研究生：顏紹文 指導教授：戴亞翔 博士

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非晶矽薄膜電晶體開電流光敏感度 非晶矽薄膜電晶體開電流光敏感度 非晶矽薄膜電晶體開電流光敏感度 非晶矽薄膜電晶體開電流光敏感度

用於背光感測之研究 用於背光感測之研究 用於背光感測之研究 用於背光感測之研究

研究生：顏紹文 指導教授：戴亞翔 博士

國立交通大學 國立交通大學 國立交通大學 國立交通大學 光電工程研究所 光電工程研究所 光電工程研究所 光電工程研究所

摘 摘 摘 摘 要 要 要 要

近年來液晶顯示器（AMLCD）的發展重心，已從早期的薄膜電晶體陣列設計 逐漸轉移到背光模組的發展。目前市面上液晶螢幕所使用的背光種類，常見的包

括有冷陰極螢光燈管（CCFL）以及發光二極體（LED）。比較其優缺點，發光二極

體不僅在色飽和度上優於冷陰極螢光燈管。從環境保護得觀點，由於發光二極體 不需使用汞原料，因此發光二極體已有逐漸取代冷陰極螢光燈管之趨勢。而目前 局部暗淡（local dimming）是發光二極體背光模組最受矚目的特有功能，其目 的在於加強畫面的對比度，亦可減少顯示器的耗電量。

在本篇論文中，我們針對發光二極體背光模組的特性來做探討。在有局部暗 淡功能的背光模組，其背光強度並非固定，而會隨著顯示的畫面做調整。因此我 們提出藉由背光感測器來偵測背光強度的想法，進而可以確定背光的強度是否達 到理想的情況。並且我們提出一種利用相同於非晶矽薄膜電晶體製成的感測電 路，故可在不變動製成步驟和不增加成本的情況下達到面板與感測電路整合的目 的。其中我們所使用的感光元件為非傳統型結構的非晶矽薄膜電晶體，並且針對 在 LED 背光照射下的光特性做仔細的研究。同時，我們也分析可能會在元件感光 上造成誤差的因素，並提出降低感測誤差的方法。

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Study on ON Current Photosensitivity of a-Si TFTs for Backlight Sensing

Student：Shao-Wen Yan Advisor：Dr. Ya-Hsiang Tai

Department of Photonics

National Chiao Tung University, Hsinchu, Taiwan Abstract

In recent years, the development focus of AMLCD shifts gradually from TFT array design to the backlight module. The most common backlight modules in LCDs include CCFL and LED. Comparison between them, LED has better color gamut and no mercury using. We find that LED has the tendency to substitute CCFL. At present, the local dimming function of LED backlight is the most paid attention. The function is for enhancing the image contrast and saving the LCD power consumption.

In this thesis, we discuss on the basis of the characteristic of LED backlight. The backlight intensity with local dimming function is not constant, and it is self-adjusting according to the frame would be shown. Therefore, we have an idea about using backlight sensors to detect the backlight intensity, and then it can be verified whether or not the backlight achieves the right intensity. The proposed light sensing circuit has the same a-Si TFTs fabrication process. Thus, the light sensing circuit can be integrated in panel without changing the mask number and extra cost. The sensing device we used is non-conventional structure of a-Si TFT, and we present a detailed experimental study of the sensing device behavior under LED backlight illumination.

Moreover, we analyze the possible factors to cause light sensing error, and propose some feasible methods to calibrate and reduce sensing error.

