在過去幾年,主動式有機發光二極體(AMOLED)顯示器做為顯示器產業的主流,其擁有著相比傳統液晶顯示器更高的對比度、更廣的視角、快速的反應時間、自發光等優點。然而隨著市場需求不斷的發展和技術的迭代,主動式有機發光二極體顯示器也面臨著限制及挑戰,其中最為主要的便是在長時間顯示靜態圖像後,可能會造成的烙印現象(burn-in),以及有機材料本身的老化,這就為近年微發光二極體(Micro-LED)技術的崛起提供了絕佳的機會,因為其並不使用有機材料,且在對比度、視角、反應時間等方面表現得比前者更優秀。
基於以上這些優勢,微發光二極體在車用顯示器、智慧型手機、攜帶式裝置、穿戴式健康監測設備、AR/VR眼鏡等領域具有相當廣闊的應用前景,若能隨著技術進一步的成熟以及生產成本的降低,微發光二極體將成為顯示技術的一個新的里程碑,並為我們提供更高品質的顯示體驗。目前微發光二極體的應用因為巨量轉移、良率及生產成本等因素,導致現階段大尺寸顯示器還無法大量普及,也因為近年穿戴式裝置的快速發展,所以主要仍偏向小尺寸的應用;而顯示器無論是為了追求更高的視覺品質,增強虛擬和擴增實境體驗,抑或是為了顯示流暢的動態內容,來改善人機之間的交互性和反應速度,微發光二極體顯示器產品朝向高畫面刷新率的方向發展都具其必要性。
為了使畫面能夠更完整的呈現,同步發光與漸進式發光(Progressive Emission, PE)相比會更有優勢,因為漸進式發光為逐行輸入資料電壓再行發光,當圖像移動速度較快時,可能會產生動態模糊(motion blur);當畫面刷新率不夠高,圖像在每一行的更新時無法足夠快速的跟上物體的運動,也會造成上述問題,影響畫面品質。然而,同步發光的畫面刷新時間,會被畫素電路的發光時間(emission period)壓縮,使其在相同解析度下,畫面刷新率會低於漸進式發光。
針對上述議題,本論文提出一個10T2C,使用低溫多晶矽薄膜電晶體(Low Temperature Poly-silicon, LTPS),使用同步發光(Simultaneous Emission, SE),並適合中小型尺寸攜帶式裝置的微發光二極體顯示器的畫素電路。當其使用脈衝寬度調變時(Pulse Width Modulation, PWM),電路的畫面刷新率可達到120 Hz,並可補償驅動電晶體之臨界電壓變異,使電路可在6 V的低壓驅動下,輸出高均一性的電流,當驅動電晶體之臨界電壓變異±0.3 V時,全資料電壓範圍的電流誤差率皆小於1.06%;而當電路使用混合模式(PWM+PAM)驅動時,會在亮度保持不變的情況下,透過將畫素電路原先的發光時間縮減50 %來將畫面刷新率提升至160 Hz,此時驅動電晶體之臨界電壓變異±0.3 V時, 全資料電壓範圍的電流誤差率皆小於0.50 %。

In the past few years, active-matrix organic light-emitting diode (AMOLED) displays have become the mainstream in the display industry, they offer advantages such as higher contrast, wider viewing angles, faster response times, and self-emission when compared to traditional liquid crystal displays (LCDs). However, as market demands and technology continue to evolve, AMOLED displays also face limitations and challenges. The most significant of these is the possibility of the burn-in phenomena after displaying static images for a long time, as well as the aging of the organic materials. These challenges have provided an excellent opportunity for the rise of micro-LED technology in recent years because it does not use organic materials and have better performance than the former in terms of contrast, viewing angle, response time, etc.
Based on these advantages, micro-LED displays have a wide range of applications such as automotive displays, smartphones, portable devices, wearable health monitoring devices, AR/VR glasses, and more. If micro-LED technology continues to mature and the reduction of production costs, it will become a new milestone in display technology, providing us with higher-quality display experiences. Currently, the application of micro-LED displays is limited for large-size displays not only attribute to factors such as mass transfer, yield, and production costs, but also the rapid development of wearable devices in recent years, that’s the reason why industry still focus on small-size applications. Whether it is to pursue higher visual quality, enhance virtual and augmented reality experiences, or display smooth dynamic content to improve the interactivity and response speed between humans and displayers, the development of micro-LED display products towards high refresh rates is necessary.
To achieve a more complete presentation of images, simultaneous emission (SE) has advantages over progressive emission (PE). Progressive emission involves sequentially inputting data voltages for each row before emission, which can result in motion blur when the image moves rapidly. Insufficient refresh rates will also cause the same problem when the speed of refreshing the data voltage of each row cannot keep up with the object's motion, which will affect the image quality. However, the refresh time of simultaneous emission is compressed by the emission period of the pixel circuit, resulting in a lower refresh rate compared to progressive emission when at same resolution.
