Device-circuit interactions and impact on TFT circuit-system design

This paper reviews the importance of device-circuit interactions (DCI) and its consideration when designing thin film transistor circuits and systems. We examine temperature- and process-induced variations and propose a way to evaluate the maximum achievable intrinsic performance of the TFT. This is aimed at determining when DCI becomes crucial for a specific application. Compensation methods are then reviewed to show examples of how DCI is considered in the design of AMOLED displays. Other designs such as analog front-end and image sensors are also discussed, where alternate circuits should be designed to overcome the limitations of the intrinsic device properties

Advanced Electrical Characterization of Oxide TFTs Design of a Temperature Compensated Voltage Reference

Any electronic device, regardless of its function, needs a reference voltage source that feeds reliably, i.e., which generates a constant voltage, upstream and regardless of external environmental conditions, such as temperature. Since such a characteristic negatively influences the behavior of the devices, whose base elements are transistors, it is essential to design a circuit that provides a voltage which is invariant over a temperature range. In this work is designed a circuit that is responsible for generating a reference voltage using only thin film transistors or TFTs, on glass substrate. However, in order to validate the concept used in the mentioned transistors, it is also dimensioned and simulated the proposed circuit in 130 nm CMOS technology, where the respective results are expected to be comparative between the two technologies. For CMOS technology, for a nominal reference voltage of 124,0 mV, Cadence simulation reveals ±2,2 ppm/ºC temperature coefficient, between -20 °C and 100 °C. The power consumptions are and 1,434 mW and 4,566 mW for both CMOS and IGZO-TFT technologies, respectively

전류 센싱 피드백 시스템을 이용한 고안정성 산화물 TFT 쉬프트 레지스터의 설계 및 제작

학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2017. 2. 정덕균.Integration of shift registers on the glass panel allows the display to be thinner, lighter, and cheaper to produce, thanks to the reduction of the number of ICs for scanning horizontal lines. Circuits of the shift register employing n-type thin film transistors (TFTs), such as hydrogenated amorphous silicon (a-Si:H) and oxide TFTs, have been reported. Recently, oxide TFTs attract much attention due to their high mobility (5~10 cm2/V∙s) compared with that of a-Si:H TFT (0.8cm2/V∙s). However, oxide TFTs often suffer from severe degradation of the threshold voltage (VTH) against the temperature and electrical stress. In this paper, in order to compensate the instability of oxide TFTs in the shift register, an oxide TFT with double gates, which can control VTH by varying the top gate bias (VTG) is adopted. The top gate of the double-gate TFT can be fabricated in the same process for the pixel IZO (Indium Zinc Oxide) so that an additional process only for the top gate is not required. Adequate VTG is provided timely, adaptively to the gate of the oxide TFTs to stabilize the threshold voltage. The fabrication result shows that the proposed shift register using VTG set at an adapted value become stable at 100℃ whereas the conventional one is mal-functioning. The optimum VTG varies from product to product and changes continuously over the lifetime of the display. Therefore, the feedback driving system suitable for the proposed shift register is required to search the optimum VTG. The system has two main functionsthe first is to sense the current of shift register and the second is the searching algorithm for finding the optimum VTG. When the transistors are degraded by an external stress, the current of the whole shift registers is changed. The information about the VTH degradation in the shift register can be gathered via current sensing circuit. The sensed current is integrated to generate the output and is forwarded to an ADC. The binary-converted current of shift register is processed by the proposed algorithm in the digital domain for obtaining an optimum VTG and then the result is converted back to analog to generate VTG. The IC implementing such functions is fabricated in a 0.18 μm BCDMOS process. When the shift register current is measured on the conventional system with increasing temperature up to 80℃, it is increased to more than 10 times than that at the room temperature. However, the proposed feedback system keeps a highly stable (<13%) current level of shift register up to 80℃ with an optimized VTG.Abstracts i Table of Contents iii List of Tables v List of Figures vi Chapter 1 Introduction 1 1.1 Background 2 1.2 Outline 7 Chapter 2 Review of oxide-based TFT device and N-type TFT circuit design 8 2.1 Overview 9 2.1.1 Characteristics of Oxide TFT 9 2.2 Oxide-based TFT 14 2.2.1 Electrical characteristics of oxide-based TFT 14 2.2.2 Stability of oxide-based TFT 18 2.3 NMOS driving circuit 24 2.3.1 Bootstrapping driving circuit 24 2.3.2 Shift register with n-type TFT 28 Chapter 3 Proposed Oxide TFT Shift Register 37 3.1 Overview 38 3.2 Characteristic of Double Gate TFT 39 3.3 Design of New shift register 46 3.3.1 Simulation Result of Conventional shift register 46 3.3.2 New shift register using Double Gate TFT 51 3.3.3 Simulation Modeling of Double Gate TFT 58 3.3.4 Simulation and Experimental Result 61 Chapter 4 Real Time Current-Sensing Feedback Compensation System 71 4.1 Overview 72 4.2 System Architecture 74 4.3 Circuit Design 77 4.3.1 Current Sensing Block 77 4.3.2 ADC/DAC Block 85 4.4 Optimum Point Searching Algorithm 100 4.5 System Verification 106 Chapter 5 Summary 116 Appendix A SPICE models 118 Bibliography 120Docto