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Acknowledgements

首先我要感謝我的指導教授 戴亞翔博士，感謝老師在我這兩年的求學期間 給予許多的教導。從學問的追求、處理問題應有的態度到做人處事的道理，老師 都以自己豐富的經驗，引導我們走向正確的道路。其中，老師常提醒我們想事情 要有階層性，一層一層的將問題切分，這樣不管遇到什麼大問題，頭腦都可以清 楚的去分析，對我來說，這是印象最深刻的也是最受用的。在此，對我最敬愛的 戴老師致上最誠摯的謝意。

接著，還要感謝學長姐們：士哲、彥甫感謝你們不吝伸出援手，遇到困難時 總是能給予最好的建議。憲哥、龍哥、曉嫻，雖然與你們共事的時間比較短，但 你們將實驗上的寶貴經驗傳承下來，也使得往後我們在實驗上更能得心應手。首 席、小黑、翔帥，謝謝你們的細心帶領，讓我們對自己所學所作的更瞭解，也因 為你們，實驗室的氣氛總是很融洽，一起打球一起吃飯，歡笑聲從不間斷。游博、

歐趴、小逼逼吸還有孫培培，從剛進研究所一起修課，到碩二為了實驗焦頭爛額，

這份革命情感回想起來令人珍惜。也要感謝學弟，小瓜呆、趴趴、少宏，有了你 們的加入，讓實驗室更有活力，。還有隔壁實驗室的小豬豬和蘇智康，雖然在不 同的實驗室，但做實驗遇到困難都能一起克服解決。還有我最要感謝的家人，有 了你們的支持讓我能無後顧之憂的完成這段學業，也期許自己將來有能力回報。

兩年的研究所生活，有了大家的陪伴，使我過的更精彩，也將是美好的回憶。

最後還要特別感謝奇美公司，有了貴公司熱情贊助以及指教，才能讓我順利 完成論文並取得學位。也感謝我的口試委員：簡昭欣教授、黃乙白教授和吳永俊 教授，感謝你們寶貴的建議和指導。

紹文 2009.06.12

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Contents

Chinese Abstract ……….……… I

English Abstract ………..…………..………. II

Acknowledgements ……..………...………...…. III

Contents ...………... IV

Figure Captions ……….... VII

Table Lists ……….... IX

Chapter 1 Introduction

1.1 Background ……… 1

1.2 Motivation ……… 2

1.3 Thesis Organization ……… 3

Chapter 2 Photo Effect on Device 2.1 Fabrication Procedure of Amorphous Silicon TFT……… 7

2.2 Photo Characteristic of Device………….……… 7

2.3 Summary……… 9

Chapter 3 Light-Sensing Circuit on TFT-LCD 3.1 Conventional Light Sensing Circuit……… 17

3.2 Light Sensing Circuit and Operation Principle……… 18

3.3 Simulation ……….……… 19

3.3.1 Simulated Method ……… 19

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3.3.2 Simulated Results ……… 20

3.4 Digitization……… 20

Chapter 4 Error Factors 4.1 Device Uniformity……… 30

4.1.1 On Current Variation ……… 30

4.1.2 Threshold Voltage Shift ……….……… 30

4.2 Temperature………….………..……… 31

4.3 Reliability……… 32

4.3.1 Staebler-Wronski Effect on Device………….……… 32

4.3.2 The Calibration of Staebler-Wronski Effect ……… 32

4.3.3 The SW Effect with Backlight Variation………..…34

4.4 Summary……… 34

Chapter 5 Conclusions ……… 45

References ……… 46

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Figure Captions

Chapter 1

Fig. 1-1 The revenue of display from 2002 to 2010……….……..……… 5 Fig. 1-2 Schematic of local dimming function by LED backlight... 5 Fig. 1-3 The backlight sensor proposed by S. H. Kim [Ref. Solid-State Electronics,

2007, pp.478-481]…… ………...6

Chapter 2

Fig.2-1 (a) Cross-section views of conventional structure a-Si:H TFT…... 11 Fig.2-1 (b) Cross-section views of gap-type structure a-Si:H TFT…………... 11 Fig.2-2 Transfer characteristics of conventional a-Si TFT……... 11 Fig.2-3 (a) Transfer characteristics of gap-type TFT of the gate-near-drain one.... 12 Fig.2-3 (b) Transfer characteristics of gap-type TFT of the gate-near-source one.. 12 Fig.2-4 (a) Drain current versus backlight illumination intensity in OFF region.... 13 Fig.2-4 (b) Drain current versus backlight illumination intensity in ON region…. 13 Fig.2-5 (a) The normalized drain current versus backlight illumination intensity of ON region and OFF region ……….……….... 14 Fig.2-5 (b) The relative photosensitivity of ON region and OFF region…...…….. 14 Fig.2-6 Transfer characteristics of gap-type TFT with different gap sizes

