Metal Oxide Thin-film Transistors Research Articles

Energy-band engineering by 2D MXene doping for high-performance homojunction transistors and logic circuits

The homojunction based on Ti3C2Tx MXene-doped In2O3 and indium oxide as the channel layer is realized in high-performance metal oxide thin film transistors (TFTs). Doping of MXene into In2O3 results in n-type semiconductor behavior, realizing tunable work function of In2O3 from 5.11 to 4.79 eV as MXene content increases from 0 to 2 wt.%. MXene-doped In2O3-based homojunction TFT presents optimal performance with electron mobilities of greater than 27.10 cm2/(V s) at 240 °C, far exceeding the maximum mobility of 3.91 cm2/(V s) for single-layer In2O3 TFTs. The improved performance originates from boosting of a two-dimensional electron gas (2DEG) formed at carefully engineered In2O3/MXene-doped In2O3 oxide homojunction interface. Besides, the transformation in conduction mechanism leads to better stability of MXene-doped In2O3 homojunction devices compared to undoped bilayer In2O3. Low-frequency noise further illustrates that doping MXene into In2O3 helps to reduce the device trap density, demonstrating excellent electrical performance. A resistor-loaded unipolar inverter based on In2O3/0.5% MXene-In2O3 TFT has demonstrated full swing characteristics and a high gain of 13. The effective doping of MXene into constructed homojunction TFTs not only contributes to improved stability, but also provides an effective strategy for designing novel homojunction TFTs for low-cost oxide-based electronics.

30.1: Invited Paper: (Invited) Recent Progress in Metal Oxide TFTs using Atomic Layer Deposition

This paper reviews recent approaches in the development of high‐ performance metal‐oxide thin‐film transistors (TFTs) through atomic‐layer deposition (ALD) process. Since metal‐oxide TFTs are contributing as one of the main backplane technology for display products such as organic light‐emitting diode (OLED) televisions, further improvements are aggressively attempted in various approaches. Among them, two mainstream approaches including cation composition ratio variation and novel structural design is introduced. Both approaches are enabled by extensive merits of the ALD process compared to the conventional sputtering process. In this paper, state‐of‐the‐art characteristics of ALD deposited oxide semiconductors are described. Results show promising potential for ALD‐derived metal oxide TFTs as a future candidate for next generation high‐end active‐matrix display devices.

Effects of Sn Doping on the Electrical Performance and Stability of Sub-V Operating Metal-Oxide Thin-Film Transistors Fabricated by Oxygen Annealing

The incorporation of Sn into a sol-gel-based indium-gallium-zinc oxide (IGZO) channel layer is investigated to improve the electrical characteristics and operational stability of amorphous metal-oxide thin-film transistors (oxide-TFTs). Sub-V operating Sn-doped IGZO-TFTs (IGZTO-TFTs) fabricated by solution-process under a high-pressure oxygen-enriched (O2) atmosphere exhibit significantly improved electrical properties such as a high linear field-effect mobility (11.5 cm2 V−1 s−1), high on/off ratio (up to 108), and low subthreshold swing (0.10 V/dec). The activation energy and density of states are determined to perform comparative analysis on the charge transport characteristics of the IGZTO-TFTs. Under both prolonged positive and negative bias illumination stresses, large shifts in threshold voltages of the IGZO-TFTs are reduced significantly after Sn doping. The suppressed formation of oxygen-related defect states affects the electrical characteristics and operational stability. This is verified by determining the proportional ratio of metal-oxygen bonding states in the O2-annealed IGZTO films.

