Active Matrix Liquid Crystal Display Research Articles

A New Gate Induced Barrier Thin-Film Transistor (GIB-TFT) for Active Matrix Liquid Crystal Displays: Design and Performance Considerations

Reducing the OFF-state leakage current, and eliminating the pseudo-subthreshold conduction in polysilicon thin-film transistors (TFTs), is a major problem since the channel current is controlled by the potential barrier associated with the grain boundaries in the undoped channel. In this paper, we present for the time, a new gate-induced barrier TFT (GIB-TFT) in which the undoped channel has no grain boundaries, while the channel current is controlled by inducing large potential barriers in the channel. We demonstrate that the proposed GIB-TFT exhibits a steep subthreshold slope and at least three orders of magnitude less OFF-state leakage current when compared to the conventional polysilicon TFTs. Using two-dimensional and two-carrier device simulation, we have analyzed the various performance and design considerations of the GIB-TFT and explained the reasons for its improved performance

A Sampling Switch Design Procedure for Active Matrix Liquid Crystal Displays

In the design of an active matrix LCD (Liquid Crystal Display), the ratio of the pixel voltage to the video voltage (RPV) of a pixel is an important factor of the performance of the LCD, since the pixel voltage of each pixel determines its transmitted luminance. Thus, of practical importance is the issue of how to maintain the admissible allowance of RPV of each pixel within a prescribed narrow range. This constraint on RPV is analyzed in terms of circuit parameters associated with the sampling switch and sampling pulse of a column driver in the LCD. With the use of a minimal set of such circuit parameters, a design procedure is described dedicatedly for the sampling switch, which intends to seek an optimal sampling switch as well as an optimal sampling pulse waveform. A number of experimental results show that an optimal sampling switch attained by the proposed procedure yields a source driver with almost 18% less power consumption than the one by manual design. Moreover, the percentage of the RPVs within 100±1% among 270 cases of fluctuations is 88.1% for the optimal sampling switch, but 46.7% for the manual design.

Full-Field Digital Mammography on LCD Versus CRT Monitors

Our purpose was to determine if the display of full-field digital mammograms on a 5-megapixel liquid crystal display (LCD) monitor is at least equivalent to the display of the same on a 5-megapixel cathode ray tube (CRT) monitor. Five radiologists evaluated normal anatomy and features of 61 abnormalities in 48 full-field digital mammograms. A 9-point Likert scale was used to compare images on two identical soft-copy review workstations, one equipped with two 5-megapixel CRTs and the other with two 5-megapixel LCDs. Outcomes were evaluated using a random-effects analysis of variance model. Means and SEs were reported. Ninety-five percent confidence intervals and p values were calculated. The two systems were equivalent for most features. The LCDs were rated better for the sharpness of mass margins (p = 0.011) and mass conspicuity (p = 0.050). For calcium features, the LCDs were rated better than the CRTs for lesion conspicuity (p = 0.010) and number of calcifications (p = 0.043). For architectural distortions, there was no statistically significant difference between the monitors in any of the features evaluated. For display characteristics, the LCDs were better for luminance (p = 0.021). The CRTs were significantly better for image noise (p = 0.001). In the overall ratings, there was no statistically significant difference between the two displays. The 5-megapixel monochrome active-matrix LCD is equivalent to and in some respects better than the 5-megapixel CRT display for full-field digital mammograms over a range of normal and abnormal findings.

Process and Characteristics of Fully Silicided Source/Drain (FSD) Thin-Film Transistors

In this paper, high-performance fully silicided source/drain (FSD) thin-film transistors (TFTs) with FSD and ul-trashort source/drain extension (SDE) fabricated by the implant-to-silicide (ITS) technique are studied thoroughly. Using the ITS technique, not only the implantation damage but also the silicide spiking is avoided so that the thermal budget can be decreased obviously. The offstate current (Ioff) of the FSD TFTs is equal to (n-channel) or smaller than (p-channel) that of the conventional TFTs. At onstate, due to the FSD and the SDE structure, the parasitic resistance of the S/D region and the carrier-injection resistance between silicide and channel are reduced. Therefore, superior onstate/offstate current ratio can be obtained. The influences of annealing temperature and time are also examined in this paper. A 600degC/30-s rapid thermal annealing is sufficient to diffuse and activate dopants and, then, fabricate high-performance FSD TFTs. Excellent short-channel behavior of the FSD TFT is also confirmed. To conclude, the high-performance FSD TFT with low parasitic resistance fabricated by low-thermal-budget process is very promising for active-matrix liquid-crystal display, active-matrix organic light-emitting-diode display, and system-on-panel applications

