Vt-Shift Compensating Amorphous Silicon Pixel Circuits for Flexible OLED Displays
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Vt-Shift Compensating Amorphous Silicon Pixel Circuits for Flexible OLED Displays Kapil Sakariya, Peyman Servati, Denis Striakhilev, and Arokia Nathan Electrical and Computer Engineering, University of Waterloo Waterloo, Ontario N2L 3G1, CANADA [email protected]
ABSTRACT In this paper, we present design considerations pertinent to amorphous silicon pixel circuits for mechanically flexible active matrix display applications. We describe both circuit topologies and pixel architectures that are amenable to surface emissive pixels in a-Si:H technology. Metastable shifts in the a-Si:H material characteristics after prolonged gate bias or mechanical stress are manifested in the form of increasing TFT threshold voltages and varying mobilities. As a result, the TFT drive current and consequently the OLED brightness decrease, and the pixel eventually turns off. The circuits presented here compensate for this decrease in current through the use of current programming and feedback, and allow for stable OLED brightness over longer time periods. They also enable the implementation of high aperture ratio (close to 100%), low leakage current, surface emissive OLED pixels. Results show that the circuits provide higher linearity and dynamic range than currently available pixel circuits while minimizing the pixel area. Charge injection effects at the gate of the drive TFT have been reduced by using smaller switching TFTs, along with circuit topologies that provide in-pixel current gain. All circuits meet the speed requirements of a QVGA 60 Hz refresh rate display, and occupy less than 300um x 300um area. INTRODUCTION Organic light-emitting diodes (OLEDs) have the potential to replace LCDs for flat panel display applications due to their many performance advantages like lower power usage, faster response time, lower cost fabrication, and a process that is ideal for mechanically flexible displays on plastic [1]. Large-area passive matrix flat panel displays on both rigid and flexible substrates have already been demonstrated [2][3], while active matrix OLED (AMOLED) displays are being developed. AMOLED displays need a pixel circuit consisting of a minimum of two thin film transistors (TFTs) to control the OLED current. Poly-crystalline silicon (poly-Si) and amorphous silicon (a-Si:H) are the two competing materials used to make these TFTs for large area displays, each having their benefits along with undesirable attributes. Amorphous silicon is the material of choice to fabricate TFTs for flexible AMOLED displays on plastic since the process temperature is relatively low. However, it suffers from threshold voltage (Vt) metastability leading to a gradual decrease in the drive current of a-Si:H TFTs. Moreover, the a-Si:H TFTs exhibit changes in mobility and threshold voltage when subjected to mechanical stress [4]. We have fabricated and modeled a-Si:H TFTs on flexible plastic substrates, and have achieved mobilities comparable to those of TFTs on glass. Using these models, we have designed a-Si:H TFT circuits that co
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