Active-matrix Light-emitting Diode Research Articles

77‐1: Invited Paper: Towards Commercialization of Vertical, Organic Light‐emittingTransistors for Active‐Matrix Displays

Vertical, organic, light‐emitting transistors (VOLETs) combine three of the four essential components of an active matrix lightemitting diode (AMOLED) pixel ‐ the drive transistor, the storage capacitor, and the OLED stack ‐ into a single, highly‐stable, selfemissive structure. VOLETs can be manufactured on amorphous silicon (a‐Si) thin‐film transistor (TFT) backplanes to enable lowcost AMOLED displays with improved brightness and lifetime characteristics. We report on the current status of VOLET development and prospective impact on the active‐matrix display industry.

A Fully-Integrated Micro-Display System With Hybrid Voltage Regulator

A fully-integrated active matrix light-emitting diode (AMLED) micro-display system is demonstrated in this paper. The system consists of a 36 × 64 AMLED array chip based on GaN-on-Silicon epilayers and a silicon driving chip integrating an on-chip hybrid voltage regulator, pixel drivers, and peripheral circuits. Then two chips are bonded with low-cost Au-free flip-chip bonding technology. As such, this system is able to directly operate with a battery without any external passive components. Multiple techniques were proposed to achieve the system integration: (1) A fully-integrated hybrid voltage regulator consisting of a step-up switched-capacitor (SC) converter cascaded by a step-down linear voltage regulator was fabricated in a 0.18 μm CMOS technology. It achieved only 1.38% output voltage ripple without external capacitors, and be able to deliver a maximum power of 216mW with peak efficiencies of 91% in the linear mode and 78% in the SC mode; (2) Micro-LEDs are fabricated on GaN-on-Silicon substrate that significantly suppresses the light crosstalk between pixels and reduces the cost; (3) An Au-free flip-chip bonding technique is developed to reduce the bonding cost and achieved reliable connections between AMLED array chip and silicon driving chip. The whole system achieved 635 pixel/in (PPI) pixel density. Images and videos with 4-bit grayscale could be rendered. This prototype achieved a high integration level and demonstrates the tremendous potential of the highly efficient, low cost fully integrated AMLED micro-display system.

Bit Depth of Drivers for Micro-LED Displays Adopting Low-Temperature Polysilicon Oxide Thin-Film Transistors

In this paper, the minimum bit depth of drivers for active-matrix light emitting diode (AMLED) displays has been specified to overcome luminance nonuniformity of LEDs for the first time. The pixel circuit comprises seven transistors and one capacitor (7T1C) based on low-temperature polysilicon oxide (LTPO) process. For the experiments, thirty green and blue LEDs were used. When the same data voltage was applied, there was luminance nonuniformity of up to 19 % and 41 % for green and blue LEDs compared to the reference LEDs. Data voltages were measured to get the same luminance as the target one. It was found that the maximum voltage intervals of 0.9 mV and 1.1 mV were necessary to overcome the luminance nonuniformity for green and blue LEDs, respectively. 1.62 V and 2.80 V were the data voltage ranges of green and blue LEDs, respectively. The bit depth of 11-bit (2,048) is the minimum one because 1.62 V / 0.9 mV for green LEDs is smaller than 2,047. In this way, at least 12-bit is necessary for the bit depth of drivers for blue LEDs.

Displaying-Synchronous Open-Loop External Compensation for Active-Matrix Light Emitting Diode Displays

External compensation approaches, using source drivers to sense and compensate the different degradation of array pixels, have been proposed for driving active matrix light emitting diode (AMLED) displays. With external compensation, much simpler pixel circuits are able to be adopted for higher manufacturing yield, and more room to improve resolution and integrate added functions. This brief proposed a displaying-synchronous open-loop (DSOL) external compensation method with a simple pixel circuit design. The sensing and compensation is performed synchronously during several continuous display frames with normal displaying. A display performance simulation flow is developed to verify the driving method. The simulation results on single pixels and overall displays with various input images and videos prove the capability of the proposed DSOL method for compensating the image quality degradation without affecting normal displaying.