Solution-deposited carbon nanotube layers for flexible display applications

We have investigated two possible fields of application for carbon nanotube (CNT) networks in flexible displays. Transparent and conductive layers of CNTs were spray coated onto glass and plastic substrates. The spectral transmission of the produced layers is almost even for all wavelengths in the visible regime. A sheet resistance of 400 Ω/□ at a transmittance of 80% was achieved. Thin-film transistors (TFT) were created on silicon wafers and glass substrates using low-density CNT networks as a semiconducting layer. The process used for device fabrication on glass substrates is fully compatible to application on plastic foils. The transistors reach on/off ratios of more than five orders of magnitude and show device charge carrier mobilities in the order of 1 cm 2/Vs. These values promise an application in active matrix liquid crystal displays (AMLCD). Issues that need to be addressed are the homogeneity and reproducibility of the device properties.

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.

SU‐FF‐I‐73: Comparison of the Effects of Viewing Conditions and Viewing Angle On Object Dectectability for Different AMLCD Displays

Purpose: The ability to interpret images displayed on active matrix liquid crystal displays (AMLCD) can be influenced by factors such as display luminance, surrounding background, room illuminance and viewing angle. We have been investigating how these parameters influence reader scores with images featuring both small objects and low contrast as typically seen in mammography. We are in a position to make some comparisons between the results obtained with displays from two different manufacturers. Method and Materials: Reader studies were conducted using a computer generated contrast detail phantom alternately presented against a display background of selected luminance levels. Luminance was also measured at different viewing angles and at four selected room illuminance levels. Image scoring was performed at each combination of background level, viewing angle and room illuminance level. Results: Image scoring performance was interpreted using k values, which reflect the contrast and diameter of the objects detected in the images. The best image scoring results were obtained when viewing angles were kept small, and also when room illuminance was at the level of 5 – 10 lux. Better scoring results were also obtained when the image background luminance was adjusted to 5 – 20 % of maximum. These results differ from what had previously been found when evaluating displays from a different manufacturer. In this case the best scoring results were obtained at zero background and the room illuminance did not seem to have a significant effect on the results obtained when kept in the range of 0 – 20 lux. Conclusion: The results support the view that while it is advisable to keep the viewing angle to a minimum, it may be advisable also to adjust the room illuminance and monitor background luminance to specific levels which may be best suited for a particular AMLCD display.

Development of High-Performance Organic Thin-Film Transistors for Large-Area Displays

AbstractOrganic thin-film transistors (OTFTs) are considered indispensable in applications requiring flexibility, large area, low processing temperature, and low cost. Key challenges to be addressed include developing solution-processable gate dielectric materials that form uniform films over large areas and exhibit excellent insulating properties, reducing contact resistance at interfaces between organic semiconductors and electrodes, and optimizing the patterning of organic semiconductors. High-performance pentacene-based OTFTs have been reported with polymeric gate dielectrics and indium tin oxide source/drain electrodes. Using such OTFT backplates, a 15-in. 1024 X 768 pixel full-color active-matrix liquid-crystal display (AMLCD) and a 4.5-in. 192 X64 pixel active-matrix organic light-emitting diode (AMOLED) have been fabricated.

The Low Leakage Current Density of MIS with SiO2 Film Made by ICP-CVD

Thin Film Transistors (TFT) made from high quality semiconductor films can be used in active matrix liquid crystal displays (AMLCD). The properties of the gate oxide are very important in the operation of MIS. In this work, crystallized SiO2 films were deposited at extremely low temperature (90{degree sign}C) by inductively coupled plasma chemical vapor deposition. The results show that the deposition rate increased approximately linearly with applied r.f. power in the ICP-CVD system. Only the (1 11) X-ray diffraction peak appeared in the range of the study. At 1000 Watts, the silicon dioxide film is amorphous which may be attributed to low deposition temperature. The value of the dielectric constant of the MIS varied from 3.75 to 4.05. From the I-V curve it can be seen that the SiO2 thin film has a lower leakage current density when prepared at 1750Watt. The lowest leakage current density obtained in this study was 6.8×10-8A/cm2 @V=1V.

