Designs of Optical Pixel Sensors for Thin-Film Transistor Liquid Crystal Displays with Optical Input Functions

  • Chia-En Wu

Student thesis: Master's Thesis

Abstract

隨著人們對於高品質、高畫質之顯示器產品需求上升,薄膜電晶體技術之進步使得顯示器可透過薄膜電晶體之特性與相對應之驅動能力進行周邊驅動電路或感測器之設計,改善顯示器整體之性能並增加其具備之功能。在目前所被廣泛應用之薄膜電晶體製程中,非晶矽薄膜電晶體因其成熟的製程、簡單的結構、高度的均勻性,及對可見光波段的高度光學反應,使得非晶矽薄膜電晶體可應用於內嵌式光學感測器及驅動電路之設計。在眾多應用中,非晶矽薄膜電晶體可依據照射光源之亮度大小而產生相對應之光電流大小,可成功將光學訊號轉換為電訊號,使外部電路可偵測接收輸入光源之光學感測電路的位置,若實現光學感測電路於顯示器面板中則可輕易達到多點光學輸入之功能。然而由於光學感測器所能感測之波段屬可見光波段,高環境白光強度之變異也會輕易使非晶矽薄膜電晶體產生較高之光電流,進而導致誤輸出之後果。因此,如何防止環境白光亮度之差異影響整體光學感測面板之輸出結果是整體設計之重點,藉此拓展光學感測電路於各式顯示器產品之應用。
本論文之主旨為針對具備光學輸入偵測功能之主動式矩陣液晶顯示器提出三個基於非晶矽薄膜電晶體之新式光學感測電路之設計,並提出一個基於非晶相銦鎵鋅氧化物電晶體之新式光學感測電路之設計,並進一步為了整體感測系統進行系統整合,設計一基於非晶相銦鎵鋅氧化物電晶體之閘極驅動電路,以產生顯示器驅動系統所需之閘極驅動訊號,並同時供給光學感測器之驅動訊號,進而達到整體感測系統整合之功能,並使整體感測系統具備高度完善性及穩定度。本論文第一個電路則是一洩流式光學感測電路,透過結合傳統彩色濾光片製程與非晶矽光學薄膜電晶體,使得光學感測器能夠區別光學輸入訊號及環境光亮度之大小,進而針對高環境白光產生補償之光電流,使光學感測電路能夠在高環境白光12280 lux的情況下維持顯著的電壓差異。本論文所提出之第二個電路為一充電式光學感測電路,同樣也透過光學薄膜電晶體結合彩色濾光片製程達到區別環境光亮度及光學輸入訊號之亮度大小,進而防止高環境白光產生誤輸出之現象。根據量測結果,所提出之充電式光學感測電路可在873 lux與12910 lux白光亮度間保持顯著之差異,充分展現其穩定性。本論文所提出之第三個電路則是針對前兩章節尚未探討之長時間穩定性進行相關光學電晶體之特性探討,透過量測光學電晶體在長時間偏壓的特性變化以設計一新式交替感測式光學感測電路,藉此延長整體感測模組之使用壽命。另外,本論文針對不同顏色之彩色濾光片進行特性量測,搭配傳統白光LED之發光頻譜可得知應用傳統白光於各光學薄膜電晶體上之能量強度,進而得到各電晶體在相同白光強度下所產生之光電流大小關係,以建立白光補償架構中各個光學薄膜電晶體之尺寸大小,藉此實現具備三原色多組輸入光源之完整光學感測模組。本論文另外提出一不同於非晶矽薄膜電晶體之非晶相銦鎵鋅氧化物薄膜電晶體製程技術進行光學感測電路之設計。由於非晶相銦鎵鋅氧化物薄膜電晶體本身具備較高之電子能量位階而對可見光頻段之反應較低,因此應用此薄膜電晶體之光學感測器較不受高環境白光環境影響。本論文也同時提出相對應之模擬等效模型,而所提出之感測器在使用低波長光源做為輸入訊號時,在高環境白光下可經由模擬結果得到大於16 V之明顯差異值。為了增進整體系統之相容性,本論文提出基於非晶相銦鎵鋅氧化物薄膜電晶體之閘極驅動電路,並在不增加額外驅動訊號之前提下實現雙向驅動之功能以保持其簡單架構。透過改善其驅動電晶體偏壓情形之結構,使所提出之閘極驅動電路可在高溫70 °C操作812小時而依然保持其輸出完整性,確保系統之穩定性及完善性。

