Low-temperature Polysilicon Thin-film Transistors Research Articles

The Fabrication and Characterization of Direct Conversion Flat Panel X-ray Imager with TlBr Film

The novel direct conversion flat panel detectors (FPDs) utilizing thallium bromide (TlBr) films are developed and characterized in this study. Thin TlBr films, deposited using an evaporation technique, achieved thicknesses of 10 μm and 50 μm. A pixel sensor array, with a pixel size of 235 × 235 μm2 and a sensitive area of 29.61 × 39.48 mm2, was developed using low-temperature polysilicon thin-film transistor (LTPS TFT) technology as the initial study to explore performance characteristics of developed imaging system. The TlBr film was characterized based on the current-voltage (I-V) profile, showing a resistivity of 2.78 × 101⁰ Ω·cm. The LTPS TFT showed a leakage current of about 10-13 A, while the noise of TlBr FPD system was around 104 e- rms, primarily limited by the readout system. The detector system was characterized using radiation from an X-ray tube with a voltage in the range of 30 kV to 100 kV and a current of 2 mA, involving an X-ray test chart as the object with the processing time of 18.15 to 18.30 ms per frame. Based on the contrast and contrast-to-noise ratio (CNR) values, the 10 μm and 50 μm TlBr FPD clearly captured the image in operation between 30 kV and 100 kV. The 50 μm TlBr FPD demonstrated improved sensitivity, showing pixel values with 10 times higher intensity and enhanced line pair detail at 30 kV X-ray operation. The 10 μm TlBr achieved spatial resolution of 267 μm full width at half maximum (FWHM), while the 50 μm TlBr achieved 250 μm FWHM. In the X-ray imaging demonstration, the shape and structure of anchovy was clearly distinguished. The TlBr film FPD presents promising potential for a room temperature X-ray imager.

A 6T1C pixel circuit compensating for TFT electrical characteristics variations, voltage drop, and OLED degradation

A voltage-programmed pixel circuit based on low-temperature poly-silicon (LTPS) thin-film transistors (TFTs) is presented for active-matrix organic light-emitting diode (AMOLED) displays. The circuit consists of six p-type transistors and one capacitor (6T1C). During the extraction stage, the circuit extracts the driving TFT’s threshold voltage and the power supply voltage. During the data input stage, the circuit generates a charging voltage, which is associated with the driving TFT’s mobility and the OLED’s threshold voltage. Consequently, the pixel circuit, using only one capacitor, compensates not only for the TFT’s electrical characteristics variations, i.e., the threshold voltage variation and mobility variation, but also for the power supply voltage drop and the OLED degradation. The circuit simulation results reveal that the OLED current error rates (CERs) are lower than 5.09% when the threshold voltage varies by ±0.5V, lower than 3.61% when the mobility varies by ±30%, and lower than 9.16% when the voltage drop varies by −0.5V in the data voltage range.

38.2: An Accurate and Stable PWM Pixel Circuit Based on LTPO TFTs for AM‐Micro‐LED Display

A pulse-width-modulation (PWM) pixel circuit for Micro-LED displays is proposed based on low-temperature poly-silicon and oxide (LTPO) thin-film transistors (TFTs). The pixel circuit features fast PWM transition, as the PWM falling-time is less than 20 μs thanks to the high-gain dynamic inverter. Furthermore, the proposed pixel circuit can compensate the degradation and non- uniformities of TFT characteristics of LTPO TFTs through auto- zeroing configuration of the dynamic inverter and a diode- connected threshold voltage compensation structure. The error rate of luminous time is less than 3.2% and the error rate of current is less than 1% under VTH shift of oxide TFT ±2 V or LTPS TFT ±1 V.

Gate Driver on Array With Multiple Outputs and Variable Pulse Widths for Low-Temperature Polysilicon and Oxide (LTPO) TFTs Driven AMOLED Displays

A gate driver on array (GOA) circuit was proposed for low-temperature polysilicon and oxide (LTPO) thin-film transistor (TFT) hybrid-driven active matrix organic light-emitting diode (AMOLED) displays with both high and low refresh rate operation. The GOA generates multiple types of scanning signals by reusing shift registers so that three signal pulses are output parallelly within a single stage to scan p- and n-type switching TFTs and emission TFT of the AMOLED pixels, respectively, leading to a 30% reduction in the layout area. Furthermore, the pulse-width of the scan signals for the emission TFT can be programed simply by changing the number of input pulses to vary refresh rates. The fabricated GOAs work well even with the pulse width reduced to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.9 \mathbf {\mu }\text{s}$ </tex-math></inline-formula> .

