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HYBRID SWITCHING MODE LIQUID CRYSTAL DISPLAYS

IP.com Disclosure Number: IPCOM000013409D
Original Publication Date: 2000-Mar-01
Included in the Prior Art Database: 2003-Jun-18
Document File: 1 page(s) / 60K

Publishing Venue

IBM

Abstract

Thin-film transistor (TFT) driving liquid crystal display (LCD) using in-plane switching (IPS) mode was demonstrated by Ohta et al. IPS mode shows wide viewing angle characteristics, but it suffers from a low optical transmission. Therefore IPS mode LCDs are not likely to be suitable for notebook application. Even for the desk top or home TV application, higher optical transmission is still preferred. In this disclosure, hybrid switching mode (HSM) LCDs were proposed. They will have wide viewing angle characteristics as well as high optical trans- mission. Hybrid switching mode (HSM) LCD uses both twisted nematic (TN) mode and IPS mode. Two unit cell structures of HSM LCD are shown in Figs. 1 and 2. For simplicity, the array structure (TFT and bus lines) is shown. Both structures have top ITO. In Fig. 1, top ITO is a continuous layer and connected to a voltage Vcom. In Fig. 2, the top ITO is pat- terned to cover the top of each pixel but top ITO of each pixel is separated from the adjacent pixels. Electrically, the top ITO is not connected to any power supply. It is floating and its voltage is determined by the area ratio of two bottom electrodes, A 1 and A 2 . If these two areas are equal, we have voltage of top ITO of each pixel equal to the arithmetic average of the voltages of the two bottom electrodes, e.i. V 1 V 2 )/2. Bottom electrode 2 of Fig. 1 and two bottom electrodes of Fig. 2 are transparent electrode, such as ITO. The electrode 1 of Fig. 1 can be any suitable metal material. The LC alignment directions (shown for a left-handed twist structure) and the polarizer directions for each structure are also shown in both figures. The dashed lines represent those on the bottom surface while the solid line represent those on o the top surface. The angle between two rubbing directions is 90 or slightly less than 900, 0 such as 85. In the field-off state (V 0 for Fig. 1 and V 1 V 2 for Fig. 2), both Figs. 1 and 2 are basically the TN structure. Due to the parallel polarizer arrangement, the field-off state is the dark state. For Fig. 1, if the voltage V 0, and it is larger than a threshold voltage, the LC molecules over the electrode 2 will tilt upward mainly due to the vertical field (TN mode) and the LC molecules over the gap between two bottom electrodes will be switched horizon- tally to the direction from electrodes 1 to 2 (or 2 to 1), i.e., perpendicular to the gap direction, mainly due to the in-plane field (IPS mode). As the voltage increases, the optical transmission increases. Because the device uses two switching modes, we name it the hybrid switching mode (HSM). The ratio of contributions to optical transmission between TN mode and IPS mode can be controlled by the ratio of the area of bottom electrode 2 to the area of the gap between two electrodes. The operation of Fig. 2 is similar to that of Fig. 1, except that the LC molecules over the bottom electrode 1 are also switchable and the ratio of contrib- utions to optical transmission between TN mode and the IPS mode can be controlled by the ratio of total area of two bottom electrodes to the area of the gap between bottom electrodes 1 and 2. In the HSM LCD, since most of the pixel area is switchable and contributes to the optical transmission, it has a high pixel average optical transmission. Also, since it uses both the TN mode and the IPS mode, it provides a wide viewing angle. 1

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HYBRID SWITCHING MODE LIQUID CRYSTAL DISPLAYS

    Thin-film transistor (TFT) driving liquid crystal display (LCD) using in-plane switching (IPS) mode was demonstrated by Ohta et al. IPS mode shows wide viewing angle characteristics, but it suffers from a low optical transmission. Therefore IPS mode LCDs are not likely to be suitable for notebook application. Even for the desk top or home TV application, higher optical transmission is still preferred. In this disclosure, hybrid switching mode (HSM) LCDs were proposed. They will have wide viewing angle characteristics as well as high optical trans- mission.

    Hybrid switching mode (HSM) LCD uses both twisted nematic (TN) mode and IPS mode. Two unit cell structures of HSM LCD are shown in Figs. 1 and 2. For simplicity, the array structure (TFT and bus lines) is shown. Both structures have top ITO. In Fig. 1, top ITO is a continuous layer and connected to a voltage Vcom. In Fig. 2, the top ITO is pat- terned to cover the top of each pixel but top ITO of each pixel is separated from the adjacent pixels. Electrically, the top ITO is not connected to any power supply. It is floating and its voltage is determined by the area ratio of two bottom electrodes, A1 and A2. If these two areas are equal, we have voltage of top ITO of each pixel equal to the arithmetic average of the voltages of the two bottom electrodes, e.i. (V1 + V2)/2. Bottom electrode 2 of Fig. 1 and two bottom electrodes of Fig. 2 are transparent electrode, such as ITO. The electrode 1 of Fig. 1 can be any suitable metal material. The LC alignment directions (shown for a left-handed twist structure) and the polarizer directions for each structure are also shown in both figures. The dashed lines represent those on the bottom sur...