Threshold‐voltage‐compensation methods for AMOLED pixel and analog buffer circuits

Abstract— New pixel‐circuit designs for active‐matrix organic light‐emitting diodes (AMOLEDs) and a new analog buffer circuit for the integrated data‐driver circuit of active‐matrix liquid‐crystal displays (AMLCDs) and AMOLEDs, based on low‐temperature polycrystalline‐silicon thin‐film transistors (LTPS‐TFTs), were proposed and verified by SPICE simulation and measured results. Threshold‐voltage‐compensation pixel circuits consisting of LTPS‐TFTs, an additional control signal line, and a storage capacitor were used to enhance display‐image uniformity. A diode‐connected concept is used to calibrate the threshold‐voltage variation of the driving TFT in an AMOLED pixel circuit. An active load is added and a calibration operation is applied to study the influences on the analog buffer circuit. The proposed circuits are shown to be capable of minimizing the variation from the device characteristics through the simulation and measured results.     

Pixel‐level data‐line multiplexing for low‐cost/high‐resolution AMLCDs

Abstract— We propose a novel data‐line multiplexing technique for low‐cost/high‐resolution active‐matrix liquid‐crystal displays (AMLCDs). This scheme reduces the number of data lines and driver chips required by one‐half without enormous multiplexing circuits. Another advantage of applying this technique is the reduction in power consumption. We demonstrated the technical feasibility of this method with application prototypes up to 15‐in. SXGA+ (1400 × 1050 pixels) AMLCDs with amorphous‐silicon (a‐Si) thin‐film‐transistor (TFT) technology. In this paper, we provide an explanation of the addressing mechanism in detail and clarify the feasibility with further technical discussion.     

A touch‐sensitive display with embedded hydrogenated amorphous‐silicon photodetector arrays

Abstract— A new touch‐sensitive hydrogenated amorphous‐silicon (a‐Si:H) display with embedded optical sensor arrays is presented. The touch‐panel operation was successfully demonstrated by fabricating a prototype of a 16‐in. active‐matrix liquid‐crystal display (AMLCD). The proposed system, obviating the need for the extraction of information from the captured images in real time, provides the location of the finger touch. Due to the simple architecture of the system, the touch‐panel operation can be readily integrated within large‐area displays.     

Electrical models of TFT‐LCD panels for circuit simulations

Abstract— As thin‐film‐transistor liquid‐crystal‐display (TFT‐LCD) panels become larger and provide higher resolution, the propagation delay of the row and column lines, the voltage modulation of Vcom, and the response time of the liquid crystal affect the display images now more than in the past. It is more important to understand the electrical characteristics of TFT‐LCD panels these days. There are several commercial products that simulate the electrical and optical performance of TFT‐LCDs. Most of the simulators are made for panel designers. However, this research is for circuit, system, and panel designers. It is made in a SPICE and Cadence environment as a commercial circuit‐design tool. For circuit and system designers, it will help to design the circuit around a new driving method. Also, it can be easily modified for every situation. It also gives panel designers design concepts. This paper describes the electrical model of a 15‐in. XGA (1024 × 768) TFT‐LCD panel. The parasitic resistance and capacitance of the panel are obtained by 3‐D simulation of a subpixel. The accuracy of these data is verified by the measured values of an actual panel. The developed panel simulation platform, the equivalent circuit of a 1 5‐in. XGA panel, is simulated by HSPICE. The results of simulation are compared with those of experiment, according to changing the width of the OE signal. The proposed simulation platform for modeling TFT‐LCD panels can be especially applied to large‐sized LCD TVs. It can help panel and circuit designers to verify their ideas without making actual panels and circuits.     

Techniques with low hardware complexity of the driver electronics

Abstract— Demand for high‐quality images in mobile phones has set a new standard for displays in such portable devices. Active‐matrix liquid‐crystal displays (AMLCDs) are replacing passive‐matrix LCDs in these applications. One of the reasons is the lack of simple techniques for displaying a large number of gray shades. The goal was to develop new addressing techniques to fill this void, and it has been achieved without increasing the hardware complexity of the drive electronics. Various techniques that are capable of displaying a large number of gray shades with low hardware complexity of the driver circuit are reviewed.     

