主動式有機發光二極體顯示器之畫素電路必須採用薄膜電晶體作為驅動與開關元件，然而不同製程的薄膜電晶體元件臨界電壓會因為製程上的誤差或者因長時間的使用而產生漂移。此外，有機發光二極體材料隨著長時間使用而老化的現象導致整體面板的亮度均勻性以及亮度下降，因而對於畫面顯示品質造成影響。
本論文即針對上述問題提出三個新式畫素補償電路。第一個電路是利用3T1C的架構且結合數位式與傳統電壓驅動法來補償薄膜電晶體臨界電壓值的變異，實驗結果顯示所提出之電路在經過18小時之60 ℃高溫測試後，電流衰減度只有原來的5%，而傳統2T1C畫素電路的電流衰減度卻是原來的35%。因此，所提出之電路具有高度電流穩定性同時也可以提高面板開口率使亮度上升。由於第一個電路無法補償有機發光二極體材料老化所造成亮度衰減的現象，第二個電路即利用4T1C的架構以及外部偵測法來補償薄膜電晶體臨界電壓值的變異和改善有機發光二極體亮度的衰減。模擬結果顯示所提出之電路在經過500小時之後，亮度誤差為原來的±2%，相較之下，第一個電路的亮度衰減度卻是原來的17%。因此，第二個電路可使有機發光二極體展現相當高的亮度穩定性。考量到改良外部偵測的複雜性，第三個電路將電壓回授補償法應用於截波反向器驅動法上，採用數位式驅動法補償薄膜電晶體臨界電壓和電子移動率的變異，同時利用電壓回授補償有機發光二極體材料老化所造成亮度衰減的現象，由模擬結果可得知電路中相關節點電壓變化值都幾乎等於相對應的薄膜電晶體和有機發光二極體臨界電壓變化值，所以本電路可以補償薄膜電晶體臨界電壓和電子移動率的變異並且增加有機發光二極體的顯像壽命。

For an active matrix organic light emitting diode (AMOLED) display, the pixel circuit utilizes a thin film transistor (TFT) as the driving and switching component. However, variation in the VTH of the driving TFT due to process variation or long-term operation, and luminance decay caused by OLED aging directly influence the image quality of the AMOLED display.
This thesis proposes three novel pixel circuits and verifies their effectiveness by experiments and simulations. The first proposed 3T1C circuit adopts a novel scheme that combines the conventional voltage-programmed method with the clamped inverter driving scheme to compensate for the VTH shift of the driving TFT. Based on the experimental results, the normalized pixel current degradation of the proposed pixel circuit is 5% of the initial current over 18 hours at 60 ℃, while that of the conventional 2T1C pixel circuit is 35%. Thus, the proposed pixel circuit has high immunity to the threshold voltage degradation of a-Si:H TFT, and thus the OLED current stability can be improved.
The second proposed 4T1C circuit compensates for threshold voltage shift using internal compensation method and ameliorates luminance decay by external detection method based on the interdependence between the luminance degradation of the OLED and its current drop under bias stress. According to the experiment results, the normalized luminance of the conventional 2T1C pixel circuit with a stable driving current decreases by 15% of its initial value for over 500 hours. However, the simulation results show that deviation of normalized luminance of the proposed pixel circuit with external detection method is within ±2% of the initial value. Thus, the proposed pixel circuit with external detection method improves the luminance stability of AMOLED.
Because of the complexity in the operation of external detection method, the third proposed 5T1C circuit adopts a novel clamped inverter pixel circuit with a voltage feedback structure that detects the OLED degradation and increases the OLED emission time in the emission period to reduce the effect of the material decay. Simulation results demonstrate that the voltages of the related nodes can be approximately set to their required values when the threshold voltages of the TFT and the OLED shift. Therefore, the proposed pixel circuit has high immunity to variations of electrical characteristics of TFTs and can compensate for OLED degradation.

[1] C. W. Tang and S. A. VanSlyke, "Organic electroluminescent diodes", Appl. Phys. Lett., vol. 51, pp. 913-915, 1987.
[2] J. H. Burrououghes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burn, and A. B. Holmes, Nature 347, pp. 539, 1990.
[3] G. Gu and S. R. Forest, “Design of flat-panel displays based on organic light-emitting devices,” IEEE J. Sel. Topics Quantum Electron., vol. 4, pp. 83-99, Jan. 1998.
