Active-matrix Display Research Articles

Study on electrical degradation of p-type low-temperature polycrystalline silicon thin film transistors with C– V measurement analysis

Laser recrystallized low-temperature poly-silicon (LTPS) films have attracted attention for their application in thin-film transistors (TFTs), which are widely used in active matrix display. However, the degradation behavior of p-type LTPS TFTs is not quite clarified yet. In this paper, the instability mechanisms of p-channel LTPS TFTs under DC bias stress have been investigated. From the IV transfer curves, it was observed that LTPS TFT's mobility increases after stress at some bias conditions. This degradation is most likely caused by interface traps between the poly-Si thin film and the gate insulator, as well as the damaged junction of the drain from stress. In this work, the assumption is examined via C– V measurement. It is found that the C GD curves of the stressed TFT slightly increase for the gate voltage smaller than the flat band voltage V FB. However, the C GS curves of the stressed device are almost the same as those before stress. By employing simulation, it is found that the degradation of p-type TFTs under this stress condition is mainly caused by the trapped charges at the interface between the gate and the drain region, which is generated by the high voltage difference applied during DC bias stress.

62.2: A Flexible 7.1-in. Active-Matrix Electrophoretic Display

We have developed a flexible 7.1-inch Quad-XGA active-matrix electrophoretic display using SUFTLA™. The whole panel circuitry including 7.3-milion transistors was successfully transferred onto a plastic substrate without damage. Integrating both scan- and data-drivers made it possible that 3-mega pixels are controlled with only 40 signal terminals. The developed panel fulfills major requirements for e-paper including true flexibility, thinness and lightness.

4.5: The Design and Driving of Active-Matrix Electrochromic Displays Driven by LTPS TFTs

We have successfully driven a high-resolution (200dpi) QVGA active-matrix electro-chromic display (AMECD) with integrated driver electronics fabricated using low-temperature polysilicon thin-film transistor (LTPS-TFT) technology. This non-volatile reflective display requires low operating voltages, and possesses the advantages of high contrast ratio, sharp image quality, very large viewing angle and an image retention of a few days.

P-20: A Short Channel Effect in Low Temperature Poly-Si Thin Film Transistor for Active Matrix Display

We have investigated a short channel (L μm) effect on an electrical reliability in low temperature poly-Si (LTPS) thin film transistor (TFT) employing excimer laser annealing (ELA). The decrease of threshold voltage due to the drain induced barrier lowering (DIBL) is observed in p-type poly-Si TFT without lightly doped drain (LDD). In the n-type poly-Si TFT with LDD, the threshold voltage is slightly decreased when a large drain bias is applied. The field effect mobility is observed on various channel length and LDD. The series resistance due to the LDD in the short channel LTPS is decreased the field effect mobility. The operating temperature dependence of the electrical characteristics is investigated. As the temperature is increase, the poly-Si TFT's characteristics is improved due to the increase the thermally activated carriers. We have investigated a reliability of short channel poly-Si TFT. The degradation of short channel poly-Si TFT under the hot carrier stress is dominant to interface state.

Novel liquid-crystal materials with negative dielectric anisotropy for TV application

— The development of materials for vertically aligned (VA) active-matrix display technology is briefly reviewed and the newest generation of liquid crystals for this application is disclosed. These new materials possess a unique combination of high dielectric anisotropy Δe and low rotational viscosity γ1 with good clearing temperatures TNI for fast-switching LC mixtures targeted for TV application.

Pentacene-based thin film transistors used to drive a twist-nematic liquid crystal display

The study addresses the factors of influence on the active matrix display that is driven by pentacene-based organic thin-film transistors (TFTs). The atmosphere and humidity conditions were found to seriously affect the performance of organic TFTs. An appropriate encapsulation layer was added to protect the organic TFTs from external damage. Organic TFTs reliability, the illumination effect and device uniformity, were also considered in the context of display application. Finally, a monochrome 3 inch 64 × 128 active-matrix twist-nematic liquid crystal display (LCD) was fabricated. The display is capable of showing video images with a refresh rate of 20 Hz. Obtained results reveal the potential of organic TFTs for active-matrix LCD technology.

16.4: A 14.1ʺ WXGA Solution Processed OLED Display with a-Si TFT

We have developed the world's largest a-Si TFT based solution processed AMOLED full color display. In our 14.1″ WXGA demonstrator, we have realized the world's best performance for solution-processed OLED materials in an active matrix display, while setting a higher standard for uniformity and image quality.