Oxide TFTs with S/D-contacts patterned by high-resolution reverse-offset printed resist layers

Besides the metal oxide thin film transistors (TFTs) in flat-panel displays that are fabricated using vacuum-processes, there is a growing interest in the fabrication of metal oxide TFTs by means of scalable, low-cost solution and printing processes for applications such as flexible displays and biosensors. Although devices with printed semiconductor and gate insulator can exhibit good electrical performance, source/drain-contacts (S/D) printed from silver (Ag) nanoparticles (NPs) typically suffer from deteriorated electrical characteristics and stability problems. On the other hand, metals providing good contacts, such as aluminum (Al), titanium (Ti) and molybdenum (Mo), cannot be formed as air-stable NPs. To overcome these issues, we have developed a patterning method based on high-resolution reverse-offset printing (ROP) of a sacrificial polymer resist layer. ROP delivers patterns with micrometer-level resolution and steep sidewalls, which are ideal for patterning vacuum-deposited metal contacts at high resolution via lift-off process. Solution-processed indium oxide (In2O3) TFTs were successfully fabricated by using ROP lift-off process for patterning of gate and S/D-electrodes using Al. The fabricated In2O3-based TFTs with Al S/D-contacts exhibit good uniformity, constant mobility (μ sat) ∼ 2 cm2 (V s)−1 over a wide range of width/length-ratios (W/L) and almost zero turn-on voltage (V on) ∼ −0.2 V. TFTs down to 5 µm channel lengths were successfully patterned. Further development of the fabrication process could lead to flexible fully-print-patterned high-resolution TFT backplanes for flexible displays, biosensors, photosensors and x-ray detectors.

Editors’ Choice—Thin Film Transistor Response in the THz Range

Novel metal oxide materials such as InGaZnO (IGZO), ZnO, SnO, and In2O3 and improved fabrication processes dramatically enhanced the achieved and projected thin film transistor (TFT) performance. The record values of the effective field-effect mobility of Metal Oxide TFT (MOTFT) materials have approached 150 cm2/Vs. We report on an improved compact TFT model based on three models: the RPI TFT model, the unified charge control model (UCCM), and the multi-segment TFT compact model. This improved model accounts for a non-exponential slope in the subthreshold regime by introducing a varying subthreshold slope and accounts for non-trivial capacitance dependence on the gate bias, and parasitic impedances. The analysis of the TFT response using this model and the analytical calculations showed that TFTs could have a significant response to impinging THz and sub-THz radiation. Using a complementary inverter and the phase-matched THz signal feeding significantly improves the detection sensitivity.

Dependence of the Transfer Characteristics of Metal-Oxide Thin-Film Transistors on the Temperature of an Extended Oxidizing Heat-Treatment

When a metal-oxide (MO) thin-film transistor (TFT), specifically that based on indium-gallium-zinc oxide (IGZO), is subjected to an oxidizing heat-treatment, its transfer characteristic is found to exhibit a parallel shift that depends on the temperature and saturates upon an extended heat-treatment. Presently a model governing the oxidation and reduction kinetics of donor-defects is applied to describe the observed temperature dependence. Quantified by the change in the saturated turn-on voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{{\text {ON}\text {Sa}\text {t}}}$ </tex-math></inline-formula> ), the parallel shift of the transfer characteristic of a TFT after an extended oxidizing heat-treatment in the temperature range from 300 to 450 °C can be precisely predicted by measurable model parameters. These results further illustrate the utility of the model as a tool for developing and optimizing MO TFT technologies by controlling <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathbf {V}_{{\text {ON}\text {Sa}\text {t}}}$ </tex-math></inline-formula> .

A high-reliability scan driver integrated circuit by MO TFTs for a foldable display

Foldable displays have become increasingly used as a solution to the portability-practicality trade-off. A new scan driver integrated with metal oxide thin film transistors (MO TFTs) is proposed for foldable displays to overcome the effects of threshold voltage shifts on the scan driver output. In the scan driver, a feedback section and two series to transistor structures are used to keep the voltage of the key node to sustain a normal output, with this structure, the lower the voltage of the negative supply, the more stable the scan driver will be. Some flexible MO TFTs and 60 stage scan drivers are fabricated by etch stop layer structures on the polyimide substrate. Under different strains and 100 000 times bending with the minimum 3.5 mm bending radius, the characteristics of MO TFTs and the output waveform of the scan driver are measured. Experimental results show that the proposed scan driver shows high reliability since only a little voltage fluctuation occurs at the outputs after the bending test and has a better output waveform with a lower voltage of the negative supply.