Compact and power‐saving polysilicon data driver with common‐decoder DA converter (CD‐DAC)

Abstract— A common‐decoder architecture for a data‐driver circuit fabricated by using a polysilicon process has been developed. The architecture achieves a compact circuit and low‐power consumption. In application to an integrated polysilicon data driver for small‐sized displays, this architecture reduces the area of the data driver by removing the vertical bus lines that occupy a large area. It also suppresses the power consumption of the data bus by reducing the number of driven lines in the data bus during word‐to‐word transitions from six to two. By using a conventional 4‐μm design rule, we fabricated an active‐matrix OLED (AMOLED) panel with an integrated six‐bit data‐driver circuit with 384 outputs. The driver circuit had a height of 2.6 mm and a pitch between output lines of 84 μm. The maximum power consumption of the driver was only 5 mW, i.e., 3.8 mW for logic‐data transfer and 1.2 mW for reference‐voltage source. Furthermore, we also fabricated an active‐matrix LCD (AMLCD) panel including driver circuits of the same type as the integrated elements. Six‐bit full‐color images were successfully displayed on both panels.

Detectability Decreases With Off-Normal Viewing in Medical Liquid Crystal Displays

To quantify the reduction in detection performance of subtle signals at off-normal viewing directions in medical active-matrix liquid crystal displays (AMLCDs). Fifty synthetic image pairs per viewing condition (a total of 350) were used in a two-alternative forced-choice experiment in which 11 trained observers viewed images at 0, 30, and 45 degrees from the display normal, along the diagonal axis of a 5 million pixel in-plane switching monochrome AMLCD. The images were generated using white-noise backgrounds. A Gaussian signal was added to the signal-present set with three different signal amplitudes (4, 8, and 12 gray levels in a 10-bit scale). The average percent correct achieved for a signal of 4 gray levels was 79.6 (95% confidence intervals based on reader and case variability: 71.6-86.9), 63.4 (CI 56.0-71.3), and 55.3 (CI 48.4-62.0), for 0, 30 and 45 degrees from the display normal, respectively. When the signal amplitude was increased by a factor of two, the performance was 76.9 and 57.0 for 30 and 45 degrees, respectively, and 95.3 and 85.3 when the amplitude was increased by a factor of three. The observers took on average about twice as long and as much as seven times as long to reach decisions in off-normal viewing. Off-normal viewing of diagnostic images in AMLCDs significantly reduces the detection of low-contrast abnormalities. Increased off-normal signal amplitudes were required to regain the detection performance measured for normal viewing. We observed this decrease in detection performance for off-normal viewing even when measured decision times were about twice as long as for normal viewing.

A Novel Self-Aligned Field Induced Drain Polycrystalline Silicon Thin Film Transistor Fabricated by using a Selective Side Etch Process

AbstractA novel T-shaped-gated (T-Gate) polycrystalline silicon thin-film transistor (poly-Si TFT) with vacuum gaps has been proposed and fabricated only with a simple process. The T-Gate structure is formed only by a selective undercut-etching technology of the Mo/Al bi-layers. Then, vacuum gaps are in-situ embedded in this T-Gate structure subsequent to capping the SiH4-based passivation oxide under the vacuum process chamber. Experimental results reveal that the proposed T-Gate poly-Si TFTs have excellent electrical performance, which has higher maximum on-off current ratio of 4.6 e107, and the lower off-state leakage current at VGS = -10 V and VDS = 5V of about 100 times less than that of the conventional one. It is attributed to the additional undoped offset region and the vacuum gap to reduce the maximum electric field at drain junction while ascribed to the sub-gate to maintain the on-current. Therefore, such a T-Gate poly-Si TFT is very suitable for the applications and manufacturing in active matrix liquid crystal displays (AMLCDs) and active matrix organic light emitting diodes (AMOLEDs).

