The Amorphous Silicon Thin Film Transistor Compensation Pixel Circuit for Threshold Voltage Shift with Application to Organic Light Emitting Diode

非晶矽薄膜電晶體在操作的同時，會因為偏壓應力而造成元件本身的衰退，其臨界電壓會隨著操作時間而漸漸增加。當非晶矽薄膜電晶體被應用在驅動有機發光二極體時，臨界電壓的增加會造成驅動電流及發光亮度都隨時間而減少。為了解決這個問題，本篇論文中提出一種含有四顆電晶體及一顆電容(4T1C)的新式臨界電壓補償式電路。在一連串改進製程參數及電晶體架構之後，終於製造出能被用於驅動有機發光二極體電路的最佳電晶體元件。不論是在模擬或實驗結果之中，我們所提出的4T1C 電路都展現出其對非晶矽薄膜電晶體臨界電壓偏移的補償效應，與傳統型兩顆電晶體一個電容(2T1C)的電路設計相比，新的電路也可以看到對驅動電流衰減的改善。在模擬結果中，當傳統型驅動電路被偏壓應力30 分鐘以後的電流衰減量在百分之7 到28 時，新提出的4T1C 驅動電路可將衰減量抑制在百分之3 之下。而在實驗中，不論在什麼操作電壓之下，4T1C 驅動電路電流的衰減也都能比傳統的2T1C 驅動電路有一半以上的改善。The threshold voltage of hydrogenated amorphous silicon thin-film transistor(a-Si:H TFT) increases with time due to bias stress. This causes the driving current and emitting brightness of organic light emitting diodes (OLED) to decay with time. Thus a new TFT voltage-programmed compensation pixel circuit for threshold voltage shift which consists of four transistors and one capacitor (4T1C) is proposed in this thesis. After a series of improvement in the design of the TFT structure and processing parameters, the TFT structure is optimized for the driving circuit of OLED. Both the simulation and experimental results reveal that the proposed 4T1C circuit can compensate the threshold voltage shift of the amorphous silicon TFT. Comparing to the traditional two transistors and one capacitor (2T1C) driving circuit, the new driving circuit gives a better compensation to the current degradation. In simulation, the theoretical current drop after a 30 minutes bias stress is in the range from 7% to 28 %, the current error of the proposed 4T1C driving circuit can be reduced to less than 3 %. The experimental results indicate that the degradation of OLED current of the proposed 4T1C driving circuit could be improved to less than half of the traditional 2T1C pixel.Chapter 1 Introduction....................................1 Chapter 2 Experiments...............................................4 2.1 Deposition System -PECVD…............................4 2.2 Substrate preparation.................................8 2.3 Deposition Procedures................................10 2.4 Measurement Techniques...............................12 2.4.1 Film Thickness and Refractive Index................12 2.4.2 IR Absorption Spectra..............................12 2.4.3 Transmittance and Reflectance......................12 2.4.4 Current–Voltage Characteristics ..................13 2.5 Design of the Voltage Driving Signals................15 2.5.1 Field Programmable Gate Array (FPGA) Board.........15 2.5.2 Voltage Step-Up Circuit............................15 2.5.3 Switching Part.....................................16 Chapter 3 Amorphous-Silicon Thin Film Transistor for Pixel Circuits ................................................20 3.1 Conventional Amorphous-Silicon TFT Structure.........20 3.1.1 Experiments........................................20 3.1.2 Results and Discussion.............................22 3.2 Improved Amorphous-Silicon TFT Structure.............23 3.2.1 Experiments........................................23 3.2.2 Results and Discussion.............................30 3.3 Double-Thick-Gate Amorphous-Silicon TFT..............31 3.3.1 Experiments........................................31 3.3.2 Results and Discussion.............................31 3.4 The Stacked-Gate Metal Amorphous-Silicon TFT.........36 3.4.1 Experiments........................................36 3.4.2 Results and Discussion.............................37 3.5 Stress-Induced Threshold Voltage Shift...............42 Chapter 4 Compensation Pixel Circuit for Threshold Voltage Shift............................................44 4.1 The Operation of the Proposed Compensation Circuit for Threshold Voltage Shift..................................44 4.1.1 Compensation Period................................44 4.1.2 Emission Period....................................45 4.1.3 The Operation of Traditional OLED Driving Circuit..47 4.2 Simulation Results...................................50 4.3 Experiments and Measurement..........................52 4.4 Results and Discussion...............................59 Chapter 5 Conclusions..............................................61 reference 6