐deposition annealing (PDA) are processed with a low‐thermal‐budget microwave (MW) heat treatment. To verify the MW effectiveness, the composition ratio of a‐IGZO thin films and the electrical characteristics of TFTs prepared by conventional thermal annealing (CTA) are compared. An X‐ray photoelectron spectroscopy (XPS) analysis reveals that MW annealing (MWA) improves the film quality more effectively than CTA. a‐IGZO TFTs treated by MWA or CTA are fabricated to evaluate their electrical characteristics. MWA is more effective than CTA in improving the performance, such as hysteresis, subthreshold swing (SS), field effect mobility (μ FE), and on/off current ratio (I on/I off). MWA provides lower interfacial trap density (D it) and volume trap density (N t) than CTA. To evaluate instability, the threshold voltage (V TH) shift is monitored using positive and negative gate‐bias stress tests. MWA demonstrates better reliability than CTA. In conclusion, high‐performance solution‐based a‐IGZO TFTs can be implemented by lowering the charge traps in the a‐IGZO channel using MWA.

Atomic Layer Deposition of Ternary Indium/Tin/Aluminum Oxide Thin Films, Their Characterization and Transistor Performance under Illumination.

Multilayered heterostructures comprising of In2O3, SnO2, and Al2O3 were studied for their application in thin‐film transistors (TFT). The compositional influence of tin oxide on the properties of the thin‐film, as well as on the TFT characteristics is investigated. The heterostructures are fabricated by atomic layer deposition (ALD) at 200 °C, employing trimethylindium (TMI), tetrakis(dimethylamino)tin (TDMASn), trimethylaluminum (TMA), and water as precursors. After post‐deposition annealing at 400 °C the thin‐films are found to be amorphous, however, they show a discrete layer structure of the individual oxides of uniform film thickness and high optical transparency in the visible region. Incorporation of only two monolayers of Al2O3 in the active semiconducting layer the formation of oxygen vacancies can be effectively suppressed, resulting in an improved semiconducting and switching behavior. The heterostacks comprising of In2O3/SnO2/Al2O3 are incorporated into TFT devices, exhibiting a saturation field‐effect mobility (μsat) of 2.0 cm2 ⋅ V−1 s−1, a threshold‐voltage (Vth) of 8.6 V, a high current on/off ratio (IOn/IOff) of 1.0×107, and a subthreshold swing (SS) of 485 mV ⋅ dec−1. The stability of the TFT under illumination is also altered to a significant extent. A change in the transfer characteristic towards conductive behavior is evident when illuminated with light of an energy of 3.1 eV (400 nm).

Enhanced thin-film transistor driven high-aperture in-plane switching liquid crystal displays without common line and black matrix

We developed active-matrix in-plane switching liquid crystal displays (IPS-LCDs) with a new vertical structure composed of thin-film transistors (TFTs) that have an aperture ratio of 60% to reduce energy consumption. The novel TFT has a channel and a back channel made of a hydrogenated amorphous-silicon semiconductor layer sandwiched by thin silicon oxide insulating layers. The transfer characteristics are enhanced by uniformly shifting the threshold voltage to be higher than the maximum LC driving voltage (typically > 5 V). The enhanced TFT characteristics provided with a new driving scheme and shielding electrodes enables both the common line and black matrix to be eliminated. We fabricated an IPS TFT-LCD panel with aperture and contrast ratios that are 160% those of the conventional pixel structure.

Compensating Nonuniform OLED Pixel Brightness in a Vertical Blanking Interval by Learning TFT Characteristics

Changes in thin-film transistor (TFT) characteristics can cause nonuniform brightness in organic light-emitting diode (OLED) displays, and external compensation circuits cannot address such changes if they occur rapidly. Our system acquires a short segment of the source voltage curve of the TFT driving each pixel, during the vertical blanking interval. It then estimates the threshold voltage ${V}_{\text {th}}$ and the mobility-related parameter ${k}$ of that TFT, using neural networks and recurrent conditional (RC) learning to perform expectation–maximization (EM). Finally, it corrects the pixel brightness accordingly. The average errors in the estimated ${V}_{\text {th}}$ and ${k}$ are, respectively, 7.3 mV and ${3.7} \times {10}^{-{11}}\,\,\text {F}/\text {V}\text {s}$ , and we demonstrate the visual effect of compensating with these estimates.

