Pixel Driver Research Articles

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.

A Flexible TFT Circuit Yield Optimizer Considering Process Variation, Aging, and Bending Effects

This paper presents a yield optimization tool, FlexiOptimizer , for flexible thin-film transistor (TFT) circuits. FlexiOptimizer considers three important effects: (1) process variation; (2) aging effect; and (3) bending effect. This SPICE-based optimizer applies response surface methodology (RSM) and orthogonal array (OA) techniques to size transistors so the yield is improved in aged and bent condition. This tool is demonstrated on two different designs: organic light emitting diode (OLED) pixel drivers and differential operational amplifiers (OPAMPs), in both amorphous silicon (a-Si) and Indium-Gallium-Zinc-Oxide (IGZO) TFT technologies.

21.2: Invited Paper: MOTFT Backpanel for 880 ppi White and 440 ppi, True Full‐Color AMOLED

AbstractMOTFT‐based backpanels and corresponding manufacture related issues are discussed for a 2.5“ diagonal, 880 ppi FHD white color AMOLED. The same design can be used for a 2.5” QFHD and 5“ FHD True‐FC AMOLED Displays with 440 ppi. We demonstrate high mobility and stability MOTFT with dimensions reach the design rules of existing manufacture line. Good TFT output impedance and long‐range uniformity over large size glass substrate enables 2T1C pixel driver circuit. Such MOTFT based AMOLED backpanel can be fabricated with 5‐6 mask steps, 2 mask steps less than those used for IPS AMLCD displays with similar pixel density. Integrating an OLED array onto the MOTFT backpanel, high pixel density AMOLED display are demonstrated with performance and form factors best fit with portable and wearable electronic devices.

MIS Sensor for Luminance Control of AMOLED Pixel

Abstract Optical feedback driving schemes can enable accurate ageing and pixel brightness non-uniformity corrections in AMOLED displays. Here, we report on a pixel driver utilizing a metal-insulator-semiconductor (MIS) sensor for luminance control of the OLED element. In the proposed pixel architecture for bottom-emission AMOLEDs, the embedded MIS sensor shares the same layer stack with back-channel etched a-Si:H TFTs to maintain the fabrication simplicity. Performance characteristics of the MIS sensor are presented and analyzed to demonstrate its feasibility. Details on the pixel circuit functionality including the sensing and programming operations are also discussed.

Flexible TFT Circuit Analyzer Considering Process Variation, Aging, and Bending Effects

This paper presents a SPICE-based simulator, FlexiAnalyzer, for flexible thin-film transistor (TFT) circuits. This simulator performs four types of analysis: yield analysis, aging analysis, performance analysis, and weak spot analysis. This simulator considers three important effects: 1) threshold voltage variation of manufacturing process; 2) threshold voltage shift due to aging effect; and 3) mobility change due to bending effect. Six different OLED pixel drivers designed in 8- μm amorphous silicon TFT technology were used in simulation results. The proposed tool provides a good solution for designers to evaluate the performance and yield of flexible TFT circuits.

Tungsten Oxide as a Gate Dielectric for Highly Transparent and Temperature‐Stable Zinc‐Oxide‐Based Thin‐Film Transistors

Tungsten oxide is currently used as gate insulator in pH-sensing ion-sensitive field-effect transistors (ISFETs) and in electrochromic devices. Its great potential as a high-κ dielectric with high transparency and temperature stability is reported. Owing to the low gate voltage sweep necessary to turn the transistor on and off, a possible application could be as a low-voltage pixel driver in active-matrix displays in harsh environments.

Current-Based Testing of Optical Feedback Pixel Driver

A testing scheme of an optical feedback pixel driver is proposed, discussed and simulated by HSPICE. A basic characteristic of the specific circuit is that the gray scales of the pixel are created by pulse width modulation. So, the integral of the output <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pixel</sub> current (which is proportional to <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oled</sub> ) has to be measured or calculated and used for testing. The testing procedure that is proposed enables the detection and identification of common processing defects up to 96%, as verified from simulation results. Although <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pixel</sub> current is tend to be very low, the measuring system used, using special instrumentation ICs, is successfully simulated and verified by HSPICE.

Zinc tin oxide based driver for highly transparent active matrix OLED displays

Zinc tin oxide (ZTO) is a promising active material for transparent electronics. We have realized the first transparent active matrix OLED pixel drivers with ZTO channels. The devices are highly transparent (>80%) in the visible part of the spectrum and suitable for see-through AM OLED displays with brightness levels of 2000 cd/m 2 at 100 Hz refresh rate and full-HD resolution.

