70.3: Current‐Scaling a‐Si:H TFT Pixel Electrode Circuit for AM‐OLEDs

We fabricated and characterized the amorphous silicon thin‐film transistor (a‐Si:H TFT) pixel electrode circuit with currentscaling function that can be used for active‐matrix organic lightemitting displays (AM‐OLEDs). As expected from previously reported simulation results, fabricated circuit showed an acceptable current‐scaling performance for a high‐resolution AM‐OLED based on a‐Si:H TFTs.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/92085/1/1.2451422.pd

Modeling of OLED degradation for prediction and compensation of AMOLED aging artifacts

Degradation is still the most challenging issue for OLED, which causes the image-sticking artifact on AMOLED displays and limits their lifetime. To overcome the demerit, OLED degradation is modeled in this thesis, and compensation based on the models is applied for AMOLEDs. A data-counting model is firstly developed to quantitatively evaluate the degradation on OLEDs, with consideration of the accumulation stress during operation. An electro-optical model is further built, based on an equivalent circuit. It can simulate the electro-optical characteristic (I-V, Eff-V) and the degradation behaviors in aging process. Besides, the correlation model is aimed to derive the current efficiency decay with measurable electrical values, delivering more dependable results at strongly aged state. The prediction and compensation are implemented based on developed models. The results show that the models exactly predict the efficiency decay during operation. The image-sticking aging artifact on AMOLED can be suppressed by applying compensation, so that the display lifetime is extended.Durch das Einbrennen von Bildern in AMOLED Displays wird deren Lebensdauer verringert; dieser Qualitätsverlust stellt nach wie vor die größte Herausforderung für die OLED Technologie dar. In dieser Thesis wird die Degradation der OLEDs modelliert und eine Kompensierung anhand der Modelle erreicht. Zunächst wurde ein Data-counting Modell entwickelt, um die Degradation von OLEDs unter Berücksichtigung der akkumulierten Belastung während des Betriebs quantitativ zu bewerten. Des Weiteren wurde ein elektro-optisches Modell entwickelt, das auf einem äquivalenten Schaltungsmodell basiert. Es kann die elektro-optischen Eigenschaft (I-V, Eff-V) und das Degradationsverhalten im Alterungsprozess simulieren. Außer den beiden Modellen wird noch ein Korrelationsmodell entwickelt, das darauf abzielt, die Abnahme der Stromeffizienz aus den messbaren elektrischen Werten abzuleiten. Dieses Modell liefert im stark gealterten Zustand zuverlässigere Ergebnisse. Aufbauend auf die entwickelten Modelle wurden die Vorhersage und die Kompensierung implementiert. Die Ergebnisse zeigen, dass die Modelle den Effizienzverlust während des Betriebes genau vorhersagen. Das Einbrennen des Bildes in das AMOLED-Display kann durch das Anwenden der Kompensierung unterdrückt werden, so dass die Lebensdauer des Displays verlängert wird

Thin-Film Transistor Integration for Biomedical Imaging and AMOLED Displays

Thin film transistor (TFT) backplanes are being continuously researched for new applications such as active-matrix organic light emitting diode (AMOLED) displays, sensors, and x-ray imagers. However, the circuits implemented in presently available fabrication technologies including poly silicon (poly-Si), hydrogenated amorphous silicon (a-Si:H), and organic semiconductor, are prone to spatial and/or temporal non-uniformities. While current-programmed active matrix (AM) can tolerate mismatches and non-uniformity caused by aging, the long settling time is a significant limitation. Consequently, acceleration schemes are needed and are proposed to reduce the settling time to 20 µs. This technique is used in the development of a pixel circuit and system for biomedical imager and sensor. Here, a metal-insulator-semiconductor (MIS) capacitor is adopted for adjustment and boost of the circuit gain. Thus, the new pixel architecture supports multi-modality imaging for a wide range of applications with various input signal intensities. Also, for applications with lower current levels, a fast current-mode line driver is developed based on positive feedback which controls the effect of the parasitic capacitance. The measured settling time of a conventional current source is around 2 ms for a 100-nA input current and 200-pF parasitic capacitance whereas it is less than 4 μs for the driver presented here. For displays needed in mobile devices such as cell phones and DVD players, another new driving scheme is devised that provides for a high temporal stability, low-power consumption, high tolerance of temperature variations, and high resolution. The performance of the new driving scheme is demonstrated in a 9-inch fabricated display intended for DVD players. Also, a multi-modal imager pixel circuit is developed using this technique to provide for gain-adjustment capability. Here, the readout operation is not destructive, enabling the use of low-cost readout circuitry and noise reduction techniques. In addition, a highly stable and reliable driving scheme, based on step calibration is introduced for high precision displays and imagers. This scheme takes advantage of the slow aging of the electronics in the backplane to simplify the drive electronics. The other attractive features of this newly developed driving scheme are its simplicity, low-power consumption, and fast programming critical for implementation of large-area and high-resolution active matrix arrays for high precision

