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Novel Current-Scaling Current-Mirror a-Si:H TFT Pixel Electrode Circuit with Cascade Capacitor
for AM-OLEDs

Hojin Lee, Juhn S. Yoo, and Jerzy Kanicki

Over last several years, it was shown that the current driving pixel electrode circuits are among the most desirable solutions for AM-OLEDs. However, as display size and resolution increase, a large timing delay can be observed at a low data current and its importance increases with the display size. To address this issue, several solutions have been proposed based on polycrystalline silicon TFT technology such as current-mirror circuit, series-connected TFT circuit, and current-mirror circuit with acceleration control line. We also proposed a-Si:H TFT based current-scaling pixel electrode circuit to address this problem. In this project, we studied an improved a-Si:H TFT current driving pixel electrode circuit with an enhanced current scaling function. A current mirror circuit with a cascaded storage capacitor is proposed here to achieve a high data-to-OLED current ratio without increasing TFT size in comparison with the conventional current mirror pixel circuit. At the same time, by removing one control signal line, this circuit is expected to have a much simpler pixel circuit layout and driving scheme than the previous cascade capacitor pixel electrode circuit.
The proposed current-driven pixel electrode circuit consists of two switching TFTs (T1 and T2), one mirror TFT (T4), one driving TFT (T3), and two storage capacitors (CST1, CST2) connected between a scan line and ground with a cascade structure. The signals of VSCAN, IDATA, and VDD are supplied by the external drivers while the anode of OLED is connected to VDD. In comparison to the cascade capacitor current-scaling pixel electrode circuit reported previously, by employing the current mirror TFT structure, the control signal line can be removed to simplify the pixel layout and driving scheme as well as to enable OLED to light up during ON-state even when top anode light-emitting device structure is used. To demonstrate the current-scaling function of the pixel electrode circuit in comparison with both the conventional current-mirror and cascade capacitor current-scaling pixel electrode circuits, we simulated all three pixel electrode circuits using H-SPICE, and measured IOLED_OFF as a function of IDATA for each pixel electrode circuit. While the conventional current-mirror pixel circuit showed only a fixed current-scaling by the ratio of T4/T3 over given IDATA range, the cascade capacitor current-scaling and the proposed current-scaling pixel electrode circuits showed non-linear current-scaling function for variable current-scaling ratio depending on IDATA. When IDATA varies from 0.2 to 5.0 μA, the proposed cascaded-capacitor pixel circuit with the ratio of CST2/CST1=1/8 can provide IOLED_OFF ranging from 1.7×10-5 to 1.7 μA. Hence much wider range of IOLED_OFF levels can be achieved by this circuit in comparison with the conventional current-mirror pixel circuit (3.0×10-2 to 1.0 μA) and the cascade capacitor current-scaling pixel circuit (8.8×10-5 to 2.0 μA). Therefore, using this pixel circuit, we expect to avoid the unnecessary pixel circuit power consumption at high current levels and minimize the programming time at low current levels with the reduced number of signal lines, which are supposed to be ideal characteristics for a high-resolution AM-OLED based on a-Si:H TFTs. The goal of this project is to design the current-driven pixel circuit based on a-Si:H TFT and to optimize the current-scaling ratio to achieve highest pixel circuit electrical performances and stability. The long-term goals include the fabrication of the pixel electrode circuits for AM-OLEDs based on this research. This project is supported by LG Philips LCD Research & Development Center, Korea.

H. Lee, J. S. Yoo, C- D. Kim, I- J. Chung, and J. Kanicki, “Novel Current-Scaling Current-Mirror a-Si:H TFT Pixel Electrode Circuit with Cascade Capacitor for AM-OLEDs,” Journal of Jpn. Appl. Phys., Vol. 46, 2007.

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