Pixel Switching Research Articles

Crosstalk‐Free Mesa‐Etched Passive‐Matrix Microscale Light‐Emitting Diode Array Using Yttrium‐Iron Garnet P‐Type Resistive‐Switching Electrodes (Advanced Optical Materials 3/2023)

Conventional passive-matrix microscale light-emitting diode (PM-μLED) arrays suffer from electrical crosstalk between adjacent pixels. In article number 2202160, Tae Geun Kim and co-workers demonstrate crosstalk-free PM-μLED arrays using yttrium iron garnet-based p-type resistive-switching (RS) electrodes with high on/off ratio (>106) and good retention (105 s). On/off switching of the μLED pixels is realized through precise control of conductive paths in the RS layers by generating and annihilating oxygen vacancies. This work provides insights for implementing a compact driving circuit by removing the space required for pixel isolation.

The rise of electrochromics through dynamic QR codes and grayscale images in screen printed passive matrix addressed displays

Electronic matrix addressed displays capable of presenting arbitrary grayscale images typically require complex device architectures including switching components to provide unique pixel addressability. Here, we demonstrate high-yield manufacturing of passive matrix addressed electrochromic displays on flexible substrates by solely using screen printing. The simple pixel architecture, obtained by printing only three active layers on top of each other, concurrently provides both the electrochromic functionality and the critical non-linear pixel switching response that enables presentation of arbitrary grayscale images in the resulting passive matrix addressed displays. The all-printed display technology exhibits unprecedented performance and is further verified by dynamic QR codes, to exemplify utilization within authentication, packaging, or other emerging Internet of Things applications requiring a low-cost display for data visualization.

Thin-Film Transistors

Thin film transistors (TFTs) are key components used in a variety of fields such as solar cell, active-matrix liquid crystal displays (AM-LCDs), pixel switches, peripheral driver circuit and flexible electronics [...]

Recent progress of oxide-TFT-based inverter technology

Oxide semiconductor-based thin-film transistor (oxide-TFT) technology have gained significant attention since the innovation of n-channel oxide-TFT using ZnO and amorphous In–Ga–Zn–O (a-IGZO) channels because of their superior device properties including excellent electrical property such as high TFT mobility over >10 cm2/Vs and the low-cost manufacturability originating from the low-temperature processability. Already, n-channel a-IGZO-TFT is widely employed as a TFT pixel switching backplane for several high-performance active-matrix flat-panel displays, and oxide-TFT technology to date is well acknowledged as the best TFT technology for future device applications in the wide range area of electronics such as sensors, Internet of things (IoT), energy-harvesting, medical/bio-interface device, etc. Therefore, the development of large-scale circuit beyond discrete TFT device level becomes increasingly important and is vital to advance the next stage of oxide-TFT technology. In particular, developing oxide-TFT-based inverter device technology is the key for developing several digital and analog circuits. In this paper, the recent progress and challenges in oxide-TFT-based inverter technology, including unipolar NMOS, PMOS, CMOS, and CMOS-like inverters using ambipolar oxide-TFT, are reviewed.

Design Method of Equivalent Driving Waveform Based on Electrowetting Response Characteristics

As a new reflective display technology, electrowetting displays (EWDs) have many important characteristics, such as high reflectivity, low power consumption, and paper-like display. However, the contact angle hysteresis, which is the inconsistency between the advancing contact angle and the receding contact angle of oil droplet movement, seriously affects the response speed of EWDs in the driving process. According to the hysteresis phenomenon of contact angle in an oil switch motion with the action of interface tension, the brightness curve of EWDs in the process of pixel switching by different driving voltages was tested in this paper, and driving voltage was changed from 30 to 100 V at the same time. Then, in order to reduce the influence of the hysteresis effect, an equivalent driving waveform design method with overdriving voltage was proposed, and the overvoltage was set to 100 V according to the hysteresis effect and driving characteristic of EWDs. Experimental results showed that the response rising time of EWDs was reduced to 21 ms by using the proposed driving waveform, and the response performance of EWDs can be effectively improved.