Active‐matrix sensor in AMLCD detecting liquid‐crystal capacitance with LTPS‐TFT technology

Abstract— An active‐matrix capacitive sensor for use in AMLCDs as an in‐cell touch screen has been developed. Pixel sensor circuits are embedded in each pixel by using low‐temperature polycrystalline‐silicon (LTPS) TFT technology. It detects a change in the liquid‐crystal capacitance when it is touched. It is thin, light weight, highly sensitive, and detects three or more touch events simultaneously.     

A novel diffractive backlight concept for mobile displays

Abstract— Power‐efficiency demands on mobile communications device displays have become severe with the emergence of full‐video‐capable cellular phones and mobile telephony services such as third‐generation (3G) networks. The display is the main culprit for power consumption in the mobile‐phone user interface and the backlight unit (BLU) of commonly used active‐matrix liquid‐crystal displays (AMLCDs) is the main power drain in the display. One way of reducing the power dissipation of a mobile liquid‐crystal display is to efficiently distribute and outcouple the light available in the backlight unit to direct the primary wavelength bands in a spectrum‐specific fashion through the respective color subpixels. This paper describes a diffractive‐optics approach for a novel backlight unit to realize this goal. A model grating structure was fabricated and the distribution of outcoupled light was studied. The results verify that the new BLU concept based on an array of spectrum‐specific gratings is feasible.     

A novel field‐sequential blue‐phase‐mode AMLCD

Abstract— Through the realization of a blue‐phase‐mode (hereinafter, the operational mode of liquid crystal having a blue phase is referred to as a blue‐phase mode), a display using an improved field‐sequential method was confirmed to be capable of display at a frame rate of 180 fps (field frequency of 540 Hz) or higher. Under this condition, an image without annoyance caused by color breakup was obtained. Moreover, a novel field‐sequential AMLCD integrated with a scan driver by combining the liquid‐crystal‐display (LCD) technology using blue phase and oxide‐semiconductor technology has been developed.     

Capturing asymmetrical spectral regrowth in RF systems using a multislice behavioral model and enhanced envelop transient analysis

Nonlinear distortion in RF and microwave systems results in spectral regrowth of digitally modulated signals. The distortion above and below the main channel can be at different levels and this is attributed to baseband effects. This article presents a new multislice behavioral‐model architecture that captures this asymmetry and can be implemented in a variety of circuit simulators, including SPICE, harmonic balance (HB), envelope transient (ET), and system simulators. The work is experimentally validated using an HBT power amplifier at 2.5 GHz driven by a WCDMA signal. The model is used with envelope transient circuit simulation which is enhanced to accommodate an arbitrary baseband transfer function. © 2006 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2006.     

Comparison of organic and inorganic alignment layers for low‐power liquid‐crystal devices using low‐frequency applied‐voltage waveforms

Abstract— Bistable displays are known to have low‐power consumption, but they usually lack the ability to display full‐color video‐rate images. Operating active‐matrix liquid‐crystal displays (AMLCDs) with an ultra‐wide refresh‐rate range, down to ～1 Hz, can lower the power consumption to that of bistable devices and offer video rates as well. A SiO^x‐layer‐based liquid‐crystal (LC) alignment technique is studied here to overcome the ion problems typically encountered with low‐refresh‐rate devices. The effect of ions on the voltage‐holding ratio (VHR) and residual DC voltage (RDC) with SiO^x‐based LC alignment is compared with those for polyimide‐based LC alignment. It is found that only SiO^x alignment provides high VHR and small RDC, with good stability over time. Therefore, it is shown that SiO^x alignment is an excellent candidate for low‐power AMLCDs operated with an ultra‐wide refresh‐rate range.     