SiNx barrier layers deposited at 250°C on a clear polymer substrate

ABSTRACTInterest is widespread in flexible thin-film transistor backplanes made on clear polymer foil, which could be universally employed for a variety of applications. All ultralow process temperatures, plastic compatible thin film transistor (TFT) technologies battle short or long term device instabilities. The quality and stability of amorphous silicon thin-film transistors (a-Si:H TFTs) improves with increasing process temperature. TFT stacks deposited at less than 250°C by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) exhibit higher threshold voltage shifts after gate bias stressing than stacks deposited at ∼300°C in the active matrix liquid-crystal display (AMLCD) industry [1]. Therefore, optically clear plastic (CP) substrates are desired that tolerate high process temperatures. The first step in a-Si:H TFT fabrication on a polymer is the deposition of a planarizing barrier and adhesion layer. For this purpose we have been using silicon nitride (SiNx) grown by PECVD.This paper discusses the substrate preparation and SiNx deposition for the process temperature of 250°C. We study the mechanical strain in the SiNx film on the CP substrate, as a function of RF power. Earlier work has shown that SiNx films deposited at low RF power are under tensile strain, and become increasingly compressed as the deposition power is raised [2]. Additionally, at very high deposition power the substrate is bombarded at the beginning of film growth to achieve good film adhesion. The goal is to identify the correct processing conditions at which the total mismatch strain between the film and the substrate is minimized, to keep the film/substrate composite flat and avoid mechanical fracture as well as peeling due to poor adhesion. Optimal deposition conditions were identified to obtain crack-free SiNx barrier layers. The barrier layers were tested by fabricating a-Si:H TFTs on them at 250°C.

10.5L: Late-News Paper: All-Organic TFT-Driven QQVGA Active-Matrix Polymer-Dispersed LCD with Solution-Processed Insulator, Electrodes, and Wires

A high-performance pentacene organic-TFT backplane with a solution-processed novel poly(4-vinylphenol)-based dielectric and a solution-processed organo-silver electrode/wire has been developed below 150°C. The all-organic-TFT backplane has been successfully applied to drive a QQVGA active-matrix (AM) polymer-dispersed (PD) LCD with a resolution of 79 dpi and a display size of 2.5 inches.

Defective Pixels in Medical LCD Displays: Problem Analysis and Fundamental Solution

Over the past few years, traditional CRT displays have gradually been replaced by active matrix LCD displays. Each pixel in an LCD display has its own individual transistor that controls the transmittance of that pixel. Occasionally, these individual transistors will short or malfunction, resulting in a defective pixel that always shows the same brightness. This article shows how defective LCD pixels can interfere with subtle features in medical images. A defective pixel affects a broad area around it therefore possibly reducing the quality of diagnosis specifically for highly demanding applications such as mammography. A specialized image processing algorithm provides an innovative solution making these defects completely invisible and recovers information from the defect so the radiologist perceives the medical image correctly.

Making defective LCD display pixels invisible

Within active matrix displays, such as LCDs, each pixel has its own individual transistor that controls its transmittance (see Figure 1). Occasionally, these individual transistors will short, or otherwise malfunction, resulting in a defective pixel. These are often visible as ‘bright’ pixels, which appear as single or several randomly-placed red, blue and/or green pixel elements on an all-black background. They can also appear as ‘missing’ or ‘dead’ pixels, black dots on all-white backgrounds. For many applications, such as LCD TV, digital cinema, and desktop monitors, the existence of defective pixels seriously degrades the image quality. In medical-imaging applications, defective pixels can even influence the critical decision-making process and therefore result in a wrong diagnosis. State-of-the art manufacturing processes are capable of producing displays with an average of no more than one faulty transistor in two million. With ever-increasing resolution, the number of defective pixels in displays increases accordingly. Typically, LCD panels are being inspected after manufacturing and panels that have too many defects are rejected. This results in a lower yield and higher panel cost. Recently, techniques have been described that make it possible to partially repair 3 defective pixels by transforming a very visible bright defect into a less visible dark defect. However, neither rejection nor partial repair is a true solution. We have been working on a completely different approach where we focus on making defective pixels invisible to the user by means of an image processing algorithm. Human observers perceive images by means of the eye, which is a complex lens system. Most lenses, including that in the human eye, are not perfect. As a result, when visual stimuli are passed through the cornea and lens, the stimuli undergo a certain degree of degradation or distortion. One typical degradation effect is the introduction of blur this means we do not perceive individual pixels, but a a low-pass-filtered combination of several neighboring pixels. Further, if one of those pixels is defective then it is possiFigure 1. An active matrix LCD works by blocking light that comes in from the back.