Mechanisms of Carrier Density Increase of Ternary Cation-Based Amorphous Oxide Semiconductors for Thin Film Transistor Applications

Amorphous oxide semiconductor (AOS) thin film transistors (TFTs) based on In2O3 have attracted much interest for use as pixel switching elements in next generation active-matrix liquid crystal (AM-LCD) and active-matrix organic light emitting diode (AM-OLED) displays. The high field effect mobility of In2O3-based AOS devices (10–25 cm2/Vsec) offers significant performance improvements over present-day a-Si TFTs (<1 cm2/Vsec) technology. Additional advantages of AOS materials include low temperature processing (RT–300 °C), isotropic wet etch characteristics, and high optical transparency (>85 % in the visible regime) all of which make this material suitable for large area, flexible, and transparent devices on inexpensive polymer substrates.There have been considerable efforts to incorporate AOSs into devices, particularly in current-driven active matrix displays such as organic light emitting diode displays as pixel-driving switching elements. Strategies to enhance amorphous phase stability, reduce bias stress-induced threshold voltage shifts, and suppress channel carrier densities have been studied and successfully applied to the switching TFT application. Third cation elements are often added to binary cation oxide systems to limit the channel carrier generation for TFT channel application. We have recently reported that the addition of Al to InZnO, a typical binary cation material system leads to enhanced amorphous phase stability, carrier suppression capability and higher carrier mobility, up to ~45 cm2/Vs (Hall Effect mobility) and ~20 cm2/Vs (field effect mobility)1. All of these characteristics are expected to be a key enabler for realizing the next generation ultra-high-definition displays.Post-process annealing is widely employed in AOS-based TFT fabrication since annealing has been shown to improve field effect mobility2-3 and channel/metallization contacts4-5 as well as reduce trap density6, which often leads to unstable device performance or unfavorable hysteresis in their transistor characterstics6-7. However, the post-annealing is often accompanied by an increase in channel carrier density that induces an unfavorable increase in the device off-state current and operation voltages2, 8. To date, the origin of the increase in channel carrier density has not been fully understood.The current study aims to identify the origin of an increase in carrier density after low-temperature annealing conducted in air, particularly for a third-cation AOS system of InAlZnO (IAZO). Through work function investigations and bandgap analysis, the carrier density of IAZO is found to be increased by > 104 times compared to that of unannealed IAZO after low temperature annealing at 200 °C in air. Photoelectron spectroscopic studies reveal that the typical intrinsic (vacancy-based native defect) or extrinsic (cation substitution) doping mechanisms are not the primary cause of the channel carrier increase. From high pressure oxidation with much enhanced reactivity of reaction gases, it is identified that the equilibrium carrier density of IAZO is much higher than those used in typical TFT channel application. The low channel carrier density tends to increase and reach the higher equilibrium carrier density in the absence of kinetic constraints. The combinatorial investigations presented herein help understand the origin of unintentional increase in channel carrier density in amorphous oxides and its effect on the operation of TFTs.The authors gratefully acknowledge the financial supports of the U.S. NSF Award No. ECCS-1931088; the Purdue Research Foundation (Grant No. 60000029); and the Improvement of Measurement Standards and Technology for Mechanical Metrology (Grant No. 20011028) by KRISS.References Reed et al. Journal of Materials Chemistry C 2020, 8 (39), 13798-13810.Nomura et al. Applied Physics Letters 2009, 95 (1), 013502.Lee et al. Thin Solid Films 2012, 520 (10), 3764-3768.Shimura et al. Thin Solid Films 2008, 516 (17), 5899-5902.Lee et al. Journal of Applied Physics 2011, 109 (6), 063702.Ide et al. physica status solidi (a) 2019, 216 (5), 1800372.Liu et al. Journal of the American Chemical Society 2010, 132 (34), 11934-11942.Lee et al. Applied Physics Letters 2014, 104 (25), 252103.

Electrochemistry on Inverse Copper Nanoantennas: Active Plasmonic Devices with Extraordinarily Large Resonance Shift

Arrays of metallic nanoparticles can be used for plasmonic color printing. However, the development of dynamic plasmonic color displays capable of controllable and reversible switching of individual pixels is still in its infancy. Here, an active plasmonic device that operates at the border between the visible and the near-infrared spectral region using inverse copper nanoantennas is introduced. This is a suitable choice, as copper can easily and reversibly be oxidized and reduced with the method of cyclic voltammetry. The inverse sample setup allows for the easy application of an electrical contact. With this configuration, a centroid wavelength shift of up to 210 nm (Δλ/λ ≈ 25%) within less than 3 min can be achieved, which is extraordinarily large. The resonance shift can be reversed, and the response increases with every voltage cycle, which is attributed to structural changes on the copper surface, leading to an increased surface area.