[4] R. Dawson, Z. Shen, D. A. Furest, S. Connor, J. Hsu, M. G. Kane, R.G. Stewart, A. Ipri, C. N. King, P. J. Green, R. T. Flegal, S. Pearson, C. W. Tang, S. Van Slyke, F. Chen, J. Shi, M. H. Lu, and J. C. Sturm, “The impact of the transient response of organic light emitting diodes on the design of active matrix OLED displays,” in IEDM Tech. Dig., 1998, pp. 875-878.
[5] C. C. Wu, C W. Chen, C. L. Lin, and C. J. Yang, “Advanced organic light-emitting devices for enhancing display performances,” IEEE Journal of Display Technology, vol. 1, no. 2, pp. 248-266, Dec. 2005.
[6] A. Nathan, A. Kumar, K. Sakariya, P. Servati, S. Sambandan, and D. Striakhilev, “Amorphous silicon thin film transistor circuit integration for organic LED displays on glass and plastic,” IEEE J. Solid-State Circuits, vol. 39, no. 9, pp. 1477-1486, Sept. 2004.
[7] M. J. Powell, C. Berkel, and J. R. Hughes, “Time and temperature dependence of instability mechanisms in amorphous silicon thin-film transistors,” Appl. Phys. Lett., vol. 54, pp. 1323-1325, Jan. 1989.
[8] T. F. Chen, C. F. Yeh, and J. C. Lou, “Investigation of grain boundary control in the drain junction on laser-crystalized poly-si thin film transistors,” IEEE Electron Device Lett., vol. 24, no. 7, pp. 457-459, Jul. 2003.
[9] A. Nathan, G. R. Chaji, and S. J. Ashtiani, “Driving schemes for a-Si and LTPS AMOLED displays,” IEEE Journal of Display Technology, vol. 1, no. 2, pp. 267-277, Dec. 2005.
[10] S. H. Jung, W. J. Nam, and M. K. Han, “A new voltage-modulated AMOLED pixel design compensating for threshold voltage variation in poly-Si TFTs,” IEEE Electron Device Lett., vol. 25, no. 10, pp. 690-692, Oct. 2004.
[11] Y. H. Tai, B. T. Chen, Y. J. Kuo, C. C. Tsai, K. Y. Chiang, Y. J. Wei, and H. C. Cheng, “A new pixel circuit for driving organic light emitting diodes with low temperature polycrystalline thin film transistors,” IEEE Journal of Display Technology, vol. 1, no. 1, pp. 100-104, Sep. 2005.
[12] J. H. Lee, J. H. Kim, and M. K. Han, “A new a-Si:H TFT pixel circuit compensating the threshold voltage shift of a-Si:H TFT and OLED for active matrix OLED,” IEEE Electron Device Lett., vol. 26, no. 12, pp. 897-899, Dec. 2005.
[13] G. R. Chaji and A. Nathan, “A stable voltage-programmed pixel circuit for a-Si:H AMOLED displays,” IEEE Journal of Display Technology, vol. 2, no. 4, pp. 347-358, Dec. 2006.
[14] C. L. Lin and Y. C. Chen, “A novel LTPS-TFT pixel circuit compensating for TFT threshold-voltage shift and OLED degradation for AMOLED,” IEEE Electron Device Lett., vol. 28, no. 2, pp. 129-131, Feb. 2007.
[15] S. Ono, K. Miwa, Y. Maekawa, and T. Tsujimura, “VT compensation circuit for AMOLED displays composed of two TFTs and one capacitor,” IEEE Trans. Electron Devices, vol. 54, pp. 462-467, Mar. 2007.
[16] C. L. Lin and T. T. Tsai, “A novel voltage driving method using 3-TFT pixel circuit for AMOLED,” IEEE Electron Device Lett., vol. 28, no. 6, pp. 489-491, Jun. 2007.
[17] S. J. Ashtiani, P. Servati, D. Striakhilev, and A. Nathan, “A 3-TFT current-programmed pixel circuit for AMOLEDs,” IEEE Trans. Electron Devices, vol. 52, no. 7, pp. 1514-1518, Jul. 2005.
[18] J. H. Lee, W. J. Nam, S. H. Jung, and M. K. Han, “A new current scaling pixel circuit for AMOLED,” IEEE Electron Device Lett., vol. 25, no. 5, pp. 280-282, May 2004.
[19] Y. He, R. Hottori, and J. Kanicki, “Current-source a-Si:H thin-film transistor circuit for active-matrix organic light-emitting displays,” IEEE Electron Device Lett., vol. 21, no. 12, pp. 590-592, Dec. 2000.
