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

Backplane System Design Considerations for Micro LED Displays

Display technologies have evolved from the bulky Cathode Ray Tube based displays to the latest lightweight and low power micro-Led (uLED) based flat panel displays. A display system consists of a device technology that either manipulates the incoming light or emits its own light and a controller circuit to control the behavior of these devices. This system makes up the backplane of a display technology. uLEDs due to their small size provide higher resolution and better contrast than all the previous display technologies like the LCDs and the OLEDs. Backplane system design considerations for a uLED flat panel display is the primary focus of this work. The uLEDs are arranged in a 2-D matrix on a glass substrate with each uLED driven by an arrangement of 2 transistor and 1 capacitor that make up a pixel circuit. Indium Gallium Zinc Oxide TFTs are used as the choice of transistors for this project. The backplane design considerations are done to support an active matrix of 10x10, 50x50 and 380x380 pixel count in both monochrome and color versions. The behavior of the pixel circuit is evaluated using existing TFT and uLED electrical device compact models to determine the optimal value of the storage capacitor needed for the pixel circuit operation at 30 & 60Hz refresh rates. A model board with shift registers, transistors and LEDs to mimic the operation of a 10x10 uLED array is made and a FPGA is used to control the operation of this board. A timing relationship between the row and column data latch is deduced and the impact of the row-line, column-line RC delay and the pixel transient response time is evaluated. The impact of IR losses due to the power and ground line resistances are evaluated with the help monochrome pixel circuit physical layout. A new pixel circuit to accommodate the RGB pixels is made and care is taken to minimize both the RC delay and IR losses. Finally, a low contact resistance (0.05Ω-mm2) modular packaging scheme to electrically bond the two-dimensional array of pixel circuits on glass with the electronics on the PCB and to reduce RC delay is given

A Semi-Empirical Compact Model for IGZO TFT to Assess the Impact of Parasitic Elements in Active Matrix Pixel Designs

In this work, an empirical off-state model was developed for amorphous IGZO TFTs with the purpose of creating a compact model in conjunction with an existing on-state model. The implementation of the compact model was done in Verilog-A to assess the impact of parasitc elements such as source/drain series resistance, and source/drain-to-gate overlap capacitances in a 2T1C pixel circuit. A novel region of operation was presented defined as a bridge between the subthreshold and the on-state regions. Two approaches were followed to solve for the fitting parameters inside this bridge region; an analytical and an empirical approach. The analytical solution provided the insight that there is a point where the derivatives of the on-state and the bridge region are equal. However, this solution showed non-physical behavior at some V_DS bias. Therefore, an empirical approach was followed where experimental data was used to find the V_DS dependence and eliminate the non-physical behavior. Ultimately, the compact model provided a remarkable R^2 in relation to experimental data and allowed for convergence during circuit simulation. The parasitic element assessment was carried out and two different phenomenon were described as they relate to these elements. Charge sharing and rise and fall time were the characteristics that were present with the introduction of parasitic elements. A capacitance ratio of C_ST/C_ov =10.6pF/265.07fF≈40 was used to diminish the former. However, the large capacitances associated in the input of the driver transistor caused the falling transient to be unable to provide full voltage swing. Therefore, proper circuit functionality was not achieved based on the presented design rules. Further work is being done to diminish overlap capacitances such as self-aligned devices

Proceedings of the Second Infrared Detector Technology Workshop

The workshop focused on infrared detector, detector array, and cryogenic electronic technologies relevant to low-background space astronomy. Papers are organized into the following categories: discrete infrared detectors and readout electronics; advanced bolometers; intrinsic integrated infrared arrays; and extrinsic integrated infrared arrays. Status reports on the Space Infrared Telescope Facility (SIRTF) and Infrared Space Observatory (ISO) programs are also included