Effects of Static and Repetitive Uniaxial Bending Strains on the Electrical Properties and Trap Characteristics of Flexible Low-Temperature Polysilicon Thin-Film Transistors

Changes in the electrical properties and trap characteristics of flexible low-temperature polysilicon thin-film transistors (TFTs) under the application of uniaxial bending strains and during repetitive bending cycles were investigated. When the bending strain increased, the transfer curve showed a negative shift, the output current decreased, and the subthreshold slope increased. After the bending strains were removed, the electrical performance showed recovery behavior, but the device performance could not return fully to its original state. The devices were also subjected to repetitive bending cycles. The transfer curves showed a positive shift after <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{{4}}$ </tex-math></inline-formula> bending cycles and the output current increased. Using the transient current method, it was established that the detrapping time constant of the traps decreased and the peak amplitudes increased as a result of both the bending strain and the bending cycles. However, while the trap activation energy was not changed by static bending, it did decrease after the repetitive bending cycles. Both the changes in the electrical properties and the time constant spectra confirm that the trap state density increased as a result of the bending processes. However, the different trapping behaviors contributed to different degradations in the device’s electrical properties.

Highly Robust Flexible Poly-Si Thin Film Transistor Under Mechanical Strain With Split Active Layer for Foldable Active Matrix Organic Light Emitting Diode Display

We report a novel active split structure of low-temperature polysilicon (LTPS) thin film transistor (TFT) on polyimide (PI) substrate showing robust electrical performance under mechanical strain. The compressive and tensile bending tests were carried out with cylinders of radii 3, 2 and 1 mm respectively. The active split TFT exhibits much more stable electrical performance than the conventional TFT. The conventional LTPS TFT shows electrical failure due to the crack generation during tensile or compressive bending test carried out with 1 mm radius cylinder. The split poly-Si islands could resist crack formation/propagation, resulting in a robust electrical performance.

A mirrored 5T1C OLED pixel circuit for compensating characteristics variations and voltage drop

A pixel circuit with p-type low temperature poly-silicon thin-film transistors (TFTs) is presented for active-matrix organic light-emitting diode displays. This circuit consists of five TFTs and one capacitor. In the circuit, the characteristics, i.e., the threshold voltage and the mobility, are extracted by the mirroring structure, and an adaptive voltage is generated at the gate of the TFT in the programming stage. Therefore, only one capacitor is needed to compensate both the threshold voltage variation and the mobility variation. Meanwhile, in the circuit, the voltage drop of the power line is also compensated. Moreover, the circuit can prevent the image flicker because the organic lighting-emitting diode (OLED) only operates in the emission stage. The circuit simulation results indicate that the OLED current error rates are lower than 4.61% when the threshold voltage variation is ±0.5V, lower than 6.92% when the mobility variation is ±30%, and lower than 3.96% when the voltage drop is −0.5V.

Effect of Biaxial Bending Strains on the Electrical Characteristics of Flexible Low-Temperature Polysilicon Thin-Film Transistors

The change of electrical properties of flexible low-temperature polysilicon (LTPS) thin-film transistors (TFTs) under biaxial bending strains was studied. Biaxial strains were applied using a ring-on-ring bending test rig. The magnitude of the strain was determined by theoretical calculations and finite-element simulations. With an increase in strain, the transfer curve shifted to a negative bias. At a biaxial strain level of 12%, the threshold voltage decreased by 27%, the field-effect mobility decreased by 13%, and the subthreshold slope increased by 25%. It had a more significant effect on the property changes than the uniaxial strain; this difference may be attributed to additional trap states generated by the biaxial strain. The analysis showed that the trap density increased when the strain level increased. The contributions of the increases in the grain-boundary trap state density and the interface trap state density to the degradation in the electrical properties were derived to be 10% and 90%, respectively. After removal of the biaxial bending strains, the electrical performance showed recovery behavior, but the performance could not return fully to its original state.

Strain distribution and electrical characteristics of flexible low-temperature polysilicon thin film transistors under biaxial bending

This paper investigates the strain distribution of flexible p-channel low temperature polycrystalline silicon thin film transistors under biaxial bending. The strain distribution of LTPS TFTs under different bending conditions is analysed by finite element analysis. \U0001d6c6r and \U0001d6c6θ are essentially equal and constant within the load ring. The finite element analysis shows that when the biaxial bending strain condition is 2.5%, the strain distribution is mainly concentrated at the contact angle between the electrode and the oxide layer. This strain is significantly greater than the overall strain of the device, which is the main reason for the bending performance of the device. We found that under biaxial bending, the threshold voltage shifts towards the negative axis as the biaxial strain increases. Within this strain range, these electrical properties are evident under biaxial strain conditions. Comparing theoretical threshold voltage shifts based on energy band changes with experimental results under biaxial strain, it was found that changes in trap density should also contribute to the threshold voltage shift.