In this thesis, we propose a pixel circuit with 10T2C structure using Low Temperature Poly-silicon thin-film transistors (LTPS-TFT) which is suitable for small and medium-sized portable micro-LED displays adopting simultaneous emission and can achieve a refresh rate of 120 Hz when using Pulse Width Modulation (PWM). It compensates for variations in the threshold voltage of the driving transistors (DTFT), allowing the circuit to output highly uniform currents under a low voltage driving of 6V. When the threshold voltage variation of the DTFTs is ±0.3 V, the current error rate for all data voltage range is less than 1.06 %. When the circuit is driven in Hybrid mode (PWM+PAM), the refresh rate can be increased to 160 Hz while maintaining the same brightness by reducing the emission period of the pixel circuit by 50 %. In this case, when the threshold voltage variation of the DTFTs is ±0.3V, the current error rate for all data voltage range is less than 0.50 %.

[1] R. M. A. Dawson et al., “The impact of the transient response of organic light emitting diodes on the design of active matrix OLED displays,” in IEDM Tech. Dig., pp. 875–878, Dec. 1998
[2] M. Stewart et al., “Polysilicon VGA active matrix OLED displays-technology and performance,” in IEDM Tech. Dig., pp. 871-874, Dec. 1998
[3] G. Cheng et al., "White organic light-emitting devices using a phosphorescent sensitizer," Appl. Phys. Lett., vol. 82, no. 24, pp. 4224-4226, Jun. 2003.
[4] A. Kumar et al., "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, Sep. 2004.
[5] Wei-Jing Wu et al., “An AC driving pixel circuit compensating for tfts threshold-voltage shift and oled degradation for amoled,” IEEE/OSA Journal of Display Technology, vol. 9, pp. 572-576, 2013
[6] Kim, Daejung et al., “High Resolution a-IGZO TFT Pixel Circuit for Compensating Threshold Voltage Shifts and OLED Degradations,” IEEE Electron Devices Society, vol.5, pp. 372-377, 2017.
[7] Chen, Qian et al., “A New Compensation Pixel Circuit with LTPO TFTs” SID 2019 Digest.
[8] Zhaojun Liu et al., “GaN-based LED micro-displays for wearable applications,” Microelectronic Engineering, vol. 148, pp. 98-103, Dec. 2015
[9] Vincent W.Lee et al., “Micro-LED Technologies and Applications,” Information Display, vol.32, pp. 16-23, Nov. 2016
[10] Chien-Chung, Lin et al., “Ultra-Fine Pitch Thin-Film Micro LED Display for Indoor Applications,” SID 2018 Digest, vol. 49, pp. 782-785, May. 2018
[11] Yong-Hoo, Hong et al., “A Novel Micro-LED Pixel Circuit Using n-type LTPS TFT with Pulse Width Modulation Driving,” SID 2021 Digest, vol. 52, pp. 868-871, May. 2021
[12] Eun Kyo, Jung et al,. “MicroLED Pixel Circuit Based on Metal Oxide Thin-Film Transistor with Progressive Emission Method Using Pulse Width Modulation,” SID 2022 Digest vol. 53, pp.1312-1315, May. 2022
[13] Young-Ju, Park et al., “Voltage-Programming-Based Pixel Circuit to Compensate for Threshold Voltage and Mobility Using Natural Capacitance of Organic Light-Emitting Diode,” Japanese Journal of Applied Physics, vol. 49, Mar. 2010
[14] C. L. Fan et al., “Electrical performance and reliability improvement of amorphous-indium-gallium-zinc-oxide thin-film transistors with HfO2 gate dielectrics by CF4 plasma treatment,” Materials, vol. 11, no. 5, p. 824, May 2018.
[15] J. J. Lih et al., “Comparison of a-Si and poly-Si for AMOLED displays,” J. Soc. Inf. Display, vol. 12, no. 4, pp. 367–371, Dec. 2004.
[16] M. Kimura et al., "Low-temperature polysilicon thin-film transistor driving with integrated driver for high-resolution light emitting polymer display," IEEE Trans. Electron Devices, vol. 46, no. 12, pp. 2282-2288, Dec. 1999.
[17] S. W. Lee and S. K. Joo, "Low temperature poly-si thin-film transistor fabrication by metal-induced lateral crystallization," IEEE Electron Device Lett., vol. 17, no. 4, pp.160-162, May 1996.
[18] T. Kamiya et al., " Sci. Technol. Adv. Mater., vol. 11, no. 4, Aug. 2010.