Dual-Gate SiO2/P3HT/SiNx OTFT

AbstractThis paper reports on a new organic thin-film transistor (OTFT) based on a dual-gate configuration. This dual-gate OTFT is useful in circuit applications from the standpoint of providing control over selected device parameters for enhanced circuit reliability. Moreover, the dual-gate structure can shield parasitic effects in vertically integrated electronics, making it particularly promising for active matrix display and imaging applications. The dual-gate OTFT also lends itself as a highly functional test structure for characterization of interface integrity of the active organic and dielectric layers. In this work, the dual-gate OTFT is fabricated using regioregular poly(3-hexylthiophene) (P3HT) as the organic semiconductor layer. The bottom-gate employs silicon dioxide (SiO2) as the gate dielectric, whereas the top-gate employs a low-temperature amorphous silicon nitride (SiNx) as the passivation dielectric. The voltage on the bottom-gate has a distinct influence on the threshold voltage, subthreshold slope, on-current, and leakage current of the top-gate TFT. Similar dependence of the bottom-gate TFT characteristics on the top-gate voltage is observed. This design provides a means of characterizing the density of states of the bottom P3HT/SiO2 and top P3HT/SiNx interfaces, and conveys insight into the underlying transport mechanisms. The ability to control selected TFT parameters (e.g., threshold voltage) using the dual-gate OTFT structure is attractive for circuit integration applications in active matrix displays and imagers.

Active-matrix field-emission display based on a CNT emitter and a-Si TFTs

— An active-matrix field-emission display (AMFED), based on carbon-nanotube (CNT) emitters and amorphous-silicon thin-film transistors (a-Si TFTs), was developed. The AMFED pixels consisted of a high-voltage a-Si TFT and mesh-gated CNT emitters. The AMFED panel demonstrated high performance for a driving voltage less than 15 V. The low-cost large-area AMFED approach using a metal-mesh technology is proposed.

3.1: Invited Paper: Rollable QVGA Active-Matrix Displays Based on Organic Electronics

A 100 μm thin QVGA active-matrix electrophoretic display is produced using organic transistors. The display can be bent to a radius less than 1 cm without any performance loss. Higher contrast and low crosstalk was achieved by addition of a field shield to our backplane.

Stability of hydrogenated amorphous silicon TFT driver

Gate and data drivers are essential for driving active matrix display. In this study, we integrate drivers with a‐Si:H to develop a compact, better reliability and cost effective display. We design and fabricate drivers with conventional a‐Si:H thin film transistors (TFTs). The output voltages are investigated according to the input voltage, temperature and operation time. Based on these studies, we propose here a new driver to prevent gate line from the floated state. For the external coupled voltage fluctuation, the proposed driver shows better stability.

33.3: Source-Gated Transistors in Amorphous Silicon for Active Matrix Displays

Compared with a FET, the Source-Gated Transistor (SGT) has a lower saturation voltage and can operate with smaller carrier concentrations and higher internal fields. OLED arrays with amorphous silicon SGT pixel drivers would dissipate less power and have better stability.

A flexible, organic QVGA display with gray‐scale capability

Abstract— A flexible 4.7‐in. QVGA active‐matrix display was demonstrated, containing 76,800 solution‐processed organic transistors. The combination of our plastic active‐matrix backplane with electrophoretic‐ink display material developed by E‐Ink resulted in a reflective low‐power display with paper‐like appearance. By using high‐performance organic transistors, it was possible to generate 2‐bit images on the display. The display can be bent to a radius below 2 cm.

39.3: Recent Advances in Printed Active Matrix Displays

AbstractPlastic logic has developed a print based process for active matrix display manufacture. This process has no vacuum or mask steps, is scalable, low cost, low temperature, and environmentally friendly. We report on our recent advances in developing our process for large area electronic paper. This includes scaling our process to print 9″ diagonal thin film transistor backplanes with >100,000 pixels (>QVGA) and printing transistors on commodity flexible plastic substrates.

Optimizing OLED Structures for a-Si Display Applications via Combinatorial Methods and Enhanced Outcoupling

