Design of LTPS Gate Driver Circuit and Simultaneous Emission Pixel Circuits for High-Resolution AMOLED Displays

  • Chin-Hsien Tseng

Student thesis: Master's Thesis

Abstract

相較於氫化非晶矽薄膜電晶體以及非晶相銦鎵鋅氧化物薄膜電晶體,低溫多晶矽薄膜電晶體具有較高載子移動率且較穩定之特性,因此適合用於閘極驅動電路以及主動式矩陣有機發光二極體畫素電路之驅動元件,然而,低溫多晶矽薄膜電晶體之技術會受到製程環境雷射溫度影響,使得分布在不同位置之面板中的電晶體之臨界電壓有所差異而降低顯示畫面品質。現今人們追求高解析度視覺體驗已成為趨勢,在高解析度面板操作中,用漸進式掃描訊號驅動畫素電路,會因畫素補償時間不足而導致畫面的變異性無法完全被補償。相較之下,用同步式訊號驅動法,補償時間在高解析度面板操作中可不受解析度影響,因此能成功補償整個面板之驅動薄膜電晶體變異性。
本論文為改善氫化非晶矽薄膜電晶體之低載子移動率問題,先提出一個9T2C閘極驅動電路以用於高解析度液晶顯示器中,其方法是藉由一簡單架構使驅動薄膜電晶體在洩流輸出點時,能保持高驅動力以縮短下降時間。模擬結果顯示,在相同尺寸之驅動薄膜電晶體條件下,輸出波型之下降時間為1.56 µs,比起傳統電路架構下降時間2.04 µs,改善了約20%。然而,針對高解析度主動式矩陣有機發光二極體顯示器之需求,再提出使用低溫多晶矽薄膜電晶體設計之兩個同步式畫素電路以及一個閘極驅動電路以應用於高解析度主動式矩陣有機發光二極體顯示器。採用同步式驅動法設計之4T2C主動式有機發光二極體畫素電路能於高解析度面板中成功補償臨界電壓變異。模擬結果顯示,此電路在5.15吋FHD的面板操作中能使發光誤差率低於5%,此外電壓線電壓下降問題以及畫面閃爍現象皆受到補償及抑制。為了使電路架構精簡化,進一步提出同步式3T2C畫素電路,藉由匹配驅動薄膜電晶體的方式,該電路僅需一條控制訊號線即可成功補償。基於模擬結果顯示,當薄膜電晶體變異±0.5 V,電流誤差率皆在3.5%以內且無畫面閃爍現象。最後則針對同步式主動式矩陣有機發光二極體畫素電路所需之驅動訊號,設計出11T1C之閘極驅動電路,使得在高速操作下確保畫素能存取精確的資料電壓值。模擬結果驗證連續八級的輸出於5.15吋240赫茲FHD的面板中,且能完整的產生同步式畫素電路所需之驅動訊號。除了考慮面板中的RC負載,此11T1C閘級驅動電路亦連同3T2C畫素電路進行模擬分析,確保所提出之閘級驅動電路輸出訊號,經由負載線依然能使主動式有機發光二極體電路成功補償±0.5 V之驅動薄膜電晶體臨界電壓變異值,以提升高解析度顯示器之性能。

Low temperature poly-silicon thin-film transistors (LTPS TFTs) have characteristic of higher mobility and more stability, compared with amorphous silicon (a-Si:H) TFTs and indium-gallium-zinc oxide (a-IGZO) TFTs. Therefore, they are suitable for use in gate driver circuits and active-matrix organic light-emitting diode (AMOLED) pixel circuits. However, the process of LTPS technology is influenced by the environment temperature of excimer laser. Therefore, the TFTs scattered on different spots of panel, have different characteristics. Nowadays that people pursue high-resolution visual experience has become a trend. Pixel circuits driven by progressive scan signals are not able to deal with the variations of LTPS TFTs because of insufficient time for completing compensation when operated in high-resolution panels. In contrast, pixel circuits with simultaneous-emission (SE) driving scheme can successfully be compensated when the circuits operate in high-resolution panels because the period time for compensation is not limited by the resolution of panels and the variations of driving TFTs.
To solve the low mobility problem of a-Si:H TFTs, this thesis firstly proposes a 9T2C gate driver circuit for high-resolution active-matrix liquid crystal display. By a simple structure, the driving TFT of the circuit can remain high driving capability for discharging output node and reduce the falling time of output signals. Simulated results show that the rising time of output signals 1.56 µs, is reduced about 20%, compared to the rising time of conventional one 2.04 µs. However, for the demand of high-resolution AMOLED pixel circuits, two SE LTPS-TFT pixel circuits and one corresponding gate driver circuit are proposed for high-resolution AMOLED displays. A 4T2C pixel circuit using SE driving scheme can successfully compensate for the VTH variation of TFTs in high-resolution panels. Simulated results revel that the current error rates are below 5% when the circuit operates in 5.15 inch FHD panel. Moreover, power line I-R drops and image flickering phenomenon are compensated and repressed. For further concise circuit structure, a SE 3T2C pixel circuit is proposed. By using matching structure, the circuit can successfully compensate for the variations with only one control signal line. Based on simulated results, the image flickering is eliminated and the related current error rates are within 3.5% when threshold voltage of driving TFT varies ±0.5 V in 5.15 inch FHD panel. Lastly, 11T1C gate driver circuit is designed for the driving scheme of SE AMOLED pixel circuits, ensuring that the pixels can collect precise data voltage value in high speed operation. Simulated results indicate that the continuous-eight gate driver circuits enable to generate complete driving scheme signals for SE pixel circuit in 5.15 inch 240 Hz FHD panel. In addition, the 11T1C gate driver circuit is applied to the 3T2C pixel circuit for simulation analyzation. Therefore, the output signals of the proposed gate driver circuit through the row line to supply for AMOLED pixel circuits are ensured for achieving uniform currents by successfully detecting ±0.5 V VTH variations of driving TFT. Thereby, the performance of high-resolution displays is enhanced.
Date of Award2018
Original languageEnglish
SupervisorChih-Lung Lin (Supervisor)

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