Backplane Requirements for Active Matrix Organic Light Emitting Diode Displays
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0910-A16-01-L09-01
Backplane Requirements for Active Matrix Organic Light Emitting Diode Displays Arokia Nathan1, Denis Striakhilev1, Reza Chaji1, Shahin Ashtiani1, Czang-Ho Lee1, Andrei Sazonov1, John Robertson2, and William Milne2 1 Electrical and Computer Engineering, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada 2 Electrical Engineering Division, Department of Engineering, University of Cambridge, Cambridge, CB3 0FA, United Kingdom
ABSTRACT Organic light emitting diode (OLED) displays are a serious competitor to liquid crystal displays in view of their superior picture quality, higher contrast, faster on/off response, thinner profile, and high power efficiency. For large area and/or high-resolution applications, an active matrix OLED (AMOLED) addressing scheme is vital. The active matrix backplane can be made with amorphous silicon (a-Si), polysilicon, or organic technology, all of which suffer from threshold voltage (VT) shift and/or mismatch problems, causing temporal or spatial variations in the OLED brightness. In addition, the efficiency of the OLED itself degrades over time. Despite these shortcomings, there has been considerable progress in development of AMOLED displays using circuit solutions engineered to provide stable and uniform brightness. Indeed the design of AMOLED pixel circuits, particularly in low-mobility TFT technologies such as a-Si, is challenging due to the stringent requirements of timing, current matching, and low voltage operation. While circuit solutions are necessary, they are not sufficient. Process improvements to enhance TFT performance are becoming inevitable. This paper will review pertinent material requirements of AMOLED backplanes along with design considerations that address pixel architecture, contact resistance, and more importantly, the VT-stability and associated gate overdrive voltage, VGS-VT. In particular, we address the question of whether conventional PECVD can be deployed for high mobility and high VT-stability TFTs, and if micro-/nanocrystalline silicon could provide the solution. 1. INTRODUCTION In the quest for ever-thinner and higher-performance displays, organic light-emitting-diodes (OLEDs) offer a promising avenue. This family of displays provides excellent brightness, brilliant color quality, wide viewing angle, fast switching times, high power efficiency and low operating voltages, all of which meet the application requirements ranging from portable mobile electronics to high definition television screens [1]. While the passive matrix OLED display structure is simple, its pulse-driven nature leads to high power consumption and low lifetime [2], restricting its use to small, low-resolution displays. For large size and/or high-resolution applications, an active matrix addressing scheme is vital. Since the OLED is a current driven device, a TFT circuit is needed at every pixel to regulate the OLED current. The circuit must compensate for VT-shift of the drive transistor and the degradation of the OLED. The active
matrix backplane can be made
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