μLEDs for optogenetics

Optogenetics is unfolding new ways for us to study the nervous system and could one day be a standard approach to treat neurological diseases like epilepsy. To selectively study the effects on a subcellular level, microscopic light sources are needed. Nanostructure, light-emitting diodes (LEDs) can realize this criteria but processing to connect and protect them is necessary before any fruitful optogenetic tests can be conducted. In this work, micron sized, III-nitride, LED light sources were created using microfabrication techniques such as lithography, etching and thin film deposition. Experimental biointegration and passivation schemes were then used to build a prototype optogenetic device for stimulation of primary neurons grown [i]in vitro[/i] onto the device, in close proximity to the light emitters. Favorable electrical and optical characteristics were obtained for the individual nanostructure LEDs, lighting up brightly at a wavelength around 470 nm. However, larger devices revealed process related and uniformity challenges to overcome. Additionally, the biointegration design would prove too complex and in need of further improvement. This effort, while not outputting a fully functioning device, has contributed to development of the utilized nanostructure LED technology so that we may see more of it in the future.Imagine if I said there was a way to control brain cells with light. You might first think of the scary mind control applications but would you also consider the potential to one day eradicate neurological diseases like epilepsy? Optogenetics is a fairly new technique in medical science and it is still a long way away from fulfilling either of these scenarios but that makes it no less interesting.[/b] Today, optogenetics allow researchers to control nerve impulses by simply shining a light on cells that have been genetically modified with light sensitive properties of fluorescent algae. A common practice in optogenetics is to make cells sensitive to blue light and as luck would have it, blue light-emitting diodes, or LEDs for short, are relatively mature and straight forward to make with high quality. However, to study optogenetic effects subcellulary, for example how stimulation affects individual synapses, light sources would have to be microscopically tiny and this is where we come in. By using tapered hexagonal platelet, gallium nitride μLEDs, less than 1 μm in diameter, situated on a small sapphire chip, we set out to make a prototype device for high resolution optogenetics. LEDs were processed in Lund Nano Lab using microfabrication equipment for lithography, etching and thin film deposition before being characterized in a probe station rig. As we also wanted to be able to test actual nerve cell stimulation, we attempted to package the LEDs and passivate them for a biological environment with conducting fluids and sensitive nerve cells, which would have been grown directly onto the device, in close proximity too the LEDs. Initial testing of the single platelet LEDs showed very promising electrical properties such as the clearly rectifying diode behavior in addition to a rather extraordinary visible light output for such small light source. Continued testing though, revealed short circuiting issues for larger LEDs with several platelets being coupled together in parallel. These issues could be explained by minute variations in original platelet height and be amended with future processing tweaks. Furthermore, actual optogenetic testing had to be abandoned as the complex packaging scheme, featuring thin film oxide passivated, wire bonds, would end up malfunctioning, suggesting a redesign is needed to remove unnecessary points of failure. While we did not fully actualize the very ambitious goals we set out to achieve, our findings have undoubtedly aided in the understanding and fixing of issues with the platelet μLED technique so that development of it can progress. In a broader perspective, the technologies we explored are still highly interesting, combined and individually. Development of smaller LEDs and their use in more and more impressive optogenetic studies are published on a regular basis and inorganic μLED products are even starting to find their way onto the consumer electronics market in direct emitting, high resolution displays. To conclude, I am certain that even if this short text would have been the first time you heard about these topics, it will definitely not be the last