Technique for Producing Metal Oxide Thin-Film Transistors at Low Temperatures

This research presents a novel method for manufacturing metal oxide thin-film transistors (TFTs) at low temperatures. The method involves coupling a metal-oxide film semiconductor to a thin film grid dielectric layer. By carefully controlling the combustion of a fuel-oxidant mixture, the desired metal-oxide film semiconductor is formed at temperatures below 350°C. This approach offers a cost-effective and scalable solution for producing high-performance TFTs on flexible plastic substrates.

Fully Sprayed Metal Oxide Transistors Utilizing Ti3C2Tx MXene Contacts

Printable large-area electronics continue to attract increasing interest from the scientific and industrial communities, owing to their potential for cost-efficient manufacturing. However, the exerted efforts primarily focus on processing semiconductors and dielectrics, with relatively limited attention toward printable electrode materials. In this respect, the family of two-dimensional (2D) transition metal carbides/nitrides, so-called MXenes, have emerged as promising electrode materials owing to their metal-like conductivity and solution processability. Despite their advantageous characteristics, however, to date, the application of MXene electrodes in the area of printed electronics has been marginally explored. Herein, we develop all-solution-processable In2O3/ZnO heterojunction metal oxide thin-film transistors (TFTs) utilizing Ti3C2Tx MXene as the gate and the electron-injecting contacts, entirely spray-coated under ambient conditions. We show that the manufacturing of these fully spray-coated devices can be scaled-up to 4 in. substrates without compromising their operating characteristics. The fabricated TFTs exhibit electron mobility of 3 cm2 V–1 s–1 with an on/off ratio of >104. Our findings demonstrate the possibility of replacing conventional metal contacts with spray-coated MXene electrodes for application in large-area electronics.

Detection of acetone vapours using solution-processed tin oxide thin-film transistors

Abnormal concentrations of volatile organic compounds (VOCs) in human breathe can be used as disease-specific biomarkers for the non-invasive diagnosis of medical conditions, such as acetone for diabetes. Solution-processed bottom gate top contact metal oxide thin-film transistors (TFTs) are used to detect acetone vapours, as part of a proof-of-concept study. The effect of increasing annealing temperature (T) and channel length (L) on electrical and sensing performance are explored. Drain current (Ids) increases following exposure as acetone undergoes a redox reaction with the adsorbed oxygen species on the semiconductor surface, which results in free electrons being released back into the conduction band. Responsivity (R) is maximized at negative bias (Vgs < 0). For L = 50 μm, the peak R of the TFT annealed at 450 °C is three times greater than that of the TFT annealed at 350 °C, with Vgs = − 37.5 V and − 33 V, respectively.Graphical abstract

Microwave-Irradiated Metal-Oxide Thin-Film Transistors With Recessed Gate Structure and Their Applications in Logic Circuits

The effects of a recessed gate structure on the device performance of sol-gel-based metal-oxide thin-film transistors (oxide-TFTs) consisting of aluminum oxide dielectric and indium-gallium-zinc oxide semiconducting films fabricated through effective microwave irradiation are investigated, as compared with elevated and non-patterned gate structures. Low-voltage operating oxide-TFTs with an optimized recessed gate structure exhibited improved device performance including OFF-state current of approximately <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{-{11}}$ </tex-math></inline-formula> A and field-effect mobility of 3.4 cm2/V-s in the linear regime, compared with those with elevated (~ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{-{9}}$ </tex-math></inline-formula> A and 1.9 cm2/V-s) and non-patterned (~ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{-{10}}$ </tex-math></inline-formula> A and 2.8 cm2/V-s) gate structures. Furthermore, the charge transport mechanism responsible for improved device performance and operational stability against prolonged positive and negative gate bias stresses in the oxide-TFTs with the recessed gate structure is investigated through activation energy and velocity distribution that are determined from direct-current and time-domain non-quasi-static transient analyses, respectively. Finally, enhancement-load-type nMOS inverters operating at 5 V are developed for logic circuits, which exhibit a relatively high dc gain of 32.2 in the recessed gate structure, compared with that of 17.1 and 27.1 in the elevated and the non-patterned gate structures, respectively.

