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

Review of Display Technologies Focusing on Power Consumption

Producción CientíficaThis paper provides an overview of the main manufacturing technologies of displays, focusing on those with low and ultra-low levels of power consumption, which make them suitable for current societal needs. Considering the typified value obtained from the manufacturer’s specifications, four technologies—Liquid Crystal Displays, electronic paper, Organic Light-Emitting Display and Electroluminescent Displays—were selected in a first iteration. For each of them, several features, including size and brightness, were assessed in order to ascertain possible proportional relationships with the rate of consumption. To normalize the comparison between different display types, relative units such as the surface power density and the display frontal intensity efficiency were proposed. Organic light-emitting display had the best results in terms of power density for small display sizes. For larger sizes, it performs less satisfactorily than Liquid Crystal Displays in terms of energy efficiency.Junta de Castilla y León (Programa de apoyo a proyectos de investigación-Ref. VA036U14)Junta de Castilla y León (programa de apoyo a proyectos de investigación - Ref. VA013A12-2)Ministerio de Economía, Industria y Competitividad (Grant DPI2014-56500-R

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

Comparison of pentacene and amorphous silicon AMOLED display driver circuits

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Amorphous In-Ga-Zn-O Thin Film Transistors for Active-Matrix Organic Light-Emitting Displays.

Active-matrix organic light-emitting display (AMOLED) is now generally viewed as the next generation display because of its vivid color, high contrast ratio, thin/light module, and low energy consumption. So far, most reported pixel circuits are either based on low temperature polysilicon (LTPS) thin film transistors (TFTs) or hydrogenated amorphous silicon (a-Si:H) TFTs. Both backplane technologies have their own shortcomings, such as nonuniformity of LTPS TFTs, low field-effect mobility and threshold voltage instability of a-Si:H TFTs. As a result, TFTs based on other semiconductor materials have been explored as an alternative approach to realize reliable, high resolution AMOLEDs. Among all, amorphous In-Ga-Zn-O (a- IGZO) TFTs possess certain advantages including visible transparency, low processing temperature, uniformity over large area, and good electrical performance, which make them very attractive for AMOLEDs. The focus of this work has been to provide a more thorough understanding of the device performance of a-IGZO TFTs, along with the underlying semiconductor physics and their possible application to AMOLEDs. Firstly, the electronic structure of crystalline In-Ga-Zn-O was studied by ab initio quantum mechanics calculation. Then the electrical properties of a-IGZO TFTs were described, including the gate voltage dependent field-effect mobility and source/drain contact resistance. The operation principles of a-IGZO TFTs were further investigated by the channel region surface potential profile obtained by scanning Kelvin probe microscopy. The effect of temperature on the electrical properties of a-IGZO TFTs was investigated. The thermally activated drain current was explored, and the density of deep states profile was calculated from measured data. Current temperature stress measurements were performed on a-IGZO TFTs. Several factors were considered when investigating the electrically stability of the devices, including the stress time, stress temperature, stress current, and TFT biasing conditions. Finally, a-IGZO TFT SPICE model was developed based on the RPI a-Si:H TFT model. Several voltage- and current-programmed AMOLED pixel circuits were simulated. The effect of threshold voltage variation on the pixel circuit performance was investigated, and the potential advantages of using a-IGZO TFTs were discussed.Ph.D.Electrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/77902/1/charchic_1.pd

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

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

Amorphous Silicon Dual Gate Thin Film Transistor & Phase Response Touch Screen Readout Scheme for Handheld Electronics Interactive AMOLED Displays

Interactive handheld electronic displays use hydrogenated amorphous silicon (a-Si:H) thin film transistor (TFT) as a backplane and a Touch Screen Panel (TSP) on top as an input device. The low mobility and instability of a-Si:H TFT threshold voltage are major two issues for driving constant current as required for Active Matrix Organic Light Emitting Ddiode (AMOLED) displays. Low mobility is compensated by increasing transistor width or resorting to more expensive material TFTs. On the other hand, the ever increasing threshold voltage shift degrades the drain current under electrical operation causing OLED display to dim. Mutual capacitive TSP, the current cell phone standard, requires two layers of metals and a dielectric to be put in front of the display, further dimming the device and adding to visual noise due to sun reflection, not to mention increased integration cost and decreased yield. This thesis focuses on the aforementioned technological hurdles of a handheld electronic display by proposing a dual-gate TFT used as an OLED current driving TFT and a novel phase response readout scheme that can be applied to a one metal track TSP. Our dual-gate TFT has shown on average 20% increase in drive current over a single gate TFT fabricated in the same batch, attributed to the aid of a top channel to the convention bottom channel TFT. Furthermore the dual gate TFT shows three times the Poole-Frenkel current than the single gate TFT attributed to the increase in gate to drain overlap. The dual-gate TFT shows a 50% improvement in threshold voltage shift over a single gate TFT at room temperature, but only ~8% improvement under 75ºC. This is an important observation as it shows an accelerated threshold voltage shift in the dual-gate. This difference in the rate of threshold voltage change under varying temperature is attributed to the difference in interface states, supporting Libsch and Kanicki’s multi-level temperature dependant dielectric trapping model. The phase response TSP readout scheme requires IC only on one side of the display. Cadence Spectre simulation results showed that both touch occurrence and touch position can be obtained using only one metal layer

Amorphous Silicon Dual Gate Thin Film Transistor & Phase Response Touch Screen Readout Scheme for Handheld Electronics Interactive AMOLED Displays

Interactive handheld electronic displays use hydrogenated amorphous silicon (a-Si:H) thin film transistor (TFT) as a backplane and a Touch Screen Panel (TSP) on top as an input device. The low mobility and instability of a-Si:H TFT threshold voltage are major two issues for driving constant current as required for Active Matrix Organic Light Emitting Ddiode (AMOLED) displays. Low mobility is compensated by increasing transistor width or resorting to more expensive material TFTs. On the other hand, the ever increasing threshold voltage shift degrades the drain current under electrical operation causing OLED display to dim. Mutual capacitive TSP, the current cell phone standard, requires two layers of metals and a dielectric to be put in front of the display, further dimming the device and adding to visual noise due to sun reflection, not to mention increased integration cost and decreased yield. This thesis focuses on the aforementioned technological hurdles of a handheld electronic display by proposing a dual-gate TFT used as an OLED current driving TFT and a novel phase response readout scheme that can be applied to a one metal track TSP. Our dual-gate TFT has shown on average 20% increase in drive current over a single gate TFT fabricated in the same batch, attributed to the aid of a top channel to the convention bottom channel TFT. Furthermore the dual gate TFT shows three times the Poole-Frenkel current than the single gate TFT attributed to the increase in gate to drain overlap. The dual-gate TFT shows a 50% improvement in threshold voltage shift over a single gate TFT at room temperature, but only ~8% improvement under 75ºC. This is an important observation as it shows an accelerated threshold voltage shift in the dual-gate. This difference in the rate of threshold voltage change under varying temperature is attributed to the difference in interface states, supporting Libsch and Kanicki’s multi-level temperature dependant dielectric trapping model. The phase response TSP readout scheme requires IC only on one side of the display. Cadence Spectre simulation results showed that both touch occurrence and touch position can be obtained using only one metal layer