An Improved Optical Feedback Pixel Driver Circuit

An improved optical feedback circuit is proposed with only a few components and a wide range of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">data</sub> voltages. Its operation is verified by simulation using the HSPICE simulator, PSIA2 thin-film transistor (TFT) model, and our previously published light-impact model of polycrystalline TFTs. In addition, the new optical feedback pixel driver (OFPD) is compared with previously published OFPDs, and its advantages are pointed out and discussed.

Design of a 16 gray scales 320 × 240 pixels OLED-on-silicon driving circuit

A 320×240 pixel organic-light-emitting-diode-on-silicon (OLEDoS) driving circuit is implemented using the standard 0.5 μm CMOS process of CSMC. It gives 16 gray scales with integrated 4 bit D/A converters. A three-transistor voltage-programmed OLED pixel driver is proposed, which can realize the very small current driving required for the OLEDoS microdisplay. Both the D/A converter and the pixel driver are implemented with pMOS devices. The pass-transistor and capacitance in the OLED pixel driver can be used to sample the output of the D/A converter. An additional pMOS is added to OLED pixel driver, which is used to control the D/A converter operating only when one row is on. This can reduce the circuit's power consumption. This driving circuit can work properly in a frame frequency of 50 Hz, and the final layout of this circuit is given. The pixel area is 28.4 × 28.4 μm2 and the display area is 10.7 × 8.0 mm2 (the diagonal is about 13 mm). The measured pixel gray scale voltage shows that the function of the driver circuit is correct, and the power consumption of the chip is about 350 mW.

NANOCRYSTALLINE SILICON THIN FILM TRANSISTORS FOR HIGH PERFORMANCE LARGE AREA ELECTRONICS

Nanocrystalline silicon ( nc - Si ) thin film transistors (TFTs) have potential for high-performance applications in large area electronics, such as next generation of flat panel displays and medical x-ray imagers, for pixel drivers, readout circuits, as well as complementary channel logic circuits for system-on-panel integration. This potential stems from reduced threshold voltage shift and higher transconductance, compared to amorphous silicon counterpart. In this paper, we discuss various TFT structures, their associated design and performance considerations, including leakage current and threshold voltage stability mechanisms.

The influence of visible light on transparent zinc tin oxide thin film transistors

The characteristics of transparent zinc tin oxide thin film transistors (TTFTs) upon illumination with visible light are reported. Generally, a reversible decrease of threshold voltage Vth, saturation field effect mobility μsat, and an increase of the off current are found. The time scale of the recovery in the dark is governed by the persistent photoconductivity in the semiconductor. Devices with tuned [Zn]:[Sn] ratio show a shift of Vth of less 2V upon illumination at 5mW∕cm2 (brightness &amp;gt;30000cd∕m2) throughout the visible spectrum. These results demonstrate TTFTs which are candidates as pixel drivers in transparent active-matrix organic light emitting diode displays.

AMOLED pixel driver circuits based on poly-Si TFTs: A comparison

In this paper, four different driver circuits for an active matrix organic light-emitting diode (AMOLED) pixel based on thin film transistor (TFT) technology are analyzed and compared. In particular, the comparison analyzed accuracy, driving speed, power consumption and area occupied. Moreover, in order to allow array simulations, the RC equivalent models of the pixel were also derived. The results were achieved considering a QVGA display (320×240), a line frequency of 60 Hz, and were verified by simulations with AIM-SPICE.

41.3: A Low‐Power AMOLED Microdisplay with Ultra‐High Pixel Density and Extended Operating Temperature Range

AbstractA low‐power, full‐color AMOLED microdisplay with an ultra‐high color pixel density of 2300 ppi and excellent image quality is reported. Based on a new pixel driver scheme, a key feature of the display is its built‐in temperature compensation circuitry that achieves instant operation at low temperatures without the need for power‐hungry heaters or complex external functionality.

Amorphous Silicon Display Backplanes on Plastic Substrates

Amorphous silicon (a-Si) thin-film transistor (TFT) backplanes are very promising for active-matrix organic light-emitting diode displays (AMOLEDs) on plastic. The technology benefits from a large manufacturing base, simple fabrication process, and low production cost. The concern lies in the instability of the TFTs threshold voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> ) and its low device mobility. Although V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> -instability can be compensated by means of advanced multi-transistor pixel circuits, the lifetime of the display is still dependent on the TFT process quality and bias conditions. A-Si TFTs with field-effect mobility of 1.1 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vmiddots and pixel driver circuits have been fabricated on plastic substrates at 150 degC. The circuits are characterized in terms of current drive capability and long-term stability of operation. The results demonstrate sufficient and stable current delivery and the ability of the backplane on plastic to meet AMOLED requirements

Towards See‐Through Displays: Fully Transparent Thin‐Film Transistors Driving Transparent Organic Light‐Emitting Diodes

