Yasuhiro Ugai

Top-Gate Staggered Amorphous Silicon Thin-Film Transistors: Series Resistance and Nitride Thickness Effects

Top-gate staggered hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) were fabricated over large-area glass substrates using a selective phosphorus-treatment (PT) of indium-tin-oxide (ITO) source/drain electrodes. The ohmic contact between a-Si:H and ITO had a specific contact resistivity of about 0.18 Ωcm2. For a 100-µm channel length TFT, the source/drain series resistance contributes less than 5% of the total drain-to-source resistance. This contribution increases to about 25% for a 10-µm channel length TFT. Our study also indicated that the interface quality of a-Si:H/a-SiNx:H is amorphous silicon nitride (a-SiNx:H) and a-Si:H thickness independent and dependent, respectively. Effective interface state densities of about 1.5×1012 cm-2eV-1 and 3.2×1012 cm-2eV-1 were obtained for top-gate TFTs with a 1300 and 300 A thick a-Si:H films, respectively. Channel conductance activation energy of about 0.1 eV was measured for this top-gate TFT with 300 A a-Si:H.

Influence of the amorphous silicon thickness on top gate thin-film transistor electrical performances

We have analyzed the influence of the hydrogenated amorphous silicon (a-Si:H) thickness on the electrical performances of top gate thin-film transistors (TFTs). We have observed that, when the a-Si:H thickness increases, the threshold voltage and the subthreshold slope decrease. The modification of the TFT apparent field-effect mobility has also been investigated: we have shown that it first increases with the a-Si:H thickness, and then decreases for thicker a-Si:H films. This change of electrical performances is most likely associated with both the variation of a-Si:H microstructure during the film depositions and the effect of parasitic source and drain series resistances. We have demonstrated that for a given TFT geometry, it is therefore possible to define an optimum a-Si:H thickness ensuring maximum TFT electrical performances, and that this optimum thickness increases significantly with the TFT channel length.

Fluorescent Liquid Crystal Color Display Using a Guest?Host UV Shutter and Phosphor Layers on the Inside of the Cell

Fluorescent liquid crystal displays (LCDs) have been demonstrated using a guest–host (GH) LC UV shutter cell. Red, green and blue inorganic phosphor lines were manufactured on a glass substrate and an indium–tin–oxide electrode was coated on a phosphor layer. The GH LC cell was composed of this substrate and the phosphor layer was inside of the cell, as well as the color filter used in the current LCD. A UV light emitting diode or a UV fluorescent lamp was used as an excitation source and the switching properties of the fluorescence intensity and their contrast ratio were measured and discussed.

High-performance top-gate a-Si:H TFTs for AMLCDs

High-performance top-gate hydrogenated amorphous silicon (a-Si:H) thin-film transistor (TFT) structures have been fabricated over a large area from plasma-enhanced chemical vapor deposition (PECVD) materials. The electrical performances of the top-gate a-Si:H TFT (μFE≅0.75cm2/Vsec, VT≅3.5V, S≅0.55V/dec) are comparable to the electrical performances observed for an inverted-staggered bottom-gate a-Si:H TFT. We have shown that the TFT field-effect mobility first increases with the a-Si:H thickness, and then decreases for thicker a-Si:H films. This change of the electrical performances can be associated either with the variation of a-Si:H microstructure with film thickness during the PECVD processes or a large density of TFT back interface states; it also involves the source/drain parasitic access resistances, especially for thick a-Si:H layers.

A Simple Ohmic-Contact Formation Technology using Phosphine Plasma Treatment for Top-Gate Amorphous-Silicon Thin-Film Transistors

The effects of exposing patterned indium-tin-oxide (ITO) on amorphous silicon oxide (a-SiOx) to a phosphine (PH3) plasma has been investigated. Phosphorous atoms react with ITO to form InPOx and InP, with a much smaller number being deposited on a-SiOx. If amorphous silicon (a-Si) is then deposited onto these layers by plasma enhanced chemical vapor deposition, phosphorous atoms from the ITO are incorporated into the a-Si in the region of the ITO. This effectively dopes the a-Si to form good ohmic contacts between the ITO and a-Si layers. In contrast, the number of phosphorus atoms incorporated into a-Si directly above a-SiOx is too small to have an effect on the electrical properties of a-Si. The doping mechanism for the ITO and a-Si is discussed, and an explanation given as to how these features can be used to mass produce high performance top-gate a-Si thin film transistors using only three photomask steps.

Development of Wide-Viewing-Angle TFT-LCDs using halftone gray-scale method

We have developed wide-viewing-angle TFT-LCDs using the halftone gray scale (HTGS) method. Compared with conventional TN-LCDs, the viewing angle of HTGS TFT-LCDs is twice as wide in the up-down direction and has also greatly improved in the diagonal direction. The gray-scale display response time of HTGS TFT-LCDs is also shorter by one-half compared with conventional TN-LCDs. We have studied the application of HTGS TFT-LCDs to passenger cabin entertainment use. In contrast to consumer use, environmental durability characteristics are required; however, the developed HTGS TFT-LCDs meet all of the requirements and have been practically used. This paper presents the principles of HTGS, the pixel design, the device structure, the electro-optical characteristics, and the application to passenger cabin entertainment.

Development of wide‐viewing‐angle full‐color TFT‐LCDs for a new type of passenger plane cockpit display

We have developed a wide-viewing-angle normally black full-color active matrix liquid crystal display (TFT-LCD) for a new type of passenger plane cockpit. The cockpit has six large-size displays (DU) with 6.71 × 6.71-inch active area and three medium-size displays (CDU) with 3.72 × 3.25-inch active area and is called a “glass cockpit.” The cockpit display is extremely important equipment in the interface between pilot (man) and airplane (machine). This paper describes the required performance for the cockpit display, the key technology to realize this performance, the performance of the developed wide-viewing-angle full-color TFT-LCD, and a comparison with CRTs.

Recent development and application of TFT-LCDs

Recent development and application of TFT-LCDs using a-Si TFT are reviewed. An in-plane- switching (IPS) mode LCD is expected to be the most promising among many candidates for economically improving the poor-viewing angle in an LCD. Our practically-unlimited viewing-angle 14.5-inch IPS TFT-LCD prototype is described, which has XGA resolution and 64-gray-scale capability. An aircraft cockpit display requires a wide viewing-angle, wide operating-temperature, and high readability. Our large-size and full-color TFT-LCD under production as a cockpit display for new types passenger plane is described. A large-size reflective-color TFT-LCDs prototype fabricated through our high-aperture-ratio technology and minimized-production process is describe. A reflective TFT-LCD is very suitable for reducing power consumption. Additionally, performance and characteristics of other TFT- LCD prototypes and products are described.

Reduced Process Method for Thin-Film-Transistor Liquid-Crystal Display (TFT-LCD) with Dry-Etching Tapered ITO Data Bus Lines

For years, many studies have been conducted for the purpose of reducing the number of TFT processes. We have developed a top-gate TFT-LCD fabricated using only indium-tin-oxide (ITO: In2O3–SnO2) data bus lines by eliminating the metal data bus line process. Substituting ITO dry-etching for wet-etching allows tapering of the side walls of thick ITO data bus lines. Controlling the tapered angle of ITO data bus lines to be less than 40° results in successful fabrication of a transistor with no offset voltage. As a result, we have developed a 6-inch-diagonal TFT-LCD fabricated with data bus lines as well as drain-source electrodes of single-layered ITO.