Abstract
近年來主動式有機發光二極體顯示器蓬勃的發展，而有機發光二極體本身為電流驅動元件，因此需要畫素電路給予其驅動電流，而畫素電路架構大多由薄膜電晶體與電容所組成，薄膜電晶體之主流製程可分為三種，包含氫化非晶矽薄膜電晶體、非晶相銦鎵鋅氧化物薄膜電晶體、低溫多晶矽薄膜電晶體。隨著顯示器產品目標尺寸的不同，將選擇使用不同的薄膜電晶體製程，對中小尺寸有機發光二極體顯示器來說，低溫多晶矽薄膜電晶體因為其極佳的電流驅動能力與較小的佈局面積而被廣泛地採用，而針對中大型尺寸有機發光二極體顯示器，氫化非晶矽薄膜電晶體與非晶相銦鎵鋅氧化物薄膜電晶體因為其較低的成本與較均勻的電性而被選擇。然而，不論是使用哪一種電晶體製程，電晶體與有機發光二極體各種元件特性上的變異需要被考慮，例如臨界電壓、載子漂移率、發光效率等，都將使顯示器畫面產生失真，因此大量的畫素補償電路相繼的被提出，然而大多數的補償電路都只能針對薄膜電晶體之臨界電壓變異做補償，並沒有考慮薄膜電晶體載子漂移率的變異與有機發光二極體的老化，此外，大多數畫素電路的補償時間因為其在補償階段需要使用資料線上的電壓，使其補償時間受限於顯示器中一列的掃描時間，當顯示器的解析度與畫面更新頻率提高時，顯示器每一列的掃描時間將減短，因此這些補償時間受限於掃描時間的畫素電路在高解析度與高掃描頻率顯示器之應用非常受限。本論文針對不同尺寸顯示器共提出四個新式畫素補償電路與一個新式閘極驅動電路，其中所提出之四個新式畫素電路皆針對高解析度與高掃描頻率顯示器做設計，其補償時間皆不受限於顯示器之掃描時間。第一個6T2C畫素電路針對小尺寸行動裝置做設計，其可以補償薄膜電晶體臨界電壓變異並有效防止畫面閃爍，提供高品質的黑畫面，此電路被實際製造成1.3吋之面板，並經由實驗結果驗證此面板有效改善傳統補償電路因為補償時間過短而造成之低畫面均勻性與低對比度之問題。第二個5T2C畫素電路也針對中小尺寸顯示器做設計，除了補償薄膜電晶體之臨界電壓變異，此電路還可以補償有機發光二極體之老化，此外，透過改變控制波形，使用同一個畫素電路架構可同時達到二維模式與三維模式之切換，在二維模式使用漸進式驅動，三維模式使用同步式驅動，使顯示器達到最大的亮度。第三個4T2C畫素電路針對中大尺寸顯示器做設計，其能夠補償增強式非晶相銦鎵鋅氧化物薄膜電晶體之臨界電壓與載子漂移率變異，並同時補償有機發光二極體之老化，此畫素電路經過下線被實際製作成電路樣本，透過實驗量測電路中各節點之電壓，證明延長補償時間可使元件變異偵測更為精準。第四個4T2C畫素電路針對大尺寸高解析度做設計，其利用反向有機發光二極體結合非晶相銦鎵鋅氧化物薄膜電晶體與上發光架構，此電路可同時補償增強型與空乏型非晶相銦鎵鋅氧化物薄膜電晶體臨界電壓與載子漂移率變異，且針對補償時間長短造成低灰階電流之影響做詳細之分析與驗證。除了畫面顯示以外，觸控已成為現今顯示器必備的功能，在觸控結構中內嵌式又為最輕薄之架構，因此本論文提出一個針對內嵌式觸控架構設計之新式雙向閘極驅動電路，其利用時間分割法增加觸控偵測的訊號雜訊比與回報率，使得觸控偵測更為精準，並透過閘極驅動電路內部之二次充電電路，使得閘極驅動電路之驅動薄膜電晶體在觸控偵測時不會被長時間的偏壓，不僅有效延長閘極驅動電路的壽命，且使得每一級閘極驅動電路的輸出波形更為一致，使得所設計之觸控功能與顯示器能夠更完美的結合，以達到高可靠度內嵌式觸控顯示器之目標。
In the recent years, active matrix organic light emitting diode (AMOLED) displays have been widely developed. Since OLED is a current-driving device, it needs a pixel circuit to supply driving currents. Most pixel circuits are composed of thin film transistors (TFTs) and capacitors. There are three mainstreams of TFT technology, which are hydrogenated amorphous silicon (a-Si:H) TFTs, amorphous indium-gallium-zinc-oxide (a-IGZO) TFTs, and low-temperature polycrystalline-silicon (LTPS) TFTs. The technology of TFT backplane is chosen according to different display sizes. For small/medium-sized AMOLED displays, LTPS TFT backplane is widely adopted because of its high current driving capability and small layout area. For medium/large-sized AMOLED displays, a-Si:H and a-IGZO TFT backplanes are better due to the low cost and high electrical uniformity. However, in each type of TFT process, the variations of device characteristics such as threshold voltage (VTH), mobility, and emissive efficiency should be considered. These variations will result in distortion of displayed images. Therefore, many pixel circuits have been proposed. Nevertheless, most compensating circuits only compensate for VTH variations of TFT. The mobility variation of TFTs and the OLED degradation are not considered. Moreover, most pixel circuits use the voltage on the data line during the compensation period, so their compensation times are limited by the scan time of a row in a display. When the resolution and frame rate of displays increase, the scan time of a row decreases. Thus, the performance of these pixel circuits are very limited for high resolution and high frame rate AMOLED applications.