Strategies for Applications of Oxide-Based Thin Film Transistors

Due to the untiring efforts of scientists and researchers on oxide semiconductor materials, processes, and devices, the applications for oxide-based thin film transistors (TFTs) have been researched and promoted on a large scale. With the advantages of relatively high carrier mobility, low off-current, good process compatibility, optical transparency, low cost, and especially flexibility, oxide-based TFTs have already been adapted for not only displays (e.g., liquid crystal display (LCD), organic light emitting diode (OLED), micro-light-emitting diode (Micro-LED), virtual reality/augmented reality (VR/AR) and electronic paper displays (EPD)) but also large-area electronics, analog circuits, and digital circuits. Furthermore, as the requirement of TFT technology increases, low temperature poly-silicon and oxide (LTPO) TFTs, which combine p-type LTPS and n-type oxide TFT on the same substrate, have drawn further interest for realizing the hybrid complementary metal oxide semiconductor (CMOS) circuit. This invited review provides the current progress on applications of oxide-based TFTs. Typical device configurations of TFTs are first described. Then, the strategies to apply oxide-based TFTs for improving the display quality with different compensation technologies and obtaining higher performance integrated circuits are highlighted. Finally, an outlook for the future development of oxide-based TFTs is given.

Analysis of self-heating-related instability in n-channel low-temperature polysilicon TFTs with different S/D contact hole densities

This study examines self-heating-related instability in n-channel low-temperature polysilicon thin-film transistors with different source/drain contact hole densities. Devices with more contact holes exhibit a higher on-current without additional parasitic capacitance, further enhancing the RC delay property. For high-current-induced self-heating stress, a device with one contact hole has one hump due to the kink effect. However, a device with six contact holes has two humps, induced by the kink effect and thermionic field emission. COMSOL simulations of heat distribution and energy bands are performed to examine the different degradation behaviors, and then physical models are proposed.

Study of traps in low-temperature polysilicon thin film transistors using a current transient method

The trap characteristics of low-temperature polysilicon thin film transistors (TFTs) are studied using a current transient method that is based on the trapping and detrapping of charge carriers from the trap sites in the device. Analysis of the measured current transient curve allowed three types of detrapping behavior to be identified, each of which displayed a different time constant, activation energy, and spatial position. This current transient method can be integrated into the negative bias temperature instability stress test used for the reliability study. The peak amplitudes of the traps increase because of the stress applied in the test, thus demonstrating that the degradation mechanism of the TFTs is closely related to changes in the traps in these devices.

Stacked-Interconnect for Monolithic Integration of Low-Temperature Polysilicon and Amorphous Metal-Oxide Thin-Film Transistors

While offering a range of practical benefits, the monolithic integration of low-temperature polysilicon (LTPS) and amorphous metal-oxide thin-film transistors presents several incompatibility issues regarding materials and processes. Presently addressed are two critical ones arising from the back-end processes of contact treatment and metallization. Both are resolved by employing a stacked-interconnect consisting of two conductor layers, with each layer forming the preferred contact electrodes for one of the two types of transistors. At the expense of a slight increase in process complexity, a narrow distribution of low specific contact resistance ( ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> Ω·cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) for both types of transistors was obtained, thus giving rise to more consistent transistor characteristics. Inverters consisting of complementary top-gate LTPS pull-up and bottom-gate indium-gallium-zinc oxide pull-down transistors were demonstrated, exhibiting a gain of 40 V/V and a rail-to-rail full swing.