Fully transparent computer displays have, until now, been the vision of science-fiction movies. Nevertheless, there are numerous applications for these devices ranging, for example, from head-mounted displays to their integration in automotive windshields. In this paper we demonstrate that this vision is on the verge of becoming reality. The realization of entirely transparent displays requires both transparent light-emitting devices, in our case organic light-emitting diodes (OLEDs) with transparent contacts, and a driving scheme based on transparent thin-film transistors (TTFTs). Since the early reports on electroluminescence from multilayer, thin-film devices composed of vacuum-sublimed small organic molecules [1] and spin-coated conjugated polymers, [2] substantial research has been devoted to the improvement of device efficiencies, color purity, and lifetime. Incitement of this development is the potential use of OLEDs in future commercial flat-panel displays. OLEDs usually consist of a layer sequence of organic functional materials (charge transporters/blockers/emitters) with an overall thickness of the order of 100 nm. Most of these materials absorb light in the deep-blue or ultraviolet spectral region and are nearly transparent in the visible part of the spectrum. Organic layers applied to emit visible light are often based on so-called guest–host systems, in which a wide-bandgap host material (absorbing in the UVonly) is doped with a few weight percent of a light-emitting dye. [3] As a result, the emitting layer appears transparent. Transparent conductive oxides, most prominently indium tin oxide (ITO) and aluminum-doped ZnO (AZO), may be used as electrical contacts to OLEDs. Therefore, OLEDs seem to be promising devices for the realization of entirely transparent visible-light emitters. [4–6] OLED displays driven in passive-matrix mode are based on conventional bottom-emitting OLEDs and are considered as an approach to fabricate small-sized, low-information-content displays with moderate pixel counts. To accomplish largerarea, high-resolution OLED displays an active-matrix-addressing scheme has been suggested. [7] Conventional a-Si:H or poly-Si TFT backplanes are not suitable as drivers for transparent displays because they are opaque in the visible part of the spectrum. Putting pixels and transistors next to each other would compromise the displays’ fill factor. Organic field-effect transistors (OFETs) as pixel drivers for OLEDs have been discussed by Sirringhaus et al. [8] Transparent OFETs have also been reported. [9] However, their performance is still poor; transparent active pixels using OFETs have not yet been demonstrated. Therefore, in this paper we focus on the production of TTFTs based on the wide-bandgap oxide semiconductor zinc tin oxide (ZnO)x(SnO2)1–x ,a s a viable alternative to previously fabricated devices. Recently, research on TTFTs with channels made from ox

Device and Circuit Level Optimization for High Performance a-Si:H TFT-Based AMOLED Displays

Active matrix organic light-emitting diode (AMOLED) displays with amorphous hydrogenated silicon (a-Si:H) thin-film transistor (TFT) backplanes are becoming the state of art in display technology. Though a-Si:H TFTs suffer from an intrinsic device instability, which inturn leads to an instability in pixel brightness, there have been many pixel driving methods that have been introduced to counter this. However, there are issues with these circuits which limit their applicability in terms of speed and resolution. This paper highlights these issues and provides detailed design considerations for the choice of pixel driver circuits in general. In particular, we discuss the circuit and device level optimization of the pixel driver circuit in a-Si:H TFT AMOLED, displays for high gray scale accuracy, subject to constraints of power consumption, and temporal and spatial resolution.

Improvement of pixel electrode circuit for active-matrix OLED by application of reversed-biased voltage

In this paper, an improved ac pixel electrode circuit for active-matrix organic light-emitting display (AMOLED) has been proposed by adding a thin-film transistor. This circuit can provide an ac driving mode for AMOLED and makes the OLED in a reversed-biased voltage during the reverse cycle. And a circuit design for understanding ac driving mode was presented. The circuit simulation results indicate that this circuit is feasible. The circuit structure is practical for the AMOLED pixel driver; it can improve the performance of OLED.

Orientation-dependent strain tolerance of amorphous silicon transistors and pixel circuits for elastic organic light-emitting diode displays

We examine the orientation dependence of strain-induced shifts in current and mobility of hydrogenated amorphous silicon thin-film transistors (TFTs). Longitudinal and transverse TFT mirrors are presented for design of strain-tolerant ΔVT-compensating pixel driver circuits for active-matrix organic light-emitting diode displays.

33.3: Source-Gated Transistors in Amorphous Silicon for Active Matrix Displays

Compared with a FET, the Source-Gated Transistor (SGT) has a lower saturation voltage and can operate with smaller carrier concentrations and higher internal fields. OLED arrays with amorphous silicon SGT pixel drivers would dissipate less power and have better stability.