As the demand of high quality display products increases in recent year, the progress in thin-film transistor (TFT) technologies make TFT be suitable for the design of driving circuits or integrated sensors. By applying these TFT-based driving circuits and integrated sensor to various display applications can realize several value-added functions. In particular, hydrogenated amorphous silicon (a-Si:H) TFTs have been widely used for driving circuits and integrated optical sensors due to its simple structure, mature process, high uniformity, and high photo-sensitivity. The a-Si:H TFT can sense the light intensity from the illumination and transform into electrical signals of photocurrents, further transmitting these signals to external drivers for position detection. These characteristics make a-Si:H TFT-based optical pixel sensor be highly suitable for achieving optical input functions with multi-point detection easily. However, since optical sensor detects the illuminations from visible lights, ambient white light illumination with high intensity also induce large photocurrents of a-Si:H TFT-based sensors and lead to serious false detection. Therefore, preventing the variation in white light from affecting the SNR is significant to TFT-based optical sensors for the display with optical input functions.
This dissertation proposes four optical sensors respectively employing a-Si:H TFTs and a-IGZO TFTs. Moreover, for increasing the whole compactness of optical sensing module, an a-IGZO TFT-based integrated gate driver circuit is proposed for generating the gate pulses of the display and the driving signals of the integrated optical sensors. This work proposes an optical pixel sensor in discharging mode. Utilizing the combination of conventional color filter process and a-Si:H photo TFTs, optical sensors can distinguish the light intensities of ambient white light and optical input signals, respectively. Therefore, the SNR of the proposed optical sensor is maintained under high ambient white light conditions of 12280 lux. This work also proposes an optical sensor in charging mode, preventing ambient white light from inducing false detections. Based on measurement results, the proposed sensor in charging mode reacts to optical input signal under the white light illuminations from 873 lux to 12910 lux. These results reveal that the proposed sensor in charging mode is highly reliable under various white light conditions. The third proposed optical sensor focuses on the degradation of photo TFTs under VDS stress. Long-term experiment is conducted for quantifying the variation in photo-induced drain currents. New optical sensor with alternately sensing structure is proposed for ensuring high long-term reliability of the proposed sensor. This work also proposes the investigation of optical responses of a-Si:H photo TFTs covered with color filters in all three primary colors. The white-light photocurrent gating (WPCG) structures with favorable aspect ratios are estimated. The transmission rates of the color filters under the illumination with various wavelengths are presented, supporting the integration of photo TFTs with conventional color filters. An optical pixel sensor through a-IGZO TFT process is proposed. Using the high energy bandgap of a-IGZO TFT, the proposed sensor is free from variations in white light. Utilizing lights with short wavelength as the input signal makes optical sensor maintain high signal-to-noise ratio (SNR) under heavy white light conditions. SPICE models of a-IGZO photo TFTs are also established for designing new optical pixel sensors. Simulation results also illustrate that the proposed sensor has difference larger than 16 V under high ambient white light, achieving position-detection functions for various display applications. For increasing the whole compactness of the sensing module, this work proposes an integrated gate driver circuit using a-IGZO TFT process. The proposed gate driver keeps the structure simple when realizing bidirectional transmission, and the circuit improves the voltage bias conditions of driving TFTs for improving the degradation of a-IGZO TFT from constant voltage stress. Experimental results demonstrate that the proposed circuit can maintain stable for over 812 hours at 70 °C, showing the robustness of the whole system.
Date of Award2018
Original languageEnglish
SupervisorChih-Lung Lin (Supervisor)

Cite this

'