New driving schemes of cholesteric liquid crystal display for high speed and uniform gray-scale performance

Cholesteric LCD (Ch-LCD) is a special kind of electronic paper display. For quite a long time, lacking of fast and effective driving schemes is a primary limitation for the enhancement of its performance. In this thesis an improved dynamic driving scheme (DDS) with the ability of driving the Ch-LCD not only into the on-off state but also into several distinct gray scales has been proposed through newly designed waveform patterns. Besides, new driving scheme called as multi-selection method (MSM) is proposed for the first time to enlarge the gray scale number. In order to further enhance the gray scale’s uniformity, a fast static driving scheme with about 2ms/line is also proposed. Multiline driving scheme for Ch-LCD has been achieved and incorporated in the enhanced DDS. All of the driving schemes have been validated by using a newly designed discrete driver system including a Vertex 5 FPGA for pattern generation. Results are quite good and consistent with the expectations.Für eine lange Zeit war das Fehlen eines effektiven Treiberschemas ein Haupthindernis für die Anwendung der Cholesterischen LCDs. Als Verbesserung führen wir ein verbessertes dynamisches Treiberschemata (genannt Enhanced Dynamic Driving Scheme) ein, das ermöglicht, das Ch-LCD nicht nur in den Ein- oder Aus-Zustand, sondern auch in mehrere verschiedene Graustufen anzusteuern. Um die Anzahl der Graustufen durch das Enhanced DDS zu erhöhen, haben wir eine neue Multi-Selection-Method (MSM), vorgeschlagen. Um die Gleichmäßigkeit der Graustufen in einer hohen Ansteuergeschwindigkeit zu verbessern, schlagen wir ein Fast Static Driving Scheme, vor, das auf dem Übergang von einem stabilen Zustand in einen anderen stabilen Zustand, aber nicht über einen meta-stabilen Zustand, basiert ist. Ein weiteres Verfahren ist das Multiline Addressing Verfahren, für das Enhanced DDS entworfen, um die vier Kombinationen der angesteuerten Zuständen zu erzielen. Um die Treiberschemata in dieser Arbeit zu validieren, wurden vier diskrete Treiber-Platinen entworfen und hergestellt

Wide Bandwidth - High Accuracy Control Loops in the presence of Slow Varying Signals and Applications in Active Matrix Organic Light Emitting Displays and Sensor Arrays

This dissertation deals with the problems of modern active matrix organic light-emitting diode AMOLED display back-plane drivers and sensor arrays. The research described here, aims to classify recently utilized compensation techniques into distinct groups and further pinpoint their advantages and shortcomings. Additionally, a way of describing the loops as mathematical constructs is utilized to derive new circuits from the analog design perspective. A novel principle on display driving is derived by observing those mathematical control loop models and it is analyzed and evaluated as a novel way of pixel driving. Specifically, a new feedback current programming architecture and method is described and validated through experiments, which is compatible with AMOLED displays having the two transistor one capacitor (2T1C) pixel structure. The new pixel programming approach is compatible with all TFT technologies and can compensate for non-uniformities in both threshold voltage and carrier mobility of the pixel OLED drive TFT. Data gathered show that a pixel drive current of 20 nA can be programmed in less than 10usec. This new approach can be implemented within an AMOLED external or integrated display data driver. The method to achieve robustness in the operation of the loop is also presented here, observed through a series of measurements. All the peripheral blocks implementing the design are presented and analyzed through simulations and verified experimentally. Sources of noise are identified and eliminated, while new techniques for better isolation from digital noise are described and tested on a newly fabricated driver. Multiple versions of the new proposed circuit are outlined, simulated, fabricated and measured to evaluate their performance.A novel active matrix array approach suitable for a compact multi-channel gas sensor platform is also described. The proposed active matrix sensor array utilizes an array of P-i-N diodes each connected in series with an Inter-Digitated Electrode (IDE). The functionality of 8x8 and 16x16 sensor arrays measured through external current feedback loops is also presented for the 8x8 arrays and the detection of ammonia (NH3) and chlorine (Cl2) vapor sources is demonstrated

Pixel Circuits and Driving Schemes for Active-Matrix Organic Light-Emitting Diode Displays