Enhancement of electrical stability of metal oxide thin-film transistors against various stresses

Metal-oxide semiconductors are considered promising alternative materials in the field of flat panel display industry due to their advantages, such as high mobility, transparency, uniformity, low production cost, and large-area processability.

Artificial Synapse Based on Oxygen Vacancy Migration in Ferroelectric‐Like C‐Axis‐Aligned Crystalline InGaSnO Semiconductor Thin‐Film Transistors for Highly Integrated Neuromorphic Electronics

AbstractArtificial synapses are a key component of neuromorphic computing systems. To achieve high‐performance neuromorphic computing ability, a huge number of artificial synapses should be integrated because the human brain has a huge number of synapses (≈1015). In this study, a coplanar synaptic, thin‐film transistor (TFT) made of c‐axis‐aligned crystalline indium gallium tin oxide (CAAC–IGTO) is developed. The electrical characteristics of the biological synapses such as inhibitory postsynaptic current (IPSC), paired‐pulse depression (PPD), short‐term plasticity (STP), and long‐term plasticity at VDS = 0.1 V, are demonstrated. The measured synaptic behavior can be explained by the migration of positively charged oxygen vacancies (Vo+/Vo++) in the CAAC–IGTO layer. The mechanism of implementing synaptic behavior is completely new, compared to previous reports using electrolytes or ferroelectric gate insulators. The advantage of this device is to use conventional gate insulators such as SiO2 for synaptic behavior. Previous studies use chitosan, Ta2O3, SiO2 nanoparticles , Gd2O3, and HfZrOx for gate insulators, which cannot be used for high integration of synaptic devices. The metal–oxide TFTs, widely used in the display industry, can be applied to the synaptic transistors. Therefore, CAAC–IGTO synaptic TFT can be a good candidate for application as an artificial synapse for highly integrated neuromorphic chips.

Impact of layer thickness on the operating characteristics of In2O3/ZnO heterojunction thin-film transistors

Combining low-dimensional layers of dissimilar metal oxide materials to form a heterojunction structure offers a potent strategy to improve the performance and stability of thin-film transistors (TFTs). Here, we study the impact of channel layer thicknesses on the operating characteristics of In2O3/ZnO heterojunction TFTs prepared via sputtering. The conduction band offset present at the In2O3/ZnO heterointerface affects the device's operating characteristics, as is the thickness of the individual oxide layers. The latter is investigated using a variety of experimental and computational modeling techniques. An average field-effect mobility (μFE) of &amp;gt;50 cm2 V−1 s−1, accompanied by a low threshold voltage and a high on/off ratio (∼108), is achieved using an optimal channel configuration. The high μFE in these TFTs is found to correlate with the presence of a quasi-two-dimensional electron gas at the In2O3/ZnO interface. This work provides important insight into the operating principles of heterojunction metal oxide TFTs, which can aid further developments.

Bilayer Metal Oxide Channel Thin Film Transistor With Flat Interface Based on Smooth Transparent Nanopaper Substrate

