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

TFTs circuit simulation models and analogue building block designs

Building functional thin-film-transistor (TFT) circuits is crucial for applications such as wearable, implantable and transparent electronics. Therefore, developing a compact model of an emerging semiconductor material for accurate circuit simulation is the most fundamental requirement for circuit design. Further, unique analogue building blocks are needed due to the specific properties and non-idealities of TFTs. This dissertation reviews the major developments in thin-film transistor (TFT) modelling for the computer-aided design (CAD) and simulation of circuits and systems. Following the progress in recent years on oxide TFTs, we have successfully developed a Verilog-AMS model called the CAMCAS model, which supports computer-aided circuit simulation of oxide-TFTs, with the potential to be extended to other types of TFT technology families. For analogue applications, an accurate small signal model for thin film transistors (TFTs) is presented taking into account non-idealities such as contact resistance, parasitic capacitance, and threshold voltage shift to exhibit higher accuracy in comparison with the adapted CMOS model. The model is used to extract the zeros and poles of the frequency response in analogue circuits. In particular, we consider the importance of device-circuit interactions (DCI) when designing thin film transistor circuits and systems and subsequently 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 reviewed to show examples of how DCI is considered in the design of AMOLED displays. Based on these design considerations, analogue building blocks including voltage and current references and differential amplifier stages have been designed to expand the analogue library specifically for TFT circuit design. The $V_T$ shift problem has been compensated based on unique circuit structures. For a future generation of application, where ultra low power consumption is a critical requirement, we investigate the TFT’s subthreshold operation through examining several figures of merit including intrinsic gain ($A_i$), transconductance efficiency ($g_m/I_{DS}$) and cut-off frequency ($f_T$). Here, we consider design sensitivity for biasing circuitry and the impact of device variations on low power circuit behaviour.Cambridge Commonwealth, European & International Trust, Chinese Scholarship Counci

Eurodisplay 2019

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

Amorphous Silicon Thin Film Transistor Models and Pixel Circuits for AMOLED Displays

Hydrogenated amorphous Silicon (a-Si:H) Thin Film Transistor (TFT) has many advantages and is one of the suitable choices to implement Active Matrix Organic Light-Emitting Diode (AMOLED) displays. However, the aging of a-Si:H TFT caused by electrical stress affects the stability of pixel performance. To solve this problem, following aspects are important: (1) compact device models and parameter extraction methods for TFT characterization and circuit simulation; (2) a method to simulate TFT aging by using circuit simulator so that its impact on circuit performance can be investigated by using circuit simulation; and (3) novel pixel circuits to compensate the impact of TFT aging on circuit performance. These challenges are addressed in this thesis. A compact device model to describe the static and dynamic behaviors of a-Si:H TFT is presented. Several improvements were made for better accuracy, scalability, and convergence of TFT model. New parameter extraction methods with improved accuracy and consistency were also developed. The improved compact TFT model and new parameter extraction methods are verified by measurement results. Threshold voltage shift (∆Vt) over stress time is the primary aging behavior of a-Si:H TFT under voltage stress. Circuit-level aging simulation is very useful in investigating and optimizing circuit stability. Therefore, a simulation method was developed for circuit-level ∆Vt simulation. Besides, a ∆Vt model which is compatible to circuit simulator was developed. The proposed method and model are verified by measurement results. A novel pixel circuit using a-Si:H TFTs was developed to improve the stability of OLED drive current over stress time. The ∆Vt of drive TFT caused by voltage stress is compensated by an incremental gate voltage generated by utilizing a ∆Vt-dependent charge transfer from drive TFT to a TFT-based Metal-Insulator-Semiconductor (MIS) capacitor. A second MIS capacitor is used to inject positive charge to the gate of drive TFT to improve OLED drive current. The effectiveness of the proposed pixel circuit is verified by simulation and measurement results. The proposed pixel circuit is also compared to several conventional pixel circuits.4 month

Amorphous Silicon Thin Film Transistor Models and Pixel Circuits for AMOLED Displays