Pentacene-based thin film transistors used to drive a twist-nematic liquid crystal display

The study addresses the factors of influence on the active matrix display that is driven by pentacene-based organic thin-film transistors (TFTs). The atmosphere and humidity conditions were found to seriously affect the performance of organic TFTs. An appropriate encapsulation layer was added to protect the organic TFTs from external damage. Organic TFTs reliability, the illumination effect and device uniformity, were also considered in the context of display application. Finally, a monochrome 3 inch 64 × 128 active-matrix twist-nematic liquid crystal display (LCD) was fabricated. The display is capable of showing video images with a refresh rate of 20 Hz. Obtained results reveal the potential of organic TFTs for active-matrix LCD technology.

A High-Performance Short-Channel Bottom-Contact OTFT and Its Application to AM-TN-LCD

We have demonstrated an organic thin-film-transistor (OTFT)-driven active-matrix twisted-nematic liquid crystal display (AM-TN-LCD) on a glass substrate, with a resolution of 160/spl times/120 pixels, 79 ppi. Substrate temperature was kept below the plastic-compatible temperature of 180/spl deg/C throughout the fabrication process. In order to realize an OTFT-driven display with fine resolution, we employed short-channel bottom-contact (BC) pentacene OTFTs. It has been known that their drivability is limited by contact resistance at source/drain (S/D). We found that the S/D contact resistance was markedly reduced when the thickness of the nonohmic Ti adhesion layer for ohmic Au S/D electrodes was reduced less than /spl sim/3 nm. We elucidate that this 3 nm corresponds to the thickness of the accumulating layer in a pentacene channel. When we use a self-assembled monolayer of mercapto-silane-coupling agent as the adhesion layer, the contact resistance becomes negligibly small and BC OTFTs scalable below 10 /spl mu/m were obtained. In addition to this OTFT-cell technology, we developed a low-damage pentacene patterning technique for integration of OTFTs and introduced low-temperature panel assembly process to suppress thermal-stress degradation of pentacene OTFTs, which are the key technologies to achieve OTFT-driven AM-TN-LCD.

Liquid Crystals Based on Hypervalent Sulfur Fluorides: Exploring the Steric Effects of ortho‐Fluorine Substituents

AbstractLiquid crystals that have ortho‐fluorinated pentafluoro‐λ6‐sulfanyl terminal groups are the most polar materials that are still compatible with current active matrix LCD technology. The synthesis of the first examples of this class of substance has been achieved by direct fluorination. The physico‐chemical characterization, supported by DFT calculations, revealed some surprising features of these new compounds: the bulky SF5 group is easily deformed and responds in a quite unusual way to the steric strain exerted by its ortho substituents. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Instability measurements in amorphous hydrogenated silicon using capacitance-voltage techniques

In the field of flat panel displays, the current leading technology is the active matrix liquid-crystal display; this uses an amorphous hydrogen silicon (a-Si:H) based thin-film transistors (TFTs) as the switching element in each pixel. However, under gate bias a-Si:H TFTs suffer from instability, as is evidenced by a shift in the gate threshold voltage. The shift in the gate threshold voltage is generally measured from the gate transfer characteristics, after subjecting the TFT to prolonged gate bias. However, a major drawback of this measurement method is that it cannot distinguish whether the shift is caused by the change in the midgap states in the a-Si:H channel or by charge trapping in the gate insulator. In view of this, we have developed a capacitance-voltage method to measure the shift in threshold voltage. We employ metal-insulator-semiconductor structures to investigate the threshold voltage shift as they are simpler to fabricate than TFTs.