Rapid progress over the last decade on thin film transistor (TFT) active matrix organic light emitting (AMOLED) displays led to the emergence of high-performance, low-power, low-cost flat panel displays. Despite the shortcomings of the active matrix that are associated with the instability and low mobility of TFTs, the amorphous silicon TFT technology still remains the primary solution for the AMOLED backplane. To take advantage of this technology, it is crucial to develop driving schemes and circuit techniques to compensate for the limitations of the TFTs. The driving schemes proposed in this thesis address these challenges, in which, the sensitivity of the OLED current to the transistor variations is reduced significantly. This is achieved by comparing the data signal with a feedback signal associated with the pixel current by means of an external driving circuit through a column feedback line. Depending on the nature of the feedback signal, (i.e. current or voltage) several pixel circuits and external drivers are proposed. New AMOLED pixel circuits with voltage and current feedback are designed, simulated, fabricated, and tested. The performance of these circuits is analyzed in terms of their stability, settling time, power efficiency, noise, and temperature-dependence. For the pixel circuits with current feedback, an operational transresistance amplifier is designed and implemented in a high-voltage CMOS process. Measurement results for both voltage and current feedback driving schemes indicate less than a 2%/V sensitivity to shifts in the threshold voltage of the TFTs. By using current feedback and an accelerating pulse, programming times less than 50 s are achieved

Active Matrix Organic Light-Emitting Displays: Novel Amorphous Silicon Thin-Film Transistors and Pixel Electrode Circuits.

Today active-matrix organic light-emitting displays (AM-OLEDs) are considered as next generation flat panel display. In this thesis, several hydrogenated amorphous silicon (a-Si:H) thin-film transistor (TFT) technologies have been developed to accelerate the AM-OLED development. Among others to address the charging time delay issue of the conventional current-driven a-Si:H TFT pixel electrode circuit, a non-linear current scaling-function by cascaded-capacitors connected to the driving TFT was investigated. To enhance the performance of fabricated pixel electrode circuit, novel design of a-Si:H pixel circuit with cascaded storage capacitors was investigated based on the current-mirror structure. The electrical and thermal stability of the proposed a-Si:H TFT pixel electrode circuits were also explored in comparison to the conventional current-driven circuit. To address the inherent electrical stability issue of the a-Si:H TFT, two novel a-Si:H TFT structures were proposed: Corbino and Hexagonal TFTs. It was shown that both a-Si:H TFT structures have the asymmetric electrical characteristics under different drain bias conditions. To extract the electrical device parameters, asymmetric geometric factors were developed for different drain bias conditions. By using multiple Hexagonal TFT structure, the output current of Hexagonal a-Si:H TFT connected in parallel increases linearly with their number within a given pixel circuit. Current-voltage measurements indicate that a high ON-OFF current ratio and a low sub-threshold slope can be maintained for multiple Hexagonal TFTs connected in parallel while the field-effect mobility and threshold voltage remain identical to a single HEX a-Si:H TFT. Due to a unique device geometry, enhanced electrical stability and larger pixel aperture ratio can be achieved in the multiple a-Si:H HEX-TFT in comparison to standard single a-Si:H TFT having same channel width. Lastly, the dynamic responses of different a-Si:H TFT structures with various storage capacitor size were explored for AM-OLEDs. The effect of dofferent storage capacitors and overlap capacitors of TFTs on the charging time and feed-through voltage characteristics of the a-Si:H switching TFT were explored. Feed-through voltage behavior of Corbino a-Si:H TFT was also discussed in comparison to normal rectangular a-Si:H TFT as a switching TFT for AM-OLEDs.Ph.D.Electrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/58418/1/hojinny_1.pd

Backplane Circuit Design with Amorphous Silicon Thin-Film Transistors for Flexible Displays

In recent years, rapid advancement in LED fabrication has enabled the possibility of using GaN micro-LEDs to be the light media in a display panel. It has superior performance in many aspects when compared with OLED technology, such as high contrast, wide viewing angle, and low power consumption. These advantages have enabled a possibility of using micro-LED technology to realize flexible displays. Currently, OLEDs need high mobility low-temperature-poly-silicon (LTPS) TFTs to be the backplane driving circuit material because lower mobility TFTs are inadequate to drive OLEDs. However, LTPS TFTs have poor uniformity over a large area due to unpredictable grain sizes and require additional fabrication processes which prevent it from being integrated onto a large-area flexible platform. On the other hand, conventional amorphous silicon (a-Si:H) technology used on LCD panels have an edge in terms of uniformity over large-area and low-cost fabrication. Even though the field-effect mobility of a-Si:H TFTs is much less than LTPS technology, it is sufficient to power up micro-LEDs with decent pixel density, which is impossible with OLEDs. However, the nature of amorphous materials gives rise to electrical instability issues. The output current of a-Si:H TFTs gradually decreases over time under electrical stress, which results in dimmer micro-LEDs in pixels. Moreover, the lack of complementary p-type TFTs in a-Si:H limits the integration of driver and control circuits onto the flexible platform to realize a full "system-on-flex". To overcome such shortcomings of a-Si:H technologies, this thesis makes a contribution in providing a solution to compensate the output current degradation by a novel pixel circuit with simple control scheme, as well as bootstrapped logic circuits that can be used as row driver and control circuits on flexible substrates. The proposed compensation pixel and row driver circuits can be combined to facilitate the realization of a "system-on-flex" backplane for a display panel with a-Si:H and micro-LED technologies