Hydrogenated amorphous Silicon (a-Si:H) Thin Film Transistor (TFT) has many advantages and is one of the suitable choices to implement Active Matrix Organic Light-Emitting Diode (AMOLED) displays. However, the aging of a-Si:H TFT caused by electrical stress affects the stability of pixel performance. To solve this problem, following aspects are important: (1) compact device models and parameter extraction methods for TFT characterization and circuit simulation; (2) a method to simulate TFT aging by using circuit simulator so that its impact on circuit performance can be investigated by using circuit simulation; and (3) novel pixel circuits to compensate the impact of TFT aging on circuit performance. These challenges are addressed in this thesis. A compact device model to describe the static and dynamic behaviors of a-Si:H TFT is presented. Several improvements were made for better accuracy, scalability, and convergence of TFT model. New parameter extraction methods with improved accuracy and consistency were also developed. The improved compact TFT model and new parameter extraction methods are verified by measurement results. Threshold voltage shift (∆Vt) over stress time is the primary aging behavior of a-Si:H TFT under voltage stress. Circuit-level aging simulation is very useful in investigating and optimizing circuit stability. Therefore, a simulation method was developed for circuit-level ∆Vt simulation. Besides, a ∆Vt model which is compatible to circuit simulator was developed. The proposed method and model are verified by measurement results. A novel pixel circuit using a-Si:H TFTs was developed to improve the stability of OLED drive current over stress time. The ∆Vt of drive TFT caused by voltage stress is compensated by an incremental gate voltage generated by utilizing a ∆Vt-dependent charge transfer from drive TFT to a TFT-based Metal-Insulator-Semiconductor (MIS) capacitor. A second MIS capacitor is used to inject positive charge to the gate of drive TFT to improve OLED drive current. The effectiveness of the proposed pixel circuit is verified by simulation and measurement results. The proposed pixel circuit is also compared to several conventional pixel circuits.4 month

Pixel design and characterization of high-performance tandem OLED microdisplays

Organic Light-Emitting Diode (OLED) microdisplays - miniature Electronic Displays comprising a sandwich of organic light emitting diode over a substrate containing CMOS circuits designed to function as an active matrix backplane – were first reported in the 1990s and, since then, have advanced to the mainstream. The smaller dimensions and higher performance of CMOS circuit elements compared to that of equivalent thin film transistors implemented in technologies for large OLED display panels offer a distinct advantage for ultra-miniature display screens. Conventional OLED has suffered from lifetime degradation at high brightness and high current density. Recently, tandem-structure OLED devices have been developed using charge generation layers to implement two or more OLED units in a single stack. They can achieve higher brightness at a given current density. The combination of emissive-nature, fast response, medium to high luminance, low power consumption and appropriate lifetime makes OLED a favoured candidate for near-to-eye systems. However, it is also challenging to evaluate the pixel level optical response of OLED microdisplays as the pixel pitch is extremely small and relative low light output per pixel. Advanced CMOS Single Photon Avalanche Diode (SPAD) technology is progressing rapidly and is being deployed in a wide range of applications. It is also suggested as a replacement for photomultiplier tube (PMT) for photonic experiments that require high sensitivity. CMOS SPAD is a potential tool for better and cheaper display optical characterizations. In order to incorporate the novel tandem structure OLED within the computer aided design (CAD) flow of microdisplays, we have developed an equivalent circuit model that accurately describes the tandem OLED electrical characteristics. Specifically, new analogue pulse width modulation (PWM) pixel circuit designs have been implemented and fabricated in small arrays for test and characterization purposes. We report on the design and characterization of these novel pixel drive circuits for OLED microdisplays. Our drive circuits are designed to allow a state-of-the-art sub-pixel pitch of around 5 μm and implemented in 130 nm CMOS. A performance comparison with a previous published analogue PWM pixel is reported. Moreover, we have employed CMOS SPAD sensors to perform detailed optical measurements on the OLED microdisplay pixels at very high sampling rate (50 kHz, 10 μs exposure), very low light level (2×10-4 cd/m2) and over a very wide dynamic range (83 dB) of luminance. This offers a clear demonstration of the potential of the CMOS SPAD technology to reveal hitherto obscure details of the optical characteristics of individual and groups of OLED pixels and thereby in display metrology in general. In summary, there are three key contributions to knowledge reported in this thesis. The first is a new equivalent circuit model specifically for tandem structure OLED. The model is verified to provide accurately illustrate the electrical response of the tandem OLED with different materials. The second is the novel analogue PWM pixel achieve a 5μm sub-pixel pitch with 2.4 % pixel-to-pixel variation. The third is the new application and successful characterization experiment of OLED microdisplay pixels with SPAD sensors. It revealed the OLED pixel overshoot behaviour with a QIS SPAD sensor

Machine Learning in Sensors and Imaging

Machine learning is extending its applications in various fields, such as image processing, the Internet of Things, user interface, big data, manufacturing, management, etc. As data are required to build machine learning networks, sensors are one of the most important technologies. In addition, machine learning networks can contribute to the improvement in sensor performance and the creation of new sensor applications. This Special Issue addresses all types of machine learning applications related to sensors and imaging. It covers computer vision-based control, activity recognition, fuzzy label classification, failure classification, motor temperature estimation, the camera calibration of intelligent vehicles, error detection, color prior model, compressive sensing, wildfire risk assessment, shelf auditing, forest-growing stem volume estimation, road management, image denoising, and touchscreens