A Multiphase Switched-Capacitor Converter for Fully Integrated AMLED Microdisplay System

A fully integrated switched-capacitor (SC) dc-dc converter is designed for powering up an active-matrix light-emitting diode (AMLED) microdisplay system. The stacking-transistor technique with three voltage conversion ratios is utilized to handle the lithium-ion battery range of 2.6-4.2 V and to reduce the switching loss. A 101-phase interleaving scheme is adopted such that, first, output voltage ripple ΔV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</sub> is significantly reduced; second, no external output capacitor is needed; and, third, SC power-cells are physically cascaded to encircle the LED array and drivers to reduce conduction loss. Two test chips were designed in the 0.18-μm 1P6M CMOS technology. The first chip is a test chip comprising the SC converter only. The output voltage is 4.2 V, and the measured maximum ΔV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</sub> is 52 mV. The measured power density is 74.4 mW/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , peak efficiency is 81%, and maximum output power is 258 mW. The second chip is a system chip integrating the SC converter with the AMLED array and drivers using flip-chip packaging. The converter delivers a maximum power of 216 mW with a 78% peak efficiency in the SC mode and a 91% peak efficiency in the linear regulator mode.

Flexible 64 × 64 Pixel AMOLED Displays Driven by Uniform Carbon Nanotube Thin-Film Transistors.

Carbon nanotube (CNT) thin-film transistors are expected to be promising for use in flexible electronics including flexible and transparent integrated circuits and in wearable chemical and physical sensors and for driving the circuits of flexible display panels. However, current devices based on CNT channels suffer from poor performance uniformity and low manufacturing yield; therefore, they are still far from being practical. This is usually caused by nonuniform deposition of the semiconducting CNTs and the rough surface of flexible substrates. Here, we report CNT thin-film transistors (TFTs) driving a flexible 64 × 64 pixel active matrix light-emitting diode display (AMOLED) by improving the formation of uniform CNT films and developing a new pretreatment technique for flexible substrates. The achieved AMOLED has uniform brightness and a high yield of 99.93% in its 4096 pixels. More than 8000 TFTs with high-purity semiconducting CNTs as the channel material show an average on-off current ratio of ∼107 and a carrier mobility of 16 cm2 V-1 s-1. The standard deviations of the on-state current and the carrier mobility are 4.1 and 6.5%, respectively. Our result shows that the panel driven by high-purity semiconducting CNTs is a promising strategy for the development of next-generation flexible, large-area displays.

Active‐Matrix GaN µ‐LED Display Using Oxide Thin‐Film Transistor Backplane and Flip Chip LED Bonding

AbstractA 2 in. active‐matrix light‐emitting diode (AMLED) display by integration of the micro‐LED onto the oxide thin‐film transistor (TFT) backplane using flip chip bonding is reported. A blue‐emitting micro‐LED (µ‐LED) with a size of 90 × 50 µm2 is fabricated on the GaN epi grown on a sapphire substrate. The amorphous indium‐gallium‐zinc‐oxide (a‐IGZO) TFT on glass exhibiting the mobility of 18.4 cm2 V−1 s−1, turn‐on voltage (V ON) of 0.2 V, and subthreshold swing 0.25 V dec−1, is used for LED backplane. A two TFT and one capacitance pixel structure is utilized for driving 128 × 384 AMLED with 120 Hz frame rate. The laser lift‐off process with flip‐chip bond allows the transfer of the µ‐LED chips with 49 152 pixels onto the TFT backplane, demonstrating a 2 in. AMLED display with a good gray scale image. The current efficiency of µ‐LED is found to be 12.9 Cd A−1 at the luminance of 630 Cd m−2. Therefore, a‐IGZO TFT backplane can be used for µ‐LED displays.

Joint Modeling of Linear Degradation and Multiple Dependent Competing Risks Data under a Step-Stress Accelerated Degradation Test

The step-stress accelerated degradation test (SSADT) is one of the most commonly used time-dependent types of stress loading tests that enables a shorter test duration. This test is more economical and flexible compared to accelerated degradation test or accelerated failure time (ADT/AFT) test plans. This test is suitable when constraints by test facilities, conditions, or samples exist. SSADT is more useful for developing products when there is inadequate knowledge for test conditions. Important aspects here are to evaluate each failure mode in the presence of the other modes by assuming some dependency structure between the failure modes due to the non-applicability of studying test units with isolated competing failure modes. This paper aims to propose a modeling approach to simultaneously analyze linear degradation data and failure time data with dependent competing risks in an SSADT experiment. We use the copula function to consider the dependency structures between two failure modes. The proposed model is applied to acceleration data, to simulation data sets and two real data sets- bus tire data set and plastic substrate active matrix light-emitting diodes (AMOLED) data set, accompanied by sensitivity analysis.