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

A Piecewise Linear Approximation D/A Converter for Small Format LCD Applications

Low power operation is a driving requirement for the advancement of portable consumer electronics. As products get smaller and have more functionality the device integration requirements get tighter. This is certainly true of small format LCD applications like PDAs and cell phones. Recent advances in LCD technology have allowed for advanced circuitry to be built on the glass. This allows for the unique opportunity to integrate the LCD column driver with other circuitry rather than the traditional flip chip mounting on the glass. The integration of these D/A converters with digital circuitry presents a new set of design considerations. These considerations allow for the exploration of non-traditional architectures and algorithms. This work will explore these design considerations in detail and present a novel algorithm for conversion as well as a system implementation of this algorithm. The system implementation is compared to a standard linear converter to weigh the relative advantages of each. A high performance dynamically biased amplifier is developed for use in the D/A converter. This amplifier has a high slew rate while consuming a small amount of quiescent power

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

Current Programmed Active Pixel Sensors for Large Area Diagnostic X-ray Imaging

Rapid progress over the last decade on large area thin film transistor (TFT) arrays led to the emergence of high-performance, low-power, low-cost active matrix flat panel imagers. Despite the shortcomings associated with the instability and low mobility of TFTs, the amorphous silicon TFT technology still remains the primary solution for the backplane of flat panel imagers. The use of a-Si:H TFTs as the building block of the large area integrated circuit becomes challenging particularly when the role of the TFT is extended from traditional switching applications to on-pixel signal amplifier for large area digital imaging. This is the idea behind active pixel sensor (APS) architectures in which under each pixel an amplifier circuit consisting of one or two switching TFTs integrated with one amplifying TFT is fabricated. To take advantage of the full potential of these amplifiers, it is crucial to develop APS architectures to compensate for the limitations of the TFTs. In this thesis several APS architectures are designed, simulated, fabricated, and tested addressing these challenges using the mask sets presented in Appendix A. The proposed APS architectures can compensate for inherent stabilities of the comprising TFTs. Therefore, the sensitivity of their output data to the transistor variations is significantly suppressed. This is achieved by using a well defined external current source instead of the traditional voltage source to reset the APS architectures during the reset cycle of their periodic operation. The performance of these circuits is analyzed in terms of their stability, settling time, noise, and temperature-dependence. For appropriate readout of the current mode APS architectures, high gain transresistance amplifiers with correlated double sampling capability is designed, simulated and fabricated in CMOS technology. Measurement and measurement based calculation results reveal that the proposed APS architectures can meet even the stringent requirements of low noise, real-time digital fluoroscopy