This thesis proposes four pixel circuits and one gate driver circuit for different display sizes. All the four pixel circuits are designed for high resolution and high frame rate AMOLED displays. Their compensation times are not limited by the scan time of a row. First 6T2C pixel circuit is designed for small-sized mobile devices. The circuit compensates for VTH variations of LTPS TFTs. Also, the circuit prevents the display from flicker, providing high quality of displayed black images. Real 1.3-inch panel is fabricated. Measured results confirm that the proposed pixel circuit effectively improves the low uniformity and low contrast ratio due to the short compensation time. Second 5T2C pixel circuit is designed for small/medium-sized displays. It compensates for the OLED degradation. Additionally, the pixel circuit can be switched between two-dimension (2D) and three-dimension (3D) modes through changing the control signals. Progressive emission (PE) method is used in 2D mode and simultaneous emission (SE) method is adopted in 3D mode, maximize the luminance of the displays. Third 4T2C pixel circuit is designed for medium/large-sized displays. It compensates for the VTH and mobility variations in a-IGZO TFTs as well as the OLED degradation. The pixel is fabricated in the test key. Node voltages in the pixel circuit have been measured, verifying that extending the compensation time makes the sensing of each variation in devices more precise. Fourth 4T2C pixel circuit is designed for 8K4K ultra high resolution and large-sized displays. The top anode OLED structure is utilized for better integration of n-type a-IGZO TFT and the top emission structure. The circuit also compensates for the VTH and mobility variations of both normally-on and normally-off a-IGZO TFTs. Furthermore, the relationship between the compensation time and low gray level current is analyzed. In addition to the image display, touch function has become indispensable in displays. Among different touch types, in-cell touch is the slimmest and lightest structure. This thesis proposed a new bidirectional gate driver circuit for in-cell touch displays. The circuit uses time division driving method (TDDM) to increase the signal-to-noise ratio (SNR) and reporting rate, making the touch sensing more precise. Also, new re-charging circuit in the gate driver circuit prevents the driving TFT in output generation circuit from long-term stress during the touch sensing. The re-charging circuit not only increase the lifetime of gate driver circuit but also makes the output waveforms produced by each stage of gate driver circuit more uniform. Therefore, the touch function can be integrated with the display better, achieving the goal of highly reliable in-cell touch displays.
| Date of Award | 2018 |
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| Original language | English |
| Supervisor | Chih-Lung Lin (Supervisor) |