An LTPS active‐matrix process with PECVD doped N+ drain/source areas

Abstract— A low‐temperature polysilicon active‐matrix process without the need for ion implantation to dope drain and source areas of TFTs has been developed. A doped silicon layer is deposited by PECVD and structured prior to the deposition of the intrinsic silicon for the channel. The dopant is diffused and activated during the excimer‐laser crystallization step. N‐channel test TFTs with different geometries were realized. The TFT properties (mobility, on/off ratio, saturation, etc.) are suitable to realize AMLCDs and AMOLED displays and to integrate driver electronics on the displays. In addition to simple TFTs, a full‐color 4‐in. quarter‐VGA AMLCD was realized. The complete display (including photolithographic masks, active‐matrix backplane, and color‐filter/black‐matrix frontplane), and an addressing system were developed and manufactured at the Chair of Display Technology, University of Stuttgart, Germany. The substitution of ion doping by PECVD deposition overcomes a major limitation for panel sizes in poly‐Si technology and avoids large investment costs for ion‐implantation equipment.     

Effect of scribe‐wheel dimensions on the cutting of AMLCD glass substrate

Abstract— The scribe‐and‐break method for glass cutting is widely used to separate individual liquid‐crystal‐display (LCD) panels from a larger motherglass substrate cell. Optimum glass‐scribing conditions including scribe‐wheel dimensions, scribing load, scribing speed, etc., have been determined based on practical manufacturing experience. However, there has been no systematic study to determine the scribing conditions necessary to avoid stray breakage. In this paper, the influence of the scribe‐wheel angle and diameter upon the scribing and breaking of an active‐matrix liquid‐crystal‐display (AMLCD) glass substrate, Corning Code1737F glass, was experimentally investigated for the case of simple single‐glass‐sheet separation. It was determined that an equation including the factors of scribe‐wheel tip angle and diameter can be used to predict median crack depth as a function of scribe load. It was further found that the breaking force of the scribed sheet was strongly influenced by the residual stress created during scribing. A wheel having a 130° tip angle and 4‐mm diameter demonstrated the best results for sheet separation in terms of the lowest breaking force without lateral crack propagation. In addition, increasing the time interval between scribing and separating was found to result in an increase in the breaking force required to separate the glass substrate.     

Motion‐blur characterization with simulation method for mobile LCDs

Abstract— A simulation method based on measured liquid‐crystal responses and human‐vision properties was proposed to characterize the motion blur of LCDs. A perceptual experiment was implemented to validate the simulation model within different viewing conditions by changing the visual angle. The results indicate that the smaller visual angle of the mobile display has no statistic significant effect on smooth‐pursuit eye tracking when perceiving a moving block on a screen. The calculation process of quantitative metric was presented based on the measured light behavior and the simulation model. In the end, the different motion‐blur reduction approaches were evaluated for mobile LCDs.     

Optimal design of optical performance of reflective ferroelectric‐ and anti‐ferroelectric liquid‐crystal displays

Abstract— In an in‐plane optical geometry, such that the average optic axis lies on the plane parallel to both substrates, the optical properties of a reflective ferroelectric liquid‐crystal (FLC) or antiferroelectric liquid‐crystal (AFLC) cell were studied within the framework of the 2 × 2 Jones matrix formalism. To obtain good achromaticity and high brightness, the cell parameters such as the molecular rotation angle and the effective phase retardation of the AFLC layer were optimized. The device performances of the AFLC cell were experimentally demonstrated in this geometry.     

Microdisplay wafer‐flatness metrology by optical interferometry

Abstract— We report a microdisplay wafer‐flatness metrology technique based on digital high‐pass filtering of topography data obtained from a commercial optical interferometer. This technique discriminates against both wafer‐scale bow/warp and pixel‐scale roughness to reveal die‐scale flatness variations that are the most relevant to microdisplay gap uniformity. We report flatness measurements of a variety of live and test silicon wafers supporting VLSI microdisplay circuitry, and show how these measurements correlate with the performance of liquid‐crystal microdisplays assembled from similar wafers. The technique is sensitive to cross‐die flatness variations as small as 25 nm in the presence of wafer bow of tens of microns. The wafer flatness variations that make the greatest contribution to liquid‐crystal cell‐gap non‐uniformity arise from interactions between the chemical mechanical planarization (CMP) process and the VLSI circuit layout. Our metrology technique can help the VLSI designer optimize microdisplay layout, and provides an objective flatness specification for wafers purchased from third‐party foundries.     