Monolayer MoS2 field-effect transistors patterned by photolithography for active matrix pixels in organic light-emitting diodes

Two-dimensional molybdenum disulfide (MoS2) has substantial potential as a semiconducting material for devices. However, it is commonly prepared by mechanical exfoliation, which limits flake size to only a few micrometers, which is not sufficient for processes such as photolithography and circuit patterning. Chemical vapor deposition (CVD) has thus become a mainstream fabrication technique to achieve large-area MoS2. However, reports of conventional photolithographic patterning of large-area 2D MoS2-based devices with high mobilities and low switching voltages are rare. Here we fabricate CVD-grown large-area MoS2 field-effect transistors (FETs) by photolithography and demonstrate their potential as switching and driving FETs for pixels in analog organic light-emitting diode (OLED) displays. We spin-coat an ultrathin hydrophobic polystyrene layer on an Al2O3 dielectric, so that the uniformity of threshold voltage (Vth) of the FETs might be improved. Our MoS2 FETs show a high linear mobility of approximately 10 cm2 V−1 s−1, due to a large grain size around 60 μm, and a high ON/OFF current ratio of 108. Dynamic switching of blue and green OLED pixels is shown at ~5 V, demonstrating their application potential.

Robust Gate Driver on Array Based on Amorphous IGZO Thin-Film Transistor for Large Size High-Resolution Liquid Crystal Displays

Amorphous IGZO thin-film transistors (TFTs) in an etch stop layer (ESL) structure was processed on 2500 mm $\times $ 2200 mm size substrate. The fabricated devices exhibit enhancement mode characteristics, and excellent uniformity over large area. The presented good operational stabilities under both positive gate bias temperature stress (PBTS) and negative gate bias temperature stress (NBTS) tests can well meet the requirements for pixel switching. However, considering even threshold voltage shift under long term positive bias stress might affect proper operation of the gate driver on array (GOA), a design with a pulse gating scheme is proposed, consisting of 13 TFTs and 1 capacitor, to avoid long term continuous bias stressing of the TFT. With the proposed GOA design, a 32-inch QUHD ( $7680\times 4320$ ) high-resolution liquid crystal display (LCD) panel with a 7 mm wide bezel is achieved. The reliability of the GOA circuit is well proved through standard aging tests.

Single-pulse ablation of multi-depth structures via spatially filtered binary intensity masks.

Digital micromirror devices (DMDs) show great promise for use as intensity spatial light modulators. When used in conjunction with pulsed lasers of a timescale below the DMD pixel switching time, DMDs are generally only used as binary intensity masks (i.e., "on" or "off" intensity for each mask pixel). In this work, we show that by exploiting the numerical aperture of an optical system during the design of binary masks, near-continuous intensity control can be accessed, whilst still maintaining high-precision laser-machining resolution. Complex features with ablation depths up to ∼60 nm, corresponding to grayscale values in bitmap images, are produced in single pulses via ablation with 150fs laser pulses on nickel substrates, with lateral resolutions of ∼2.5 μm.

Characterization of Unstable Blinking Pixels in the AisaOWL Thermal Hyperspectral Imager

The AisaOWL thermal hyperspectral instrument, manufactured by Specim, is a relatively new push-broom sensor well suited to airborne environmental surveys. The sensor covers the 7.6– $12.6~\mu \text{m}$ part of the long-wave infrared region with 102 continuous bands, and is capable of imaging in low-light conditions. The detector array is a mercury cadmium telluride (MCT) semiconductor, which has an inherent randomly varying dark current for random pixels. This manifests in the raw data as a pixel switching between different intensity levels. These pixels are termed “blinkers” by the manufacturer. For each data acquisition, the pixels need to be tested for blinking behavior as different pixels are affected during each acquisition. However, little is known about the number of blink events, the duration of frames, or the optimal length of data acquisition. This paper presents the characterization of the blinking nature of pixels in the MCT detector array to provide guidance on data acquisition and processing. This paper finds that blinking behavior is not completely random, with some pixels more prone to blinking behavior than others. Most blinking pixels have only a few short blinks; therefore, there is still a considerable amount of good data in a blinking pixel.