[19] K. Nomura et al., “Amorphous oxide semiconductors for high-performance flexible thinfilm transistors,” Jpn. J. Appl. Phys., vol. 45, no. 5B, pp. 4303–4308, May 2006.
[20] E. Fortunato et al., "Oxide Semiconductor Thin-Film Transistors A Review of Recent Advances," Adv. Mater., vol. 24, no. 22, pp. 2945-2986, May 2012.
[21] C. L. Lin et al., “Pixel Circuit With Leakage Prevention Scheme for Low-Frame-Rate AMOLED Displays,” IEEE J. Electron Devices Soc., Vol. 8, pp. 235–240, Nov. 2020.
[22] S. H. Jung et al., "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.
[23] J. Wager, “Flat-Panel-Display Backplanes: LTPS or IGZO for AMLCDs or AMOLED Displays?,” Information Display, Mar. 2014
[24] H. Chen et al., “Liquid crystal display and organic light-emitting diode display: present status and future perspectives,” Light Sci. Appl., vol. 7, 17168, Mar. 2018.
[25] Kim, Daejung et al., “High Resolution a-IGZO TFT Pixel Circuit for Compensating Threshold Voltage Shifts and OLED Degradations,” IEEE Electron Devices Society, vol.5, pp. 372-377, 2017.
[26] Wenxue, Huo et al., “A New Pixel Circuit for Active Matrix Mini&Micro Light Emitting Diodes,” 5th IEEE International Conference on Electronics Technology, pp. 152-155, May. 2022
[27] Yi-Cheng, Liu et al., “Mini-LED displays pixel circuit design,” 2020 IEEE International Conference on Consumer Electronics, Nov. 2020
[28] Tan, Guanjun et al., “High dynamic range liquid crystal displays with a mini-LED backlight,” Optics Express, vol. 26, pp. 16572-16584, Jun. 2018
[29] Ming-Yang Deng et al., “Reducing Power Consumption of Active-Matrix Mini-LED Backlit LCDs by Driving Circuit,” IEEE Transactions on Electron Devices, vol. 68, pp.2347-2354, May. 2021
[30] Ke, Cheng-Han et al., “A Novel Mini-LED Backlit Driving Circuit Using PWM Mechanism with VTH and VSS I-R Rise Compensation,” AM-FPD 2022, pp.25-28, Jul. 2022
[31] Lin, Chih-Lung et al., “AM PWM Driving Circuit for Mini-LED Backlight in Liquid Crystal Displays,” IEEE Journal of the Electron Devices Society, vol. 9, pp. 365-372, 2021
[32] Wenxue Huo et al., “The Limitation of Threshold-Voltage Compensation Range for Internal Compensation Circuit in the AMMiniLED Pixel Structure,” 2022 The 4th Asia Energy and Electrical Engineering Symposium, 2022
[33] Lin, Chih-Lung et al., “AM Mini-LED Backlight Driving Circuit Using PWM Method With Power-Saving Mechanism,” IEEE Journal of the Electron Devices Society, vol. 10, pp. 256-262, 2022
[34] Jung, Eun Kyo et al., “A New Pixel Circuit Based on LTPO Backplann Technology for Micro-LED Display Using PWM Method,” SID 2021 Digest, 2021
[35] Wang, Ting et al., “A New PWM Pixel Circuit for Micro-LED Display wuth 60Hz Driving and 120Hz Lighting,” SID 2020 Digest.
[36] Bin Zhao et al., “A novel compensation pixel circuit for high bits of AM mini/micro-LED based on PWM method,” Energy Reports, Apr. 2021
[37] Yong-Hoo, Hong et al., “A Novel Micro-LED Pixel Circuit Using n-type LTPS TFT with Pulse Width Modulation Driving,” SID 2021 Digest, vol. 52, pp. 868-871, May. 2021
[38] Lin, Yi-Zhen et al., “Active-Matrix Micro-LED Display Driven by Metal Oxide TFTs Using Digital PWM Method,” IEEE Transactions on Electron Devices, vol. 68, pp. 5656-5661, Nov. 2021
[39] Oh, Jongsu et al., “Pixel circuit with P-type low-temperature polycrystalline silicon thin-film transistor for micro light-emitting diode displays using pulse width modulation,” IEEE Electron Device Letters, vol.42, pp. 1496-1499, Oct. 2021
[40] Zou, Pei-An et al., “”A New Analog PWM Pixel Circuit With Metal Oxide TFTs for Micro-LED Displays,” IEEE Transactions on Electron Devices, vol. 69, pp. 4306-4311, Aug. 2022
[41] Bin Zhao et al., “A novel compensation pixel circuit for high bits of AM mini/micro-LED based on PWM method,” Energy Reports, Apr. 2021
[42] Fu, Jia et al., “Two-mode pwm driven micro-led displays with dual-gate metal-oxide tfts,” Digest of Technical Papers - SID International Symposium, vol.53, pp. 1070-1073, May. 2022
[43] Wang, Limin et al., “A Novel Hybrid Driving Method for Mini/Micro LED Display,” Digest of Technical Papers - SID International Symposium, vol.53, pp. 314-317, Jul. 2022
[44] Lin, Yi-Zhen et al., “Active-Matrix Micro-LED Display Driven by Metal Oxide TFTs Using Digital PWM Method,” IEEE Transactions on Electron Devices, vol. 68, pp. 5656-5661, Nov. 2021
[45] Hong, Yong-Hoo et al., “Improvement of the low gray-level expression using hybrid pulse width modulation and pulse amplitude modulation driving method for a micro light-emitting diode pixel circuit,” Journal of Information Display, vol. 23, pp. 243-249, 2022
[46] B. W. Lee et al., "Novel Simultaneous Emission Driving Scheme for Crosstalk-free 3D AMOLED TV," J. Soc. Inf. Disp., vol. 41, no. 1, pp. 758-761, May. 2010.