Effect of NBTI on the Electrical Characteristics and Trap States of Low Temperature Poly-silicon Thin Film Transistor

Polysilicon thin film transistors have broad application prospects in high-performance flat panel displays and flexible wearable devices due to their high carrier mobility and good stability. However, due to process conditions and other reasons, there are traps inside, and under some electrical stress conditions, its electrical properties will degrade. Here we report the changes in electrical characteristics of p-type polysilicon thin film transistors with stress time at a negative gate voltage of -20V and a temperature of 60°C. It is obtained that the transfer characteristics of the p-type polysilicon thin film transistor move in the negative direction with the increase of Negative Bias Temperature Instability time, and with the increase of SS degradation, and the total trap state inside it is calculated by SS, which provides a theoretical basis for improving the reliability of the device.

Remote Doping Effects of Indium-Gallium-Zinc Oxide Thin-Film Transistors by Silane-Based Self-Assembled Monolayers.

Oxide thin-film transistors (TFTs), including indium–gallium–zinc oxide (IGZO) TFTs, have been widely investigated because of their excellent properties, such as compatibility with flexible substrates, high carrier mobility, and easy-to-fabricate TFT processes. However, to increase the use of oxide semiconductors in electronic products, an effective doping method that can control the electrical characteristics of oxide TFTs is required. Here, we comprehensively investigate the effect of silane-based self-assembled monolayer (SAM) doping on IGZO TFTs. Instead of a complex doping process, the electrical performance can be enhanced by anchoring silane-based SAMs on the IGZO surface. Furthermore, differences in the doping effect based on the structure of SAMs were analyzed; the analysis offers a systematic guideline for effective electrical characteristic control in IGZO TFTs.

Observation of Hydrogen-Related Defect in Subgap Density of States and Its Effects Under Positive Bias Stress in Amorphous InGaZnO TFT

In amorphous oxide semiconductors, hydrogen-related defects have been unable to be observed from the electrical characteristics of thin-film transistors (TFTs) in comparison to oxygen-related defects. Here, the hydrogen-related defect is observed as the Gaussian distributed density of states (DoS) in ${E}_{\text {C}}-{0.6}$ eV level by using the photoresponse of C-V characteristics in In-Ga-Zn-O (IGZO) TFTs. Coupled with X-ray photoelectron spectroscopy (XPS), it is identified as hydroxide ion (OH−), which both passivates oxygen vacancy defects and supplies the carriers as shallow donors. Furthermore, the evolution of DoS under positive bias stress and its origin are comparatively investigated for IGZO TFTs with three types of gate insulators (GIs), such as low-temperature atomic layered deposited Al2O3, thermally grown SiO2, and double-layered GIs.

Optimizing the plasma oxidation of aluminum gate electrodes for ultrathin gate oxides in organic transistors

A critical requirement for the application of organic thin-film transistors (TFTs) in mobile or wearable applications is low-voltage operation, which can be achieved by employing ultrathin, high-capacitance gate dielectrics. One option is a hybrid dielectric composed of a thin film of aluminum oxide and a molecular self-assembled monolayer in which the aluminum oxide is formed by exposure of the surface of the aluminum gate electrode to a radio-frequency-generated oxygen plasma. This work investigates how the properties of such dielectrics are affected by the plasma power and the duration of the plasma exposure. For various combinations of plasma power and duration, the thickness and the capacitance of the dielectrics, the leakage-current density through the dielectrics, and the current–voltage characteristics of organic TFTs in which these dielectrics serve as the gate insulator have been evaluated. The influence of the plasma parameters on the surface properties of the dielectrics, the thin-film morphology of the vacuum-deposited organic-semiconductor films, and the resulting TFT characteristics has also been investigated.

The effect of substrate curvature on capacitance and current–voltage characteristics in thin-film transistors on flexible substrates

Mechanically flexible electronics are devices designed to operate under significant physical deformations such as bending, twisting, and stretching. While the materials systems and devices compatible with flexible substrates have been extensively studied, the mathematical framework for analysis remains identical to that of traditional planar silicon-based electronics. However, the non-planar and dynamic form factors desired from flexible electronics invalidate assumptions made in these models. For electronic devices to be predictable and ultimately commercially viable, they must be understood in any physical form. Here we employ the method of moments to calculate the capacitance between two electrical conductors of arbitrary shape. Combined with a model for source–drain current in thin-film transistors (TFTs) on the surface of a cylinder, we are able to calculate the current–voltage characteristics in curved TFTs as a function of bending angle. We demonstrate how deformations to device geometry are expected to lead to non-negligible changes in current–voltage characteristics. This work represents the first step towards a new framework for understanding and characterizing electronics with any physical form factor, ultimately bringing flexible electronics closer to commercial viability.