In brief

PAGE 192 Researchers in the USA propose a two-dimensional pixelating technique for actuating liquid metal. Continuous electrowetting is used to move the liquid metal between reservoirs, which resulted in ON and OFF switching of adjacent pixels. This method removes the need for a physical switch to be employed in pixelated antennas. PAGE 227 An indoor positioning method based on a single LED and camera is given in work from China. By simultaneously measuring both the strength and the angle of arrival of the LED light, ‘fingerprint’ signals of the receiver position are identified. This work is proposed as an optimum solution for indoor positioning systems using optical communication. Liquid metal actuation between pixels using continuous electrowetting. PAGE 229 Work from Iran presents a low power, all-optical switch consisting of a microfibre loop resonator in one arm of a microfibre Mach–Zehnder interferometer. The design is numerically predicted to offer easy configuration, small dimensions, low threshold power and fast switching times in optical signal processing and communications applications. A single camera and LED are used to determine receiver positions indoors. PAGE 194 Researchers in Egypt are developing an accumulated grey level image (AGLI) method for lung cancer detection. The method works by classifying genetic mutations in DNA sequences of effective genes in lung cancer, using a combination of principal component analysis and AGLI. The reported computational approach could be used as a diagnostic aid to classify genetic mutations associated with lung cancer. Numerical predictions propose an efficient all-optical switch design for optical communication applications. PAGE 187 A simple polarisation reconfigurable printed monopole antenna is presented by researchers in China. The antenna structure consists of a printed monopole, two switching circuits, and a reflector, and is shown to have four operational states that are accessed by controlling the ON/OFF states of the switching circuits. The antenna is a candidate for use in multi-frequency and multi-polarisation systems. A computational diagnostic approach for identifying genetic mutations associated with lung cancer. Four operational states reported for a simple polarisation reconfigurable printed monopole antenna.

Highly Stable Zinc Oxynitride Thin-Film Transistors with Field-Effect Mobility Exceeding 100 cm2/Vs

Ultra definition, large-area displays with three-dimensional visual effects represent the megatrend in the current and future display industry. On the hardware level, such a “dream” display requires faster pixel switching and higher current driving, which in turn necessitate thin-film transistors (TFTs) with high field-effect mobility values. Amorphous oxide semiconductors (AOSs) such as In-Ga-Zn-O have recently emerged as candidate materials that may satisfy the above requirements, but the trade-off between device performance (i.e., field-effect mobility) and stability (i.e., threshold voltage shift) under illumination critically limits their usability. Here we report on amorphous-like Zn-O-N (ZnON) TFTs that are Indium-free but still exhibit high field-effect mobility exceeding 100 cm2V-1s-1. Such devices have a superior stability over conventional AOS devices with respect to light radiation, which can be understood by studying the fundamental electronic band structure of ZnON. It is anticipated that high-mobility and high-stability ZnON TFTs will be extensively applied to next-generation flat-panel displays.

An Automatic Gain Control System for ISFET Array Compensation

This paper presents an automatic gain calibration system for ion-sensitive field effect transistors (ISFETs) sensing arrays which compensates for mismatch in the gain of the pixels. The system utilizes the high frequency spectrum of the sensed signal to superimpose a reference sine wave through the reference electrode, which allows identification and calibration of the local gain. The complete system has been fabricated in a 0.35 μm CMOS process and has been designed to achieve very high accuracy and a small settling time to allow fast pixel switching. Measured results confirm good performance with only a 1.15% deviation in gain caused by noise and harmonic distortion and a rapid settling time of 70.5 μs which is suitable for CMOS-based ISFET arrays.

Two-phase microfluidics in electrowetting displays and its effect on optical performance.

Driving microfluidic flow in micropixels by electrowetting to realize light switches and displays is of both practical and fundamental significance. The electro-optical performance related to microfluidic behavior needs to be clarified to optimize device functions. In this article, the microfluidic performance in electrowetting display devices was categorized according to the oil-water interface shape and response. The oil film movement was divided into vertically "thinning" and transversally "opening," for which the "thinning" process was found the key factor determining the pixel switching speed rather than the "opening" process. Therefore, the breakup point and the oil film thickness were critical, which could be controlled by surface wettability and oil volume. We have also realized a new oil filling method with controllable dosing volume assisted by the microfluidic creation of microdroplets. This study could help quantitatively understand electrowetting display performance in both its theoretical and practical aspects.