Active-matrix organic light-emitting displays driven by amorphous silicon (a-Si) thin-film transistors (TFT) require superior organic light-emitting device (OLED) performance in order to achieve sufficient pixel brightness and to avoid degradation of the a-Si transistors. In addition, the a-Si TFT circuitry of high-resolution full-color displays can become rather complex and therefore occupies a large amount of the pixel area, leading to a significantly reduced aperture ratio with conventional bottom-emitting OLEDs. A top-emitting device architecture is therefore advantageous. In this paper we describe general concepts for optimizing top-emitting OLEDs via combinatorial methods and for enhancing outcoupling. To optimize OLED performance, we use combinatorial device fabrication. Our system allows the simultaneous fabrication of 10 × 10 individual devices on one substrate, enabling a systematic variation of material combinations and electrodes as well as of device parameters such as layer thickness, layer sequence, and dye dopant concentrations. In the first part, we present an overview of the capabilities of combinatorial methods for electrical and electro-optical device optimization. We show results on multilayer OLEDs ranging from trilayer devices to phosphorescent five-layer OLEDs. The second part describes an outcoupling concept that allows the emission characteristics of top-emitting OLEDs to be tailored. We demonstrated that and analyzed how the angular intensity distribution and the spectral characteristics can be tuned. Light outcoupling can be enhanced by simply varying the optical thickness of a dielectric layer deposited on top of a semitransparent metal electrode. Using this capping-layer concept the outcoupled light intensity in forward direction was increased by a factor of 1.7 while concomittantly achieving a high color purity. The strength of the capping-layer concept is in particular that the optical and the electrical device performance can be optimized separately. Using optimized layer thicknesses and the capping concept, we achieve an efficiency of NTSC (National Television Standard Committee) white from separate red, green and blue sub-pixels exceeding 22 cd/A under realistic display-driving conditions, thus enabling the world's largest (20-inch) a-Si active-matrix OLED display. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

45.1: System‐on‐Plastic with Organic Electronics: A Flexible QVGA Display and Integrated Drivers

AbstractA flexible 4.7‐inch QVGA active‐matrix electrophoretic display is demonstrated, containing 76800 solution‐processed organic transistors. In the same technology, 32‐ and 120‐stage row drivers are produced with adequate speed to drive a QVGA display. Both the display and the drivers are highly flexible. Bending the display to a radius below 2cm does not result in any performance loss.

P‐34: A 5‐in. QQVGA CNT‐FED with a Glass‐Mesh Triode Driven by a‐Si TFT

AbstractActive‐matrix field emission display (AM‐FED) with a glass‐mesh gate plate was developed by using single‐wall carbon nanotube (CNT) as an electron emission source and a‐Si TFT as a control switch, respectively. The a‐Si TFT and screen‐printed CNT emitters were successfully integrated to fabricate the active‐matrix cathode. We fabricated the first prototype AM‐triode CNT FED panel with 5‐in. in diagonal and 160×3×120 subpixels through the vacuum packaging of the AM cathode, the glass‐mesh gate, and the anode. The fabricated AM‐CNT FED panel showed good uniformity, high stability and reliability, and high brightness with a driving voltage of below 15 V.

Development of OLED With High Stability and Luminance Efficiency by Co-Doping Methods for Full Color Displays

We propose co-doping systems in emission layers of red and green organic light-emitting diodes (OLEDs). The luminance-voltage, luminous and power efficiency-voltage characteristics, operational stability, and the energy bands of materials were measured. In red OLED devices, we propose an emitting assist (EA) dopant for better luminance efficiency and power efficiency with pure red emission and improved operational stability. The EA dopant (rubrene) did not emit itself but assisted the energy transfer from a host (Alq) to an emitting dopant (DCJTB). By doping rubrene, the luminance efficiency increased from 1.7 to 4.3 cd/A (from 0.6 to 1.9 lm/W) with chromaticity of (x=0.64, y=0.36) unchanged. An improved lifetime was also observed. In green OLED devices, we introduced hole transporting material (NPB) into an emission layer for better charge injection balance. The green devices with the emitting dopant (C545T) achieved the luminance efficiency of 8.5 cd/A compared with 6.9 cd/A without NPB. We studied the co-doping methods and use of this approach for active-matrix full color display. The power consumption of white emission at 100 cd/m/sup 2/ was reduced by 32%. The effectiveness of these co-doping methods was demonstrated for practical applications.

34.1: Active Matrix Displays Made with Printed Polymer Thin Film Transistors

AbstractWe present a process for active matrix flat panel display manufacture by printing polymer thin film transistors. In this process, transistors are fabricated using soluble semi‐conducting and conducting materials. Accurate definition of the transistor channel and other circuit components is achieved by direct inkjet printing combined with surface energy patterning. We have used this process to create a 4,800 pixel, 50 dpi, active matrix display. This process is in principle environmentally friendly, low temperature, compatible with flexible substrates, cost effective and advantageous for short run length and large display sizes.

Organic Transistor Circuits for Application to Organic Light-Emitting-Diode Displays

Organic thin-film transistor circuits based on the small molecule, copper phthalocyanine, have been demonstrated for application to an active-matrix display with organic light-emitting diodes. The circuit consists of two organic thin-film transistors and one storage capacitor which provide continuous current under pulsed operation. The transistors for the circuit have been designed on the basis of the current characteristics of the individual transistors. Continuous output current has been obtained under pulsed voltage operation by using the organic transistor circuit.