Chapter 6 Characterization of Thin film Transistor for Optical
6.3 Light Sensor for Detecting Uniformity of Backlight Intensity
For back light illumination application, low power consumption is very important.
Portable consumer electronic products, such as mobile phone, PDA, always move toward a tendency of constantly reducing their power consumption because users expect that they can operate for a long time. For LCD-based products, most power consumption is attributed to backlight, accordingly, backlight power saving is considered one of the most effective ways to reduce LCD energy dissipation. In recent years, LED backlighting was sought to replace the fluorescent backlighting because
112
the TFT-LCDs with LED backlighting have advantages of greater efficiency, long lifetime and environmental compatibility. Since the LED backlight modules could be non-uniform in panel or degrade after a long time operation. These situations all have the same phenomenon that is the changing of backlight intensity. In this work, we also can use our back light research to propose the backlight sensor for the accurate backlight intensity. We can use the backlight sensors to detect the light intensity, and determine the intensity whether or not to achieve the value that it should be.
6.4 Conclusion
We proposed system connect with forward and reverse measurements can be used to set up sensing direction in active-matrix displays. Due to sensing disparity capability of gate metal shielding by itself, it has potential used to photo transistors with embedded optical sensors to capture three dimensional images near the panel.
This system is without extra novel device process development and it is expected that the integration in the pixels with the same device of sensing system onto the panel.
Meanwhile, a newly developed light sensing circuit using the identical LTPS TFTs fabrication processes has been proposed. It can perform sensing operation and trustworthy readout operation through amplifying small photo leakage current to analog voltage. Due to the poor uniformity of LTPS TFTs as experimental results shown, we also proposed calibration methods to reduce the illumination intensity error from 4700lux to 1200lux and compensate the Vth shift variation. On the other hand, we study on the feasibility of LTPS TFTs for light sensing application. The off region would be more appropriate for the purpose that is to accurately quantify the light intensity. Both front light researches would provide the possibility for the light sensor array integrated in the pixels with the same device of LTPS TFTs.
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Figure 6-1 The I
D-V
Gtransfer characteristics under illumination from
dark to 31320 lux. (Figure insert) rotatable probe station experimental
setup.
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Figure 6-2 Gate metal shielding by itself for poly-Si TFTs.
115
(a)
(b)
Figure 6-3 The forward and reverse measured photo currents verses
negative gate bias with several incident angles under (a) 5100 lux (b)
20500 lux illumination conditions.
116
Figure 6-4 Forward and reverse photo currents ratio verses incident
angles under 5100 lux and 20600 lux illumination conditions.
117
Figure 6-5 Using LTPS TFTs as oblique light sensors for three
dimensional interaction display (a) Single light source (b) Multiple light
sources.
118
Figure 6-6 Schematic diagram of proposed 2T1C light sensing circuit and
its timing sequence.
119
Figure 6-7 Illumination dependence of ID-VD characteristic and its fitting formula.
120
Table 6-1 I
0(L) and R
0=1/ A
0(L) at V
GS=0.5V with the illumination intensity variation.
Brightness I0 (L) R0=1/A0 (L) Dark 2.40E-12 1.81E+11 203 lux 9.34E-12 1.58E+11 837 lux 1.80E-11 1.18E+11 3632 lux 3.82E-11 7.06E+10 9113 lux 6.59E-11 4.71E+10 17199 lux 9.80E-11 3.47E+10 31320 lux 1.52E-10 2.50E+10
121
Figure 6-8 SPICE simulation results of TFT (W/L=20um/5um).
T1 I
0(L)
R
0122
Figure 6-9 (a) The modified 2T1C light-sensing circuit model for
simulation (b) its time diagram.
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Figure 6-10 Simulation results under illumination and in the dark.
Vout Vin
Vg
Dark
31320lux
124
Figure 6-11 The comparison of the current ratio of under illumination and
in the dark R
L/Damong on, subthreshold, and off region and that of
current level (inset).
125
(a)
(b)
Figure 6-12 Measured waveforms of output voltages of proposed 2T1C
light-sensing circuit illuminative variations from dark to 31320 lux on (a)
subthreshold region and (b) off region.
126
Figure 6-13 Measured output slopes of fifteen proposed light-sensing
circuits in off region (dash line) and their average curve (solid line).
127
(a)
(b)
Figure 6-14 The influence of V
thshift operated on (a) subthreshold region
(b) off region.
128
Figure 6-15 Average slopes versus illumination intensity with five
samples as an average unit.
129
Figure 6-16 (a) Schematic of our proposed light-sensing circuit with compensation part and (b) time diagram.
Ф1
Ф2
130
(a)
(b)
Figure 6-17 Fifty times of Monte Carlo simulation results of the proposed
2T1C light-sensing circuit when V
thshift is ±0.5V (a) before
compensation (b) after compensation.
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Fig. 6-18 (a) Simplified block diagram of digitization circuit and (b) its
signal diagrams.
