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A NEW THIN FILM TRANSISTOR STRUCTURE FOR INCREASING STORAGE CAPACITANCE IN THE PIXEL ELEMENT BH. MIN, H.S. CHOI, J.S. PARK, M.K. HAN Department of Electrical Engineering, Seoul National University, San 56-1, Shinrim-dong, Kwanak-ku, Seoul 151-742, Korea

ABSTRACT In a high resolution TFT LCD, the ratio of a parasitic capacitance between a gate and a source electrode to the liquid crystal capacitance increase. Increase of the Cg/CIC makes the voltage shift of the pixel electrode large. This also results in degradation of the display quality such as image sticking or flicker. The voltage shift can be decreased by increasing the value of a storage capacitor, however, it decreases the aperture ratio. We present a new thin film transistor structure to increase the storage capacitance without reducing the aperture ratio. In the simulation results, we have observed that the pixel voltage shift is remarkably reduced compared with the conventional one.

INTRODUCTION The polycrystalline silicon(poly-Si) thin film transistor(TFTs) are becoming important for active matrix liquid crystal displays(AMLCDs) [1][2]. The use of poly-Si material is compatible with well-established processes for monocrystalline silicon. The mobility of polySi is higher and the photosensitivity is lower than those of a-Si. Also, the requirement of many external leads for a high quality active matrix LCD causes a serious problem of an increase in size and complexity. Reducing the number of external leads is key to improving the reliability, yield and costs. The final step in the progress toward reducing the number of leads is the full integration of self-scanned devices. This was accomplished with a poly-Si TFTs fabrication process because the poly-Si TFTs can be used for both the switching elements of the active matrix and the peripheral drive circuitry. However, the poly-Si TFTs used in the display have parasitic capacitance between the gate and source electrodes. This parasitic capacitance affects the pixel voltage potential that determines the electro-optical property of the liquid crystal cell, degrades constant ratio, display uniformity and reliability, and sometimes causes visible flicker on the display panel [3][4]. Generally, TFT liquid crystal displays contain a storage capacitor in each pixel element, parallel to the liquid crystal capacitance in order to nullify the parasitic capacitance effects. Unfortunately, this scheme diminishes an aperture ratio because a large storage capacitance area is required to reduce the Cgs/Cs ratio. In this paper, we propose a new TFT structure of which the storage capacitance is at least twice than that of the conventional TFTs without sacrificing the aperture ratio. We simulate the proposed TFT by SPISCES and SPICE simulator. The on-current is found to be increased considerably compared with the conventional one. We have observed that a drastic reduction of the pixel voltage shift when a high gate voltage is applied to the pixel switching transistor. Also, the characteristic of the T-V curve (Transmission ve