Influence of adsorbed oxygen concentration on characteristics of carbon-doped indium oxide thin-film transistors under bias stress

To clarify a factor on the reliability, we investigated the characteristics of carbon-doped indium oxide (InO1.16C0.04) thin-film transistors by varying the O2 concentration from 0.001% to 100% at atmospheric pressure under negative bias stress (NBS) and positive bias stress (PBS). A positive threshold voltage (V th) shift was observed when the bias stress was changed from NBS to PBS. The positive V th shift increased with increasing bias voltage irrespective of the O2 concentration. This behavior was attributed to the reaction between adsorbed O2 molecules on the back side of the InO1.16C0.04 channel and the electrons in the channel being strongly enhanced under PBS. We have found the magnitude and direction of the bias stress play an important role in the positive V th shift under environments included O2 at concentrations as low as 0.001%.

Improvement of device characteristics of plasma-treated indium gallium zinc oxide thin-film transistors through thermal annealing

In this study, an Ar/O2 plasma mixture treatment with different proportions of O2 was used to reduce the oxygen vacancy density in an amorphous indium gallium zinc oxide (a-IGZO) thin film. The objective was to enhance the field-effect carrier mobility in a thin-film transistor (TFT) with the IGZO film as the channel layer. Atomic force microscopy revealed that the roughness of the IGZO film after plasma treatment was higher than that of the untreated film; however, the surface roughness of the IGZO film decreased after the proportion of O2 was increased in the plasma. The Hall measurement results showed that the resistivity of the plasma-treated IGZO film increased with a decrease in the electron concentration in the film; in addition, the carrier mobility considerably increased. The IGZO TFT fabricated from this film exhibited a high field-effect carrier mobility of 36 cm2 V−1 s−1, a subthreshold swing (SS) of 1.25 V/decade, an I OFF current of 4.58 × 10−11 A, and an I ON/I OFF current ratio of 7.55 × 105. To further improve the device performance, the plasma-treated IGZO films were subjected to thermal annealing with the annealing temperature ranging from 100 °C to 300 °C. After the annealing process, the plasma-treated IGZO TFTs demonstrated a further improvement in the device performance with a field-effect carrier mobility of 38.8 cm2 V−1 s−1, SS of 0.7 V/decade, I OFF current of 1.04 × 10−11 A, and an I ON/I OFF current ratio of 9.93 × 106. In addition, a reliability test was performed to evaluate the stability of the IGZO TFT devices, which revealed that the threshold voltage maintained a high degree of stability during the long-term tests. Therefore, the plasma-treated IGZO TFTs with subsequent postgrowth annealing could be helpful for the fabrication of next-generation flat-panel displays.

Recent Developments of Flexible InGaZnO Thin‐Film Transistors

Flexible InGaZnO thin‐film transistors (TFTs) have been extensively investigated over the last decade with an aim to transferring electronic devices from rigid substrates to light‐weight, soft and flexible ones. Firstly, an introduction to flexible InGaZnO TFT is provided, where the superiority over its rigid counterparts is illustrated. Then, the TFT structures are exhibited with their primary film layers, and the material choice and process selection are presented for each layer to explain the fabrication of flexible InGaZnO TFTs with high performance. Afterward, the recent advances on the electrical performance of the flexible InGaZnO TFTs achieved by either material optimization or structure innovation are summarized, and their operating principles and improvement mechanisms are clarified. Next, the recent progresses on the mechanical flexibility of flexible InGaZnO TFTs are presented according to improvement methods. All these improvements enable the flexible InGaZnO TFTs to endure smaller bending radius and more bending cycles even under electrical and illumination stresses. In particular, the mechanisms concerning mechanical bending are demonstrated in detail, which successfully explain the bending‐induced instability in electrical characteristics of flexible InGaZnO TFTs in numerous studies. Finally, the challenges in this area are summarized as guidance for future research.