[20] Y. C. Lin, H. P. D. Shieh, and J. Kanicki, “A novel current-scaling a-Si:H TFTs pixel electrode circuit for AM-OLEDs,” IEEE Trans. Electron Devices, vol. 52, no. 6, pp. 1123-1131, June 2005.
[21] M. Mizukami, K. Inukai, H. Yamagata, T. Konuma, T. Nishi, J. Koyama, S. Yamazaki, and T. Tsutsui, “6-bit digital VGA OLED,” in SID Tech. Dig., 2000, pp. 912-915.
[22] H. Kageyama, H. Akimoto, T. Ouchi, N. Kasai, H. Awakura, N. Tokuda, and T. Sato, “A 3.5-in. OLED display using a 4-TFT pixel circuit with an innovative pixel driving scheme,” in SID Tech. Dig., 2003, pp. 96-99.
[23] H. Kageyama, H. Akimoto, Y. Shimizu, T. Ouchi, N. Kasai, H. Awakura, N. Tokuda, K. Kajiyama, and T. Sato, “A 2.5-inch OLED display with a three-TFT pixel circuit for clamped inverter driving,” in SID Tech. Dig., 2004, pp. 1394-1397.
[24] C. H. Chen, K. H. Chen, C. C. Chen, Y. F. Chen, and H. P. D. Shieh, “AMOLED panel driven by time-multiplexed clamped inverter circuit to reduce complex control signals,” Journal of the Society for Information Display, pp. 787-791, Jul. 2008.
[25] Y. C. Lin and H. P. D. Shsieh, “Improvement of brightness uniformity by AC driving scheme for AMOLED display,” IEEE Electron Device Lett., vol. 25, no. 11, pp. 728-730, Nov. 2004.
[26] C. L. Lin, T. T. Tsai, and Y. C. Chen, “A novel voltage-feedback pixel circuit for AMOLED displays,” J. Display Technol., vol. 4, no. 1, pp. 54-60, Mar. 2008.
[27] A. Giraldo, M. J. Childs, D. Fish, M. T. Johnson, M. Klein, H. Lifka, W. Oepts, W.A. Steer, and N.D. Young, “Optical feedback in active matrix polymer OLED displays,” in The 16th Annual Meeting of the IEEE, vol. 2, 2003, pp. 529-530.
[28] D. Fish, N. Young, S. Deane, A. Steer, D. George, A. Giraldo, H. Lifka, O. Gielkens, and W. Oepts, “Optical feedback for AMOLED display compensation using LTPS and a-Si:H technologies,” in SID Tech. Dig., 2005, pp. 1340-1343.
[29] D. A. Torres, P. F. Lister, and P. Newbury, “LUT-based compensation model for OLED degradation,” Journal of the Society for Information Display, vol. 13, issue 5, pp. 435-441, May 2005.
[30] S. J. Ashtiani, G. R. Chaji, and A. Nathan, “AMOLED pixel circuit with electronic compensation of luminance degradation,” IEEE Journal of Display Technology, vol. 3, no. 1, pp. 36-39, Mar. 2007.
[31] P. E. Burrow, S. R. Forrest, T. X. Zhou, and L. Michalski, “Operating lifetime of phosphorescent organic light emitting devices,” Appl. Phys. Lett., vol. 76, no. 18, pp. 2493-2495, May 2000.
[32] G. R. Chaji, C. Ng, A. Nathan, A. Werner, J. Birnstock, O. Schneider, and J. B. Nimoth, “Electrical compensation of OLED luminance degradation,” IEEE Electron Device Lett., vol. 28, no. 12, pp. 1108-1110, Dec. 2007.
[33] H. J. In and O. K. Kwon, “External compensation of nonuniform electrical characteristics of thin-film transistors and degradation of OLED devices in AMOLED displays,” IEEE Electron Device Lett., vol. 30, no. 4, pp. 377-379, Apr. 2009.
[34] O. K. Kwon and H. J. In, “External compensation of non-uniform image quality and image sticking in active matrix organic light emitting diode displays,” in International Display Manufacturing Conference 2009 (IDMC’ 09), S01-01.
[35] K. S. Karim, A. Nathan, M. Hack, and W. I. Milne, “Drain-Bias Dependence of Threshold Voltage Stability of Amorphous Silicon TFTs,” IEEE Electron Device Lett., vol. 25, no. 4, pp. 188-190, Apr. 2004.
[36] B. H. You, J. H. Lee, and M. K. Han, “Polarity balanced driving scheme to suppress the degradation of Vth in a-Si:H TFT due to the positive gate bias stress for AMOLED,” IEEE Journal of Display Technology, vol. 3, no. 1, pp. 40-44, Sep. 2007.