[47] C. S. Chiang et al., "Electrical Instability of Hydrogenated Amorphous Silicon Thin-Film Transistors for Active-Matrix Liquid-Crystal Displays," Jpn. J. Appl. Phys., vol. 37, no. 11, pp. 4704-4710, Sep. 1998.
[48] M. J. Powell et al., "Time and temperature dependence of instability mechanisms in amorphous silicon thin ‐ film transistors," Appl. Phys. Lett., vol. 54, no. 14, pp. 1323-1325, Apr. 1989.
[49] S. Sambandan and A. Nathan, "Equivalent Circuit Description of Threshold Voltage Shift in a-Si:H TFTs From a Probabilistic Analysis of Carrier Population Dynamics," IEEE Trans. Electron Devices, vol. 53, no. 9, pp. 2306-2311, Aug. 2006.
[50] C. L. Fan et al., "New pixel circuit compensating poly-si TFT threshold-voltage shift for a driving AMOLED," J. Korean Phys. Soc., vol. 55, no. 4, pp. 1185-1189, Apr. 2010.
[51] C. L. Fan et al., "A novel LTPS TFT pixel circuit to compensate the electronic degradation for active-matrix organic light emitting diode displays," Int. J. Photoenergy, Apr. 2013.
[52] J. P. Lee et al., "Threshold voltage and IR drop compensation of an AMOLED pixel circuit without a VDD Line," IEEE Electron Devices Lett., vol. 35, no. 1, pp. 72-74, Nov. 2014.
[53] C. L. Fan et al., "LTPS-TFT pixel circuit compensating for TFT threshold voltage shift and IR-drop on the power line for AMOLED displays," Adv. Mater. Sci. Eng., May 2012.
[54] Y. Sohn et al., “Effects of TFT mobility variation in the threshold voltage compensation circuit of the OLED display,” J. Inf. Display, vol. 18, pp. 25–30, Jan. 2017.
[55] C. L. Lin et al., "New Voltage-Programmed AMOLED Pixel Circuit to Compensate for Nonuniform Electrical Characteristics of LTPS TFTs and Voltage Drop in Power Line, "IEEE Trans. Electron Devices, vol. 61, no. 7, pp. 2454-2458, Jul. 2014.
[56] K. V. Chellappan et al., "State of the Art in Stereoscopic and Autostereoscopic Displays," Proc. IEEE Inst. Electr. Electron Eng., vol. 99, no. 4, pp. 540-555, Apr. 2011.
[57] J. Hong et al., "Three-dimensional display technologies of recent interest Principles, status, and issues," Appl. Opt., vol. 50, no. 34, pp. 87-115, Dec. 2011.
[58] P. Fuchs et al., “Virtual Reality Headsets - A Theoretical and Pragmatic Approach”, CRC Press, Feb. 2017.
[59] Jie Shan et al., “AUTOSTEREOSCOPIC MEASUREMENT: PRINCIPLES AND IMPLEMENTATION,” Medicine, Jan. 2003
[60] E. Lueder, 3D Displays, John Wiley & Sons, Jan. 2012.
[61] Kim, Jin-Ho et al., “PWM pixel circuit with LTPS TFTs for micro-LED Displays,” Digest of Technical Papers - SID International Symposium, vol.50, pp.192-195, May. 2019
[62] Kang, Kyeong-Soo et al., “A Compact Amorphous In-Ga-Zn-Oxide Thin Film Transistor Pixel Circuit With Two Capacitors for Active Matrix Micro Light-Emitting Diode Displays,” IEEE Journal of the Electron Devices Society, vol. 11,pp. 204-209, 2023