Journal of the Society for Information Display

Jiun-Haw Lee Editor in Chief Distinguished Papers, Best of Display Week 2021 Fifteen papers won the title of Distinguished Papers of Display Week 2021 and are published in the May 2021 issue of the Journal of the Society for Information Display (JSID). Congratulations!! The program committee of SID Display Week nominated these papers, which cover different research topics, including active-matrix devices, liquid crystal technology, flexible displays, display electronics, display systems, display measurements, micro light-emitting diode (microLED), and augmented reality (AR) displays. After strict and expedited reviews, JSID has published these high-quality papers before Display Week 2021 (May 17–21). Note that these papers will remain open access until the end of 2021. They are available at https://onlinelibrary.wiley.com/doi/toc/10.1002/(ISSN)1938-3657.DW21. The June 2021 issue of JSID contains two invited papers. The first paper is about microLED technologies by Sony Corporation, which is the pioneer in this field that demonstrated the prototype of microLED in 2012 (called crystalLED at that time). Technology details were disclosed in this manuscript, which provided clear illustrations of LED devices, pixel design, driving innovations, and assembly of this emerging display technology. The second invited paper is a review that compares three major display technologies: liquid crystal display (LCD) lit by miniLED backlight, organic light-emitting diode (OLED), and microLED. Note that the same group (Shin-Tson Wu's research group at the University of Central Florida) had published a similar review paper entitled, “Prospects and challenges of mini-LED and micro-LED displays,” in the July 2019 issue of JSID, which has attracted 41 citations thus far. The 2021 paper provides updates on the progress during the past two years, broadens the topic focus, provides an overview comparison from different display performances, and provides insight on future perspectives. 9.4-inch 228-ppi flexible micro-LED display | Seok-Lyul Lee, et al. | DOI: 10.1002/jsid.1022 This flexible microLED display was fabricated on a low-temperature polysilicon thin-film transistor (LTPS-TFT) backplane, which achieved a contrast ratio over 1,000,000:1 because of the self-emission characteristics of microLED. Additionally, the microLED display's device structure and process flow are disclosed. No degradation was observed after bending tests were conducted (1,000 times with a bending radius of 40 mm). This display also exhibited higher brightness and less color shift than OLED displays when observed from oblique angles. In other words, this microLED display had better viewing-angle performances compared to OLED display. Prospects and challenges of mini-LED, OLED, and microLED displays | En-Lin Hsiang, et al. | DOI: 10.1002/jsid.1058 LCD with mini-LED backlight, OLED, or microLED: who wins? It is an interesting question. This invited review paper attempts to provide an apples-to-apples comparison based on recent products and technologies from nine display performances, including ambient contrast ratio (ACR), motion picture response time (MPRT), viewing angle, color gamut, resolution, power consumption, cost, lifetime, and flexibility. Each display has its own advantages and disadvantages. For the current stage, flexibility, lifetime, and cost are the main issues for miniLED/LCD, OLED, and microLED, respectively. JSID is published monthly by Wiley. Subscription fees apply, but SID members and student members have free online access via sid.org/Publications.aspx. Many universities also have an institutional subscription. JSID is indexed in the Web of Science. Submit your original manuscript online via mc.manuscriptcentral.com/sid. Author guidelines can be found on the Journal's homepage: tinyurl.com/jsidhome. Editorial board: tinyurl.com/jsideb. Please direct any questions about JSID to its editor in chief at editor@sid.org. EarlyView: accepted papers about to be published can be accessed online via tinyurl.com/jsidev. The Journal is soliciting review papers on any display-related topic. If you have a great idea for a review paper, please contact the editor at editor@sid.org. Page charges for invited review papers will be waived.

(Invited) Integration of Organic Photodiode with Low Temperature Poly Silicone Thin Film Transistors for a Conformable Image Sensor

With the rapid aging of Japanese society, how to increase quality of life (QoL) while controlling rising medical expenses has become an urgent issue. To solve this difficult issue, the acquisition and utilization of biological information using new technologies such as wearable devices is more expected. In particular, self-care and home medical care, in which patients and their families are responsible for their own health, are considered as one of the way to solving the issues of a super-aging society. Indeed, wearable sensors that can constantly monitor health conditions and home-use blood pressure monitors with communication functions are being introduced to the market one after another to prepare for the advent of a self-care era.On the other hand, when designing a new insurance system or incentive system using biometric information from wearable sensors, it is important issue how to confirm whether data measured at home is the patient's own. Furthermore, the risk of patient mix-ups must be reduced as more wearable devices come to be used in hospitals and welfare facilities in the future. Therefore, measuring vital signs simultaneously with biometric authentication of the user is an urgent issue.We have developed a sheet-type image sensor that enables high-resolution and high-speed reading. This sheet-type image sensor can take the high-resolution image of fingerprints and veins used for biometric authentication. In addition, the same sheet-type imager can measure the pulse wave which is one of the vital signs, and its distribution.Although there are many reports of sheet-type image sensors, it has not been achieved both high-resolution imaging and high-speed readout, and static biometric data and dynamic vital signs cannot be measured by one sheet-type image sensor. This is because high-sensitivity photodetectors and high-speed switching elements could not be integrated on a polymer substrate without damaging of the switching elements.The developed sheet-type image sensor is fabricated by densely integrating a high-efficiency readout circuit using an active matrix of a low-temperature polysilicon thin film transistor and a photodetector that uses a highly efficient organic semiconductor as a photosensitive layer. The resolution of the image sensor achieves 508 dots per inch (dpi) required for fingerprint authentication, and the organic photodetector consists of bulk hetero structure organic layer which has high photosensitivity to near infrared light with a wavelength of 850 nanometers (external quantum efficiency of 50% or more). It it easy to integrate the sheet-type image sensor into equipment and attach it to curved surfaces due to the thickness of the polymer base material is 10 micrometers, and the total thickness of the sheet-type image sensor is 15 micrometers (Figure 1). By developing a process technology that integrates photodetectors and thin-film transistors without damage to each other, it has become possible to realize a sheet-type image sensor that achieves both high-resolution imaging and high-speed reading.Evaluation of vein and fingerprint images taken by this sheet-type image sensor showed that the contrast difference of the veins was less than 5% compared to images using a general CMOS imager. We confirmed that the conformable imager has high image quality equivalent to that of conventional CMOS imagers.Since the sheet-type image sensor is thin and bendable, it can be easily integrated into wearable devices, and it is possible to measure health condition and perform biometric authentication at the same time. As a result, it is expected that prevention of “spoofing” and patient from being mixed up.