(i) i = 5um (ii) i = 12um (iii) i = 30um……... 15 Fig.2-7 (a) Schematic diagram of different open-ratio structures……...………….16 Fig.2-7 (b) Drain current versus open-ratio………..…………...…….16

Chapter 3

Fig.3-1 Original backlight sensing circuit design……….……….…………...22 Fig.3-2 Source follower as the readout part of sensor………….……….22

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Fig.3-3 Photograph of the fabricated 2T1C light-sensing circuit……….23

Fig.3-4 (a) Experiment result of light sensing circuit in the dark……….…24

Fig.3-4 (b) Experiment result of light sensing circuit under illumination…………24

Fig.3-5 (a) Proposed 4T2C light sensing circuit and (b) The time diagram….…....25

Fig.3-6 I_D-V_D curve for photo current simulation……….………25

Fig.3-7 Simulation of the readout part……….……….………26

Fig.3-8 Simulation results of light sensing circuit……….………….…..…27

Fig.3-9 Equivalent circuit and layout of the proposed sensor………..….…28

Fig.3-10 Schematic diagram of sensor array embedded in panel………..….…28

Fig. 3-11 (a) Simplified block diagram of digitization circuit and (b) its signal diagrams……….………..…29

Chapter 4 Fig. 4-1 (a) The drain current versus illumination intensity curves of 8 devices…..35

Fig. 4-1 (b) Error analysis of on current variation between the measured light intensity and the illuminated light intensity...35

Fig. 4-2 (a) The drain current versus illumination intensity at Vgs = 9,10,11V…....36

Fig. 4-2 (b) Error analysis of threshold voltage shift between the measured light intensity and the illuminated light intensity………...36

Fig. 4-3 (a) The drain current versus illumination intensity at temperature = 35℃, 40℃, 45 ℃………..….……….…...37

Fig. 4-3 (b) Error analysis of temperature variation between the measured light intensity and the illuminated light intensity………..………...37

Fig. 4-4 (a) The drain current versus temperature at Vgs=10V, Vds = 10V…….….38

Fig. 4-4 (b) The TC versus temperature curves of 4 devices..………...…....38

Fig. 4-5 (a) The drain current versus illumination intensity curves with different optical stress times..….………...39

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Fig. 4-5 (b) Error analysis of SW effect between the measured light intensity and the illuminated intensity………...39 Fig. 4-6 Drain current versus open ratio (a) before stress and (b) after stress…….40 Fig. 4-7 ID/Open-ratio versus open-ratio (a) before stress and (b) after stress…….41 Fig. 4-8 I_D/Open-ratio versus open-ratio under 50000 lux illumination with different stress times………..……….……….…...42 Fig. 4-9 Error analysis of SW effect after calibration between the measured light

intensity and the illuminated light intensity………...…..……….…...42 Fig. 4-10 (a) Optical stress with different light intensities………..……….…...43

Fig. 4-10 (b) ID/Open-ratio versus open-ratio under 50000 lux illumination with different stress times and different stress light intensities.…...……….…...43 Fig. 4-11 Comparison of the maximum error from each factors………...…………..44

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Table Lists

Chapter 3

Table 3-1 I0(L) and R0 at VGS=10V with the illumination intensity variation……… 26 Table 3-2 Circuit simulated conditions ……….……….……… 27

Chapter 4

Table 4-1 The parameters of the proposed model at 50000 lux stress intensity….….44