Unusual instability mode of transparent all oxide thin film transistor under dynamic bias condition

We report a degradation behavior of fully transparent oxide thin film transistor under dynamic bias stress which is the condition similar to actual pixel switching operation in active matrix display. After the stress test, drain current increased while the threshold voltage was almost unchanged. We found that shortening of effective channel length is leading cause of increase in drain current. Electrons activate the neutral donor defects by colliding with them during short gate-on period. These ionized donors are stabilized during the subsequent gate-off period due to electron depletion. This local increase in doping density reduces the channel length.

Self-Aligned Top-Gate Amorphous Indium Zinc Oxide Thin-Film Transistors Exceeding Low-Temperature Poly-Si Transistor Performance

Thin-film transistor (TFT) is a key component of active-matrix flat-panel displays (AMFPDs). These days, the low-temperature poly silicon (LTPS) TFTs are to match with advanced AMFPDs such as the active matrix organic light-emitting diode (AMOLED) display, because of their high mobility for fast pixel switching. However, the manufacturing process of LTPS TFT is quite complicated, costly, and scale-limited. Amorphous oxide semiconductor (AOS) TFT technology is another candidate, which is as simple as that of conventioanl amorphous (a)-Si TFTs in fabrication but provides much superior device performances to those of a-Si TFTs. Hence, various AOSs have been compared with LTPS for active channel layer of the advanced TFTs, but have always been found to be relatively inferior to LTPS. In the present work, we clear the persistent inferiority, innovating the device performaces of a-IZO TFT by adopting a self-aligned coplanar top-gate structure and modifying the surface of a-IZO material. Herein, we demonstrate a high-performance simple-processed a-IZO TFT with mobility of ∼157 cm(2) V(-1) s(-1), SS of ∼190 mV dec(-1), and good bias/photostabilities, which overall surpass the performances of high-cost LTPS TFTs.

Study of Low Temperature (180 oC) Dual-Select-Diode for Applications to Pixel Switching Devices for Flexible Displays

Thin film dual select diodes (TF-DSDs) were fabricated at a low temperature (180 oC) for applications to pixel switching devices for flexible displays. The Silicon-rich silicon nitride (SRSN) films were prepared by catalytic chemical vapor deposition (Cat-CVD) technique. Composition of the source gas mixture affected directly the photoluminescence spectra and the current-voltage characteristics of the SRSN films. We observed non-linear, asymmetric current-voltage characteristics that can be tuned by controlling the film deposition parameters.

Anion control as a strategy to achieve high-mobility and high-stability oxide thin-film transistors

Ultra-definition, large-area displays with three-dimensional visual effects represent megatrend in the current/future display industry. On the hardware level, such a “dream” display requires faster pixel switching and higher driving current, which in turn necessitate thin-film transistors (TFTs) with high mobility. Amorphous oxide semiconductors (AOS) such as In-Ga-Zn-O are poised to enable such TFTs, but the trade-off between device performance and stability under illumination critically limits their usability, which is related to the hampered electron-hole recombination caused by the oxygen vacancies. Here we have improved the illumination stability by substituting oxygen with nitrogen in ZnO, which may deactivate oxygen vacancies by raising valence bands above the defect levels. Indeed, the stability under illumination and electrical bias is superior to that of previous AOS-based TFTs. By achieving both mobility and stability, it is highly expected that the present ZnON TFTs will be extensively deployed in next-generation flat-panel displays.

46.3: Improved Oil Motion Control and Hysteresis‐Free Pixel Switching of Electrowetting Displays

AbstractHysteresis‐free pixel switching and a high control of the fluid flow are beneficial for making optimal use of electrowetting display technology. As described in the paper, the pixel design is shown to play a dominant role to optimize the pixel switching characteristics and to allow improved gray‐scale reproducibility. Use of pixel features such as a hydrophilic patch or a staircase, are shown to be effective. Advantages and drawbacks of these methods are discussed.