P‐214: Late‐News Poster: Stretchable Active‐Matrix Light‐Emitting Diode Array Using Printed Electric Components on Plastic and Elastomer Hybrid Substrate

We present stretchable active‐matrix light‐emitting diode (AMLED) array using printed components, such as stretchable electrodes, electrochemical transistor, contact pads, on plastic and elastomer hybrid substrate. Inkjet‐printing and facile chip‐bonding methods were used to implement stretchable AMLED array that stably operates under biaxial strain of 20%

DL Prof. Patrick Yue Speaks at IMECAS [Chapters

IEEE Solid-State Circuits Society (SSCS) Distinguished Lecturer (DL) Prof. Patrick Yue visited the Institute of Microelectronics of the Chinese Academy of Sciences (IMECAS), China, on 12 October 2017. He gave an inspiring talk, "Recent Developments in Visible Light Communication-Applications and SoC Design," which drew over 70 attendees, including IMECAS faculties, graduate students, undergraduate students, and local professionals. After the presentation, many interesting questions were asked and led to a thoughtful discussion. This talk presents two advanced visible light communication (VLC) modulator system-on-chips (SoCs). The first is an IEEE 802.15.7 PHY-I standard-compliant VLC transmitter. The second is an active matrix light-emitting diode (LED) microdisplay driver SoC with embedded VLC function.

A Driving Method of Pixel Circuit Using a-IGZO TFT for Suppression of Threshold Voltage Shift in AMLED Displays

A driving method of pixel circuit using amorphous indium gallium zinc oxide (a-IGZO) thin-film transistor (TFT) is proposed to improve the image quality of active matrix light-emitting diode displays. The proposed pixel circuit employs a diode-connected structure to compensate for variation in threshold voltage ( $V_{\sf {th}}$ ) of the a-IGZO TFT. In addition, the proposed driving method adopts negative bias annealing to suppress the $V_{\sf {th}}$ shift. The annealing time is optimized based on the experimental observation of the minimum $V_{\sf {th}}$ shift. After a stress time of 30000 s, the measurement results show that the $V_{\sf {th}}$ shift is reduced by 29.6%, using an optimized annealing time of 5% of one frame time. In addition, the maximum deviation in the emission current using the proposed driving method was measured to be less than 4.32% after a stress time of 30000 s.

Design and Characterization of Active Matrix LED Microdisplays With Embedded Visible Light Communication Transmitter

This paper presents the design and characterization of the first active matrix light-emitting diode (AMLED) microdisplay with an embedded visible light communication (VLC) transmitter, enabling LED digital signage for location-based applications such as information broadcasters and indoor positioning beacons. The driver system-on-a-chip (SoC) integrates four identical macro-cells, each containing a pixel driver array, a row driver, a column driver, and a first-in first-out memory, to drive a wide quarter-VGA (WQVGA) display featuring 400 × 240 blue micro-LED (µLED) pixels fabricated on a single gallium nitride (GaN) substrate. The size of each µLED pixel is 30 × 30 µm2. At the system level, pulse-width modulation (PWM) superimposed with on–off keying modulation is proposed to accomplish grayscale control for display and simultaneously transmit VLC signal by modulating the µLED array. At the circuit level, a pixel driver cell composed of three transistors and one capacitor (3T1C) with a novel VLC function is employed to implement the control scheme. Flip-Chip bonding is adopted to establish connections between the WQVGA microdisplay and the AMLED driver SoC. Implemented in a 0.5-µm 2P3M CMOS process, the driver SoC enables a high-resolution microdisplay module to achieve 4-bit grayscale at a 100-Hz frame rate, while supporting 1.25-Mb/s VLC for a bit error rate 500 cm.

Reliability Estimation from Linear Degradation and Failure Time Data With Competing Risks Under a Step-Stress Accelerated Degradation Test

The accelerated degradation test (ADT) has become critical for product reliability assessment. In performing the ADT for newly developed products, a constant-stress ADT may be impractical or sometimes impossible where the available size of testing units and the testing duration are heavily bounded to meet the short development period of the products. As an alternative, a step-stress accelerated degradation test (SSADT) can be a useful tool for satisfying the test limitation, and for making up for uncertainty in selecting appropriate levels of stress. Occasionally, the elevated stress under SSADT not only accelerates the performance degradation of products, but it may also expedite traumatic failures. This paper proposes a modeling approach to simultaneously analyze linear degradation data and traumatic failures with competing risks in an SSADT experiment. Under the modeling approach, a cumulative exposure model is considered. The failure rate corresponding to each failure mode is described as a function of the degradation level at the moment of failure. No parametric assumptions are made regarding the failure-time distribution to extend the proposed method to more general cases. We derive maximum likelihood estimates of the model parameters, then estimate failure rates and product reliability based on the degradation level to failure. Asymptotic properties of the maximum likelihood estimates are also discussed. The proposed model is applied to accelerated degradation data from plastic substrate active matrix light-emitting diodes (AMOLEDs), along with sensitivity analysis.

Improvement of uniformity in active-matrix light-emitting diodes

Improvement of uniformity in active-matrix light-emitting diodes