Comparison of characteristics of thin-film transistor with In2O3 and carbon-doped In2O3 channels by atomic layer deposition and post-metallization annealing in O3

Characteristics of thin-film transistors (TFTs) with amorphous In2O3 (InO1.2) and carbon-doped In2O3 (InO1.16C0.04) channels by post-metallization annealing (PMA) process were investigated. The InO1.2 TFT changed from metallic to switching behavior after PMA at 200 °C. In contrast, the InO1.16C0.04 TFT exhibited superior properties such as a threshold voltage (V th) of 3.2 V and a high mobility of 20.4 cm2 V−1 s−1 at PMA 150 °C because of the reduction of excess oxygen vacancies. A large negative V th shift was observed for the InO1.2 TFT for 10 800 s in N2 under zero bias voltage while there was no V th change for the InO1.16C0.04 TFT.

Tuning the electrical performance of solution-processed In2O3TFTs by low-temperature with HfO2-PVP hybrid dielectric

Here in, we investigated the solution-processed and hysteresis-free indium oxide (In2O3) thin-film transistors (TFTs) fabricated with hafnium oxide-poly(vinyl)phenol (HfO2-PVP) hybrid thin film as gate dielectric by a low temperature sol-gel method. The hybrid dielectric thin film exhibits unique dielectric properties of a low leakage current density of 1.2 × 10−8 A/cm2, gate areal capacitance of 44.4 nF/cm2 and a dielectric constant (k) of 6.5 at 1 kHz. In addition, the hybrid films show a high-quality homogeneous and pin-hole free surface with a low surface roughness (Rq) of 0.75-nm and display a low surface energy of 36.7 mJ/m2 with hydrophobic behavior. This dielectric material is then used used in In2O3 TFTs as the gate insulator. Here, the In2O3 semiconductor as the channel layer is examined at 200 °C and 230 °C temperatures for TFT characteristics. The final TFT device fabricated at 230 °C showed much improved electrical performance with the mobility (μsat) of 2.6 cm2/V.s, Ion/Ioff ratio of 105, subthreshold swing (SS) of 330 mV/dec, threshold voltage (VT) of 0.1 V at a low operating voltages because of a better interface between dielectric and semiconductor with fewer charge carriers traps. This study could be an effective approach for the next generation of all-solution fabrication of TFTs that could play a vital role in optoelectronics applications.

Effect of PECVD Gate SiO2 Thickness on the Poly-Si/SiO2 Interface in Low-Temperature Polycrystalline Silicon TFTs

This study investigates the effect of the gate SiO2 thickness (80, 100, and 130 nm) deposited by plasma enhanced chemical vapor deposition on the interface and reliability characteristics of low-temperature polycrystalline silicon thin film transistors. Field effect mobility is significantly degraded as the gate oxide thickness decreases. The border trap density (Nbt) extracted from capacitance–voltage hysteresis exhibits no trend with respect to the gate oxide thickness, indicating that field effect mobility is not governed by Nbt. The quantitative interface trap density (Nit) was obtained using a 3-terminal charge pumping method; results showed that Nit decreased as the gate oxide thickness increased. However, it was observed that the threshold voltage (Vth) shift during negative bias temperature stress is worse in the thicker SiO2 film, which has a low Nit. After activation annealing, the amount of hydrogen in the gate oxide increased as the thickness of the insulator was raised. This in turn caused a larger shift in Vth. To validate this mechanism, the amount of hydrogen with respect to the device depth was analyzed via secondary ion mass spectroscopy. It has been found that the presence of more hydrogen concentration in the SiO2 film and the interface to the thicker SiO2 results in more Vth shifts under bias temperature stress.

Characteristics of a Multiple-Layered Graphene Oxide Memory Thin Film Transistor with Gold Nanoparticle Embedded as Charging Elements

In this study, we report the highest mobility in the reduced graphene oxide- (rGO-) based TFTs embedded with Au NPs. In addition, we fabricated a reduced graphene oxide memory device (rGO-capacitor), a reduced graphene oxide thin film transistor (rGO-TFT), and a reduced graphene oxide memory thin film transistor (rGO-MTFT) and characterized their electrical performances. While the rGO-TFT device was investigated for nonambipolar channel performance, the rGO-capacitor and rGO-MTFT were examined for nonvolatile memory capabilities in a metal-graphene-insulator-silicon (MGIS) structure. The incorporation of the gold nanoparticles (Au NPs) between the rGO and an insulator silicon dioxide (SiO2) layer served as a charging element. The rGO-capacitor revealed the memory effect of hysteretic capacitance-voltage (C-V) loops, and the flat-band voltage shift ( Δ V FB ) was measured as 0.1375 V after 100 s retention time. The rGO-TFT shows the p-channel characteristics with high hole mobility of 16.479 cm2/V⋅s. The threshold voltage shift ( Δ V th ) of the rGO-MTFT was detected as 5.74 V from 10 V to -30 V sweep, demonstrating high mobility of 22.887 cm2/V⋅s.