P‐20: 32‐inch 8K4K 120Hz LCD with LTPS TFT

The world's largest LTPS 8K monitor with Low Temperature Poly‐Silicon (LTPS) thin‐film transistor (TFT) is developed to have high performance and ready for mass‐production. With new process and panel design, we have realized the 32‐inch 8K LTPS display with 120Hz high frame rate and GOA driving. In this article, the challenges and related technology for large‐size 8K monitor will be discussed. Furthermore, a new LTPS panel design will be proposed to reduce gate loading and improve the uniformity of common voltage (Vcom).

P‐67: Foldable Ultra‐High‐Resolution AMOLED Panel with Cascade Display‐Driver Integrated Circuit

Smart portable device equipped with active matrix organic light emitting diode (AMOLED) display is a trend in modern consumer market. In this paper, we present not only a 15.6‐inch foldable 4K ultra high definition (UHD) resolution of low temperature poly‐silicon thin film transistor (LTPS‐TFT) AMOLED panel, but also a display driver integrated circuit (DDIC) module which integrate the cascade structure and high‐magnification demura 6 times compression technology.

9.4‐inch 228‐ppi flexible micro‐LED display

AbstractA 9.4‐inch 228‐ppi full‐color micro‐LED display using the flexible low temperature polysilicon thin‐film transistor (LTPS‐TFT) backplane has been successfully developed. In order to solve the interconnection of tiny micro‐LEDs, those size are less than 30 μm, we introduced the flip‐chip soldering technology for high‐resolution flexible micro‐LED displays. Contrast ratio of &gt;1,000,000:1 with a brightness of 700 nits was realized by the full‐color micro‐LED display with uniform brightness and color shift free at any off‐axis viewing angles. These supreme micro‐LED visual performances will provide a more comfortable user experience for high‐resolution flexible automotive applications.

Reducing Power Consumption of Active-Matrix Mini-LED Backlit LCDs by Driving Circuit

An active-matrix (AM) driving circuit using a low-temperature poly-silicon thin-film transistor (LTPS TFT) is developed to reduce the power consumption of mini-light-emitting diode (LED) backlit liquid-crystal displays (LCDs). By lowering the drain-source voltage ( V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> ) of the switching TFT on the driving current path, the range of required voltage across the proposed circuit can be reduced, decreasing the power consumption of the mini-LED backlight. The proposed circuit also can compensate for V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TH</sub> variation in the LTPS TFT and the I- R rise of V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SS</sub> to provide a uniform driving current to flow through the mini-LED. By measuring the I- V curves of a fabricated LTPS TFT, we build a reliable simulation model and compare the proposed circuit to that of the state-of-the-art. Simulated results indicate that the proposed circuit can improve the power consumption by 16.67% and reduce the current error rates below 8% when the V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TH</sub> of TFT varies by ±0.3 V and V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SS</sub> rises +1 V. Measured results further confirm that the V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DS</sub> of the switching TFT is reduced by 2.208 V compared to a 6T2C compensating circuit. Therefore, widespread applications of the proposed circuit for AM mini-LED backlit LCDs to produce high-dynamic-range images are foreseeable.

The time response for the low-temperature poly-silicon thin-film transistors to x-ray irradiation pulse

In this study, the time response behavior of low-temperature polycrystalline silicon (LTPS) thin-film transistors (TFTs) to x-ray irradiation pulses with different frequencies is analyzed. The formulas for the time response of excited and recovery behaviors are proposed for possible use in circuit performance enhancement in real x-ray irradiation situations. The predicted and measured results fit fairly well, which is important in the development of x-ray image sensors using LTPS TFTs.