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

GaN Micro-LED Integration with Thin-Film Transistors for Flexible Displays

The research presented provides a systematic attempt to address the major challenges for the development of flexible micro-light-emitting diode (LED) displays. The feasibility of driving GaN-based micro-LEDs with a-Si:H-based thin-film transistors by using a thin-film bonding and transfer process was initially proposed. This approach was implemented to create an inverted pixel structure where the cathode of the LED is connected directly to the drain contact of the drive TFT resulting in a pixel circuit having more than 2× higher brightness compared to a standard pixel design. This “paste-and-cut” technique was further demonstrated for the development of flexible displays, enabling the study of the effect of mechanical strain and self-heating of the devices on plastic. Through a finite-element analysis, it was determined that the applied stress-induced strain near the quantum wells of the micro-LEDs are negligible for devices with diameters smaller than 20 microns. Thermal simulation of the LEDs on plastic revealed that a copper bond layer thicker than 600 nm can be used to alleviate self-heating effects of the micro-LEDs. Using these design parameters, micro-LED arrays with 20 micron diameter were integrated onto flexible substrates to validate the theoretical predictions. Further scaling of the LED size revealed substrate bending also tilts the direction of the LED structure, allowing further extraction of light. This effect was demonstrated using nanowire LEDs with a 250 nm diameter transferred onto plastic, where the light output could be enhanced by 2× through substrate bending. Finally, through the removal of bulk defect and surface states, fabrication of highly efficient micro-LEDs having > 400% increase in light output (compared to conventional diodes) was achieved. This outcome was accomplished through the removal of the defective buffer region adjacent to the active layers of the LED and minimization of the non-radiative recombination at the sidewalls. The former was accomplished through the removal of the buffer layer after separation of the LED from the process wafer while the latter is accomplished using a surround cathode gate electrode to deplete free carriers from the sidewall of the forward-biased LED. The resulting performance enhancements provided a basis for high-brightness flexible micro-LED displays developed in this dissertation

Electroluminescence in epitaxial thin film zns and znse

The application of the metalorganic chemical vapour deposition technique to the production of II-VI compound semiconductor electroluminescent devices is discussed. Both low field MIS minority carrier injection devices and high field impact excitation structures are considered, and comparisons are drawn with more commiercially orientated electroluminescent displays. The epitaxial growth of ZnS and ZnSe onto (100) orientated GaAs substrates, using the reactions between dimethyl zinc and the hydrides HgS and H2Se, is described. Details are given of a novel epitaxial MISi device processing technology, in which a ZnS I-layer also acts as an etch-stop, thus enabling chemical removal of the GaAs substrate. Metal electrodes deposited directly onto the ZnS and ZnSe allow the electrical and electroluminescent characteristics of these epitaxial II-VI compound layers to be investigated in the absence of any influence from the substrate material. X-ray diffraction and reflection high energy electron dififraction confirm that the structures are epitaxial and of excellent crystallinity. It is demonstrated in an electron beam induced current study that conduction in the epitaxial MIS devices is highly uniform, and this is manifested in a uniform spatial distribution of electroluminescence. A description is given of high field impact excitation electroluminescent devices, in which the ZnS layer is doped with manganese during MOCVD growth. The spatial distribution of EL in these devices is shown to be non-uniform, and thus indicative of filamentary conduction in the ZnS:Mn, in accordance with a recently proposed dielectric breakdown model of instability. It is demonstrated that the transient characteristics of the epitaxial structures correlate with those of commercial polycrystalline devices, and are also consistent with the predictions of a dynamic model of instability. As a result of filamentary conduction, both epitaxial and polycrystalline devices are prone to degradation through localised dielectric breakdown. These breakdown events generally result in a gradual erosion of the active electrode area, although, under certain operating conditions, mobile filaments can cause rapid destruction of epitaxial structures. The columnar microstructure of sputtered devices appears to prevent such filament mobility, and it is concluded that, although filamentary conduction is a result of the carrier injection mechanism and is independent of the crystallinity, the associated damage is strongly influenced by the microstructure of the device

Automatic exposure control for space sequential camera

The final report for the automatic exposure control study for space sequential cameras, for the NASA Johnson Space Center is presented. The material is shown in the same sequence that the work was performed. The purpose of the automatic exposure control is to automatically control the lens iris as well as the camera shutter so that the subject is properly exposed on the film. A study of design approaches is presented. Analysis of the light range of the spectrum covered indicates that the practical range would be from approximately 20 to 6,000 foot-lamberts, or about nine f-stops. Observation of film available from space flights shows that optimum scene illumination is apparently not present in vehicle interior photography as well as in vehicle-to-vehicle situations. The evaluation test procedure for a breadboard, and the results, which provided information for the design of a brassboard are given

Eurodisplay 2019

The collection includes abstracts of reports selected by the program by the conference committee