132
133
Chapter 7
Conclusions and Suggestions for Future Work
In previous chapters, the characterizations of low-temperature poly-silicon thin film transistor for optical sensor application are studied. In chapter 3, a new photo behavior parameter Unit-Lux-Current, which is the ability of photo leakage current induced per unit-photo flux, is used to analyze the effects of illumination on LTPS TFTs. An equation is provided to properly describe ULC under various bias and temperature conditions for further exploration of photo leakage mechanism. In addition, since LTPS TFTs suffer from huge variation owing to the diverse and complicated grain distribution in the poly-Si film, the ULC variation will also be discussed. A possible future work which may try to insert such photo equations in SPICE models for design.
In chapter 4, the probable degradation cases for the device under DC operation are considered. We apply both stress conditions which are hot carrier and self heating stress deliberately to manipulate the defect-related photo behaviors and modify ULC equations in LTPS TFTs. Comparatively, this work focused on how additional non-uniform defects and the photo leakage mechanism influence both lateral and gate-drain overlap depletion. For the viewpoint of future application, the empirical adjusted equation of ULC also meanwhile provides a potential modeling for simulation of LTPS TFT circuitry.
In chapter 5, in our previous studies, under back light illumination, we propose both unstressed devices which can be seen as effective medium approach and stressed devices which have additional non-uniform defects photo-induced leakage model,
134
comparatively. The different characteristics of front light and back light ULC are also in comparison. Meanwhile, we provide new insight which use energy level of trap defect behaviors connected with photo induced current to further make sure the existence of tail state after self heating degradation. Furthermore, a more accurate model after self-heating degradation is proposed. Due to LTPS TFTs are top gate structures, for future sensor design consideration, such photo leakage current which suffer from back light illumination are must be amend.
In chapter 6, we study on sensor application of the low-temperature poly-silicon thin film transistor. For front light illumination application, first, a three dimensional embedded optical sensor employs low temperature poly-silicon thin film transistor which used gate metal shielding by itself characteristics was proposed. The system connect with forward and reverse measurements can be used to set up sensing direction. It provides sensing disparity characteristics of adopted devices under illumination. It’s expected the integration of sensing system onto the panel without extra components sensors and extra change in the fabrication process. Then a circuit of source follower type based on the LTPS TFTs which can sense the illumination condition is proposed to be used as an ambient light sensor. Some kinds of variation effect can be calibrated by statistical and compensation circuit methods. For back light illumination application, we can use the backlight sensors to detect the uniformity of light intensity. All research would provide the possibility for the light sensor array integrated in the pixels with the same device of LTPS TFTs.
135
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簡 歷
姓 名：郭 彥 甫
性 別：男
出生年月日：民國 69 年 12 月 03 日
籍 貫：台灣省新竹縣
住 址：新竹縣竹北市大眉里9鄰中華路1483巷9號
學 歷：
國立中山大學物理學系學士 (88.09-92.06) 國立交通大學光電工程研究所碩士班 (92.09-94.06)
國立交通大學光電工程研究所博士班 (94.09-99.08)
博士論文題目：
低溫多晶矽薄膜電晶體元件特性應用於光感測器之研究
Study on Characterization of Low-Temperature Poly-Silicon Thin
Film Transistor for Optical Sensor Application
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Publication List International Journals:
[1] Ya-Hsiang Tai and Yan-Fu Kuo, “Statistical study on the temperature dependence of the turn-on characteristics for p-type LTPS TFTs,” Solid-State Electronics, vol.51, no.8, pp.1092-1095, 2007.
[2] Ya-Hsiang Tai, Yan-Fu Kuo, and Yun-Hsiang Lee, “Dependence of Photosensitive Effect on the Defects Created by DC Stress for LTPS TFTs,”
IEEE Electron Device Letters, vol. 29, no. 12, pp. 1322-1324, Dec. 2008.
[3] Ya-Hsiang Tai, Yan-Fu Kuo, and Yun-Hsiang Lee, “Photosensitivity Analysis of Low Temperature Poly-Si Thin Film Transistor Based on the Unit-Lux-Current,” IEEE Transactions on Electron Devices, vol 56, no 1, pp.
50-56, Jan 2009.
[4] Ya-Hsiang Tai, Yan-Fu Kuo, Guo-Pei Sun, “An Empirical Defect-Related Photo Leakage Current Model for LTPS TFTs Based on the Unit-Lux-Current,” IEEE Transactions on Electron Devices, vol. 57, no. 5, pp.
1015- 1022, 2010.
[5] Ya-Hsiang Tai, Yan-Fu Kuo, Shao-Wen Yen, “Gap-type a-Si TFTs for Backlight Sensing Application,” submitted to IEEE Journal of Display
[5] Ya-Hsiang Tai, Yan-Fu Kuo, Shao-Wen Yen, “Gap-type a-Si TFTs for Backlight Sensing Application,” submitted to IEEE Journal of Display