With the rapid development of electronic devices, a lot of electronic waste is generated yearly. They posed sereve environmental concerns. Integrating sustainable materials into electronic devices is one of promising solutions to achieve the balance between technological development and ecological protection. Cellulose nanopaper has emerged as a hot sustainable material for thin film transistors (TFT) in the flexible display. However, the performance of nanopaper TFT is generally poor. In this study, a transparent nanopaper with a root-mean-square surface roughness of 3.49 nm is prepared by spin coating cellulose nanofibrils on glass and applied as a substrate for metal oxide TFTs. Bilayer channel (IGZO/Al <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{{2}}\text{O}_{{3}}$ </tex-math></inline-formula> ) TFTs with bottom gate structure are then deposited on transparent nanopaper by physical vapor deposition at room temperature. Consequently, the as-prepared nanopaper TFT not only exhibits a saturation mobility of as high as 21.98 cm2/(V·s), an <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{I}_{\text {on}}/\text{I}_{\text {off}}$ </tex-math></inline-formula> ratio of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${5.07}\times {10} ^{{6}}$ </tex-math></inline-formula> , and a subthreshold swing of 0.75 V/dec, but shows good bias stability. This work brings a step closer to green, sustainable, and inexpensive transparent paper electronics.

Performance Enhancement for Indium-Free Metal Oxide Thin-Film Transistors with Double-Active-Layers by Magnetron Sputtering at Room Temperature.

We prepared an indium-free metal oxide thin-film transistor (TFT) using a double-active-layers structure at room temperature. We changed the growth sequence of Al-doped zinc oxide (AZO) and zinc oxide (ZnO) double-active-layers on Si/SiO2 substrates by magnetron sputtering deposition to regulate the field-effect performance of TFTs. According to the analysis of field-effect properties before and after annealing in different atmospheres, the performance of TFT devices with ZnO/AZO/SiO2/Si double-active-layers was obviously better than that with single AZO or ZnO active layer and inverted AZO/ZnO/SiO2/Si double-active-layers in the device structure. The active layer with higher carrier concentration (AZO in this case) was closer to the dielectric layer, which was more favorable for carrier regulation in TFT devices. In addition, the annealed device had a lower on/off ratio (Ion/Ioff), easier-to-reach on-state, and higher mobility. Furthermore, the performance of the devices annealed under vacuum condition was obviously better than that annealed under air atmosphere. The Ion/Ioff could reach 6.8 × 105 and the threshold voltage was only 2.9 V.

Overall enhancement of electrical properties of indium oxide thin-film transistors by surface doping

Doping is a powerful tool for modulating the charge carrier density in metal oxide thin-film transistors (TFTs). However, one crucial limitation is the disruption of microstructure after doping of crystalline semiconductors, which dramatically decreases the charge carrier mobility. Here, we report on a surface doping method that utilizes organic acceptor molecules to efficiently modulate the carrier concentration without disrupting the lattice of thin indium oxide channel layers. For the first time, we achieve electron transfer from indium oxide channel to the organic layer, instead conventional strategy of organic to metal oxide, to control the intrinsic carrier concentration and enhance the electrical performance of the indium oxide TFTs. Moreover, we use a complementary tool based on spectroscopic techniques to prove the fundamental charge transfer processes between metal oxide semiconductors and organic molecular dopants. The results indicate that surface doping is a simple and effective way to improve the performance of oxide TFTs and suitable for potential applications in large-area electronics. • Indium oxide TFTs were fabricated by solution process method. • Surface doping modulates the carrier concentration of the channel layer. • The electrical performances of the devices were enhanced by surface doping.

Rational design of oxide heterostructure InGaZnO/TiO2 for high-performance thin-film transistors