Reflective LCD with high reflectance and color reproducibility for reduced eye strain

We fabricated a prototype of a 434‐ppi 5.9‐in reflective liquid crystal display for use as an e‐book reader. With high‐level optical characteristics with a reflectance of 28.7% and an NTSC ratio of 37%, we achieved eye‐friendly reflective liquid crystal display.     

Novel liquid‐crystal materials for AMLCDs

Abstract— We have developed novel liquid‐crystal (LC) materials with a difluoromethyleneoxy CF₂O) moiety as the linkage group in order to satisfy the various requirements of active‐matrix liquid‐crystal displays (AMLCDs). We measured the physical properties of the CF₂O LC materials. The novel CF₂O LC materials have excellent physical properties, that is, high dielectric anisotropy, low viscosity, a broad nematic range, high reliability, etc. It was revealed that several advantages can be obtained by the introduction of CF₂O moiety into the LC molecules. The mixture containing CF₂O LC materials for various applications indicated a decrease in rotational viscosity of about 15–25% compared with that for conventional LC mixtures. Also, these novel mixtures with CF₂O LCM demonstrated a higher voltage‐holding ratio (VHR). It was confirmed that mixtures containing CF₂O LC materials have suitable characteristics for various applications, for example, low‐driving‐voltage note‐book PCs, quick response monitors, and transflective applications.     

Simulation and fabrication of a fast fringe‐field switching liquid crystal with enhanced surface anchoring enabled by controlled polymer topology

We demonstrate a fringe‐field switching nematic liquid crystal with electro‐optical behavior modulated by both bulk and surface polymer stabilization. The polymer is formed by ultraviolet irradiation‐induced phase separation of various amounts of a reactive monomer in the planar‐aligned nematic liquid crystal. Simulation is carried out to verify the effect of anchoring energy. Experimental evidence validates the effect of monomer concentration on transmittance–voltage and response times curves of fringe‐field switching cells. The polymer‐stabilized alignment with a higher polymer concentration escalates the interaction between the liquid crystal and the polymer structure and increases the surface anchoring energy. The polymer stabilization also improves the dynamic response times of liquid crystal. The enabling polymer‐stabilized alignment technique has excellent electro‐optical properties such as a very good dark state, high optical contrast, and fast rise and decay times that may lead to development of a wide range of applications.     

Active in‐cell touch circuit using floating common electrode as sensing pad for the large size in‐plane‐switching liquid crystal displays

In this paper, we propose an in‐cell active touch circuit using the concept of floating common electrode for large size in‐plane‐switching liquid crystal displays. Compared with the conventional passive in‐cell touch circuit, the proposed method can greatly reduce the readout channel numbers by sharing the sensing bus for one column. Because the touch signal is amplified in the pixel, the touch panel can be easily scaled up in size. Simulation results show that the difference between touch and no‐touch signals can be as large as 10 μA even with threshold voltage shift of ±1 V. The possible issues of the proposed method are experimentally verified.     

Processes,characterizations, and system applications of color‐filter liquid‐crystal‐on‐silicon microdisplays

Abstract— A color‐filter liquid‐crystal‐on‐silicon (CF‐LCOS) microdisplay that integrates color filters on silicon for color will be presented. The color‐filter process on silicon was optimized to achieve fine resolution and precise alignment of the color filters on the pixel array, good adhesion to the silicon suface, and a flat surface for the liquid‐crystal cell assembly. Important optical and electrical parameters of the color filters were extracted to establish an electro‐optical model of the CF‐LCOS microdisplays for device simulation. Thermal, chemical, and light‐stability characterizations were performed to ensure the stabilty of the color filters and CF‐LCOS microdisplays. With color CF‐LCOS microdisplays already available, the projection or viewing optics is greatly simplified. This CF‐LCOS microdisplay is ideal for near‐to‐eye displays because of its low‐power consumption and compactness. The CF‐LCOS microdisplay could also withstand medium light illumination for medium‐sized projectors. A single‐panel projector based on one CF‐LCOS microdisplay of 1280 × 768 × RGB resolution was demonstrated.