Effect of Hydrogen on Long-Term Reliability of InZnO TFTs Characterized by Low-Frequency Noise

The long-term reliability of InZnO (IZO) thin film transistors (TFTs) under a hydrogen-containing environment is researched. Hydrogen incorporation induces hydroxyl groups and oxygen vacancies, leading to the generation of electrons and extra random trap/emission processes in the IZO films. Consequently, the electrical properties and low frequency noise characteristics of IZO TFTs are deteriorated under the long-term hydrogen treatment. Additionally, the recovery characteristics of IZO TFTs after the hydrogen treatment demonstrate that the hydrogen induced hydroxyl groups and oxygen vacancies remain stable at room temperature. The oxygen vacancies can be eliminated, while the hydroxyl groups may still exist at a high temperature. These results may provide useful guidelines in the design and application of IZO TFTs.

Analysis of low frequency noise characteristics of praseodymium doped indium gallium oxide thin film transistor

Metal oxide thin film transistors (MOTFTs) have been extensively investigated in the display industry because of their attractive characteristics, including high performances, low processing temperatures, and simple fabrication. However, under the actual working condition, the characteristics of TFTs are easily affected by the light irradiation caused the negative gate bias stress (NBIS). Therefore, the NBIS stability of MOTFT is a crucial issue that must be solved before their commercialization into an optoelectronic device. In this article, praseodymium-doped indium gallium oxide (PrIGO) is employed as the channel layer of thin film transistor (TFT). The TFTs with Pr doping exhibit a remarkable enhancement in NBIS stability. The structure and chemical composition of PrIGO film are analyzed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. Besides, to further explore the mechanism for the improvement of NBIS stability, the low-frequency noise characteristics of IGO-TFT device and PrIGO-TFT device are studied. According to the low frequency noise characterization and analysis results, the correspondence between the normalized drain current noise power spectral density(<inline-formula><tex-math id="Z-20210823100314-1">\begin{document}$ S_{\rm ID}/I_{\rm DS}^2 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210368_Z-20210823100314-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210368_Z-20210823100314-1.png"/></alternatives></inline-formula>) and frequency shows 1/<i>f<sup>γ</sup></i> (<i>γ</i> ≈ 0.8) low frequency noise behavior for IGO-TFT device and PrIGO-TFT device. In addition, by studying the influences of different channel lengths on the low frequency noise of the IGO-TFT and PrIGO-TFT devices, it can be concluded that the low frequency noise of the device comes mainly from the channel region rather than from the source/drain contact region. In the linear region of the IGO-TFT device and PrIGO-TFT device, according to the linear fitting of the <inline-formula><tex-math id="Z-20210823100337-1">\begin{document}$ S_{\rm ID}/I_{\rm DS}^2 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210368_Z-20210823100337-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20210368_Z-20210823100337-1.png"/></alternatives></inline-formula> versus the overdrive voltage (<i>V</i><sub>GS </sub>– <i>V</i><sub>th</sub>), it is proved that the low frequency noise of the IGO-TFT device and the PrIGO-TFT device are mainly affected by the carrier number fluctuation model. Finally, based on the carrier number fluctuation model, the defect state density at the interface between active layer and gate insulating layer is extracted to be 7.76 × 10<sup>17</sup> cm<sup>–3</sup>·eV<sup>–1</sup> and 9.55 × 10<sup>17</sup> cm<sup>–3</sup>·eV<sup>–1</sup> for IGO-TFT and PrIGO-TFT devices, respectively. We speculate that the Pr element can induce defect states in the IGO system, and the trap states induced by Pr ions facilitate the capture of free electrons by positively charged oxygen vacancies, which lead the photo-induced carrier in conduction band to be suppressed.