In this study, hetero-structured and self-passivated oxide thin film transistors (TFTs) were fabricated, wherein the channel comprised a N2O plasma (a-IGZO:N2O) treated a-IGZO front layer and TiO2 ultra-thin back layer. A high field-effect mobility (μFE) of 40.5 cm2/Vs and small sub-threshold swing (SS) of 250 mV/decade upon a SiO2/Si substrate were achieved using a rational design. Moreover, small threshold voltage shifts (ΔVth) of 0.5 (−0.7) and 0.8 (−0.9) V under gate bias and light illumination stress tests were obtained. Using Si3N4/HfO2 gate dielectrics, μFE and SS were enhanced to 53.6 cm2/Vs and 75 mV/decade, respectively. Experimental observations indicate that the electrons transfer from TiO2 to a-IGZO:N2O layer resulted in the accumulation of free carriers near a-IGZO:N2O/TiO2 interfaces. Additionally, an appropriate N2O treatment reduced the oxygen-related defects and bulk/interface trap density, while controlling the carrier density in the bilayer a-IGZO:N2O/TiO2 TFTs. Furthermore, the ultra-thin TiO2 layer acted as a surface passivation layer that reduced the leakage current and protected the channel from ambient influences. The proposed high-performance hetero-structured a-IGZO:N2O/TiO2 TFTs with superior stability are expected to provide a new and effective approach to obtaining high-performance metal oxide TFTs for application in thin film electronics.

High-performance ITO/a-IGZO heterostructure TFTs enabled by thickness-dependent carrier concentration and band alignment manipulation

Utilization of highly conductive metal-oxide (MO) film such as indium-tin-oxide (ITO) in a channel layer has been considered as a promising strategy to realize high-mobility thin-film transistors (TFTs). However, achieving high-mobility is typically restricted by severe negative threshold voltage (Vth) shift and large off-current which are consequences of channel thickness increment. Here, to realize high-mobility MO TFTs with low Vth and off-current level, a heterogeneous ITO/amorphous indium-gallium-zinc-oxide (a-IGZO) channel structure was implemented. In the channel, the ultrathin (4 nm) ITO layer contributes to retain high electron concentration and boost the mobility, while the overlayered a-IGZO layer mitigates Vth shift and off-current increase. The ITO/a-IGZO TFTs optimized via the thickness-dependent carrier concentration of ITO and band alignment manipulation in the bilayer considerably improved the device performance showing saturation field-effect mobility of >61 cm2/V·s (average of 58.2 ± 2 cm2/V·s), subthreshold slope of <120 mV/decade (average of 129 ± 12 mV/decade), and current on/off ratio of >5 × 1010. Various electrical characterization and technological computer-aided design simulation were performed to establish a plausible mechanism explaining enhanced mobility and Vth regulation in the ITO/a-IGZO TFTs. Additionally, systematic stability tests and spectroscopic analysis were carried out to evaluate the operational stability of the device, and it is suggested that Sn ion diffusing from ITO to the heterogeneous interface can be responsible for enhanced stability by reducing the oxygen vacancy defects.

Readily Accessible Metallic Micro-Island Arrays for High-Performance Metal Oxide Thin-Film Transistors.

Thin-film transistors using metal oxide semiconductors are essential in many unconventional electronic devices. Nevertheless, further advances will be necessary to broaden their technological appeal. Here, a new strategy is reported to achieve high-performance solution-processed metal oxide thin-film transistors (MOTFTs) by introducing a metallic micro-island array (M-MIA) on top of the MO back channel, where the MO is a-IGZO (amorphous indium-gallium-zinc-oxide). Here Al-MIAs are fabricated using honeycomb cinnamate cellulose films, created by a scalable breath-figure method, as a shadow mask. For IGZO TFTs, the electron mobility (µe ) increases from ≈3.6 cm2 V-1 s-1 to near 15.6 cm2 V-1 s-1 for optimal Al-MIA dimension/coverage of 1.25 µm/51%. The Al-MIA IGZO TFT performance is superior to that of controls using compact/planar Al layers (Al-PL TFTs) and Au-MIAs with the same channel coverage. Kelvin probe force microscopy and technology computer-aided design simulations reveal that charge transfer occurs between the Al and the IGZO channel which is optimized for specific Al-MIA dimensions/surface channel coverages. Furthermore, such Al-MIA IGZO TFTs with a high-k fluoride-doped alumina dielectric exhibit a maximum µe of >50.2 cm2 V-1 s-1 . This is the first demonstration of a micro-structured MO semiconductor heterojunction with submicrometer resolution metallic arrays for enhanced transistor performance and broad applicability to other devices.