Chapter 1
Introduction
1.1 General Background
Thin film transistor (TFT) is a metal-oxide-silicon field effect transistor (MOSFET) fabricated on an insulator substrate by employing all thin film constituents. Thin film transistors have been widely used as switching devices in flat panel display, such as active-matrix liquid crystal display (AMLCD) [1.1-1.5] and active-matrix organic light emitting diode (AMOLED) display [1.6-1.10]. The active layers of thin film transistors can be mainly divided into two types, amorphous silicon (a-Si) and poly-crystalline silicon (poly-Si), according to the crystallization status. The hydrogenated a-Si (a-Si:H) TFT is commonly applied in large size active matrix displays (AMDS) due to its highly mature process, low manufacturing cost and good device uniformity. However, the threshold voltage of a-Si:H TFT would increase with long time operation. Therefore, the poor stability limits the application of a-Si TFT in AMDs. On the contrary, poly-Si TFT is suitable for the high-resolution, compact size active matrix display in mobile electrical products. Since mobility and stability of poly-Si TFT is higher than a-Si TFT, it offers a promising solution to realize the
“System on Panel” technology. The high driving capability and existence of complementary devices lead to integrate functional and driver circuits on the glass substrate. The improvement of electrical characteristics and understanding of degraded mechanism of poly-Si TFTs is important for development of advanced mobile display technology. In addition, AMLCD requires back-light source to display
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input image. However, poly-Si TFT operated in illumination environment exhibits an undesired high leakage current to affect the function of pixel switch [1.11].
Furthermore, AMOLED technology with high brightness, high color saturation and fast response time has attracted more and more attention worldwide. Since the brightness of OLED is strongly dependent of driving current, it needs a well-controlled current source to provide the uniform brightness among numerous pixels. Since the main backplane technologies of AMLCD, a-Si TFT and ploy-Si TFT have been developed in the past 20 years, they can be directly taken as the pixel switch and the driving current source for AMOLED. Using excimer laser to re-crystalline a-Si active layer, poly-Si TFT can offer very high current capability.
However, the laser re-crystallization process also generates plenty of the grain boundaries in poly-Si TFT, leading to poor uniformity and very huge variation due to the narrow laser process windows for producing large grain size poly-Si TFT. The fluctuation of pulse-to-pulse laser energy and non-uniform laser beam profile make laser energy density hard to hit the super lateral regime everywhere. The random grain boundaries and traps exist in the channel region [1.12-1.14]. This will cause serious non-uniformity of brightness in AMOLED panel. Since the device-to-device uniformity is hard to control, it is essential to develop circuits to compensate the variation. Another one of promising approaching for AMOLED backplane is to use a-Si TFT because of its many advantages, including simple manufacturing process low-cost in large size panel and good device uniformity. However, it had been reported that the threshold voltage of a-Si TFT would shift during operation with increasing time [1.15-1.16]. The increasing threshold voltage of a-Si TFT would reduce the current driving capability to result in the lower brightness of OLED after long time operation. In addition, the threshold voltage of OLED is also shifted with
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time. Therefore, the current originated from a-Si TFT would be very sensitive to the OLED degradation.
1.2 Motivation
Hydrogenated amorphous silicon (a-Si:H) technology is quite attractive in AMDs due to its low processing temperature and low manufacture cost. The bottom gate inverted-staggered back-channel-etched (BCE) type of a-Si:H TFT shown in Fig 1.1 has been widely used as a switching element to control the gray level in AMLCD and to drive AMOLED. AMLCD panels are usually used in an illumination environment such as under the back-light. Therefore, the leakage current of TFT under back-light illumination in TFT-LCD displays should be reduced to avoid losing the storage charges in the pixel. Although photo leakage current of inverted staggered a-Si:H TFT was also a serious problem, it had been solved by light-shielding structure proposed by Akiyama et al.
Compared with a-Si:H TFT technology, the poly-Si TFT technology has some distinct advantages but its manufacture is more complex and high cost. The major advantage of poly-Si TFT is the higher field effective mobility than that of the amorphous silicon (a-Si) based devices. The high carrier mobility and the existence of complementary pairs permit the integration of drive circuits and the smaller area of pixel transistor. The integration of drive circuits could reduce manufacturing costs, and increase the functionality of large-area microelectronics [1.17-1.18]. The smaller area of pixel transistor leads to a larger aperture ratio for a given pixel size, or enables a high resolution display for a given aperture ratio, resulting in fine image quality for mobile display. Recently, the demand of high-end mobile electronic products such as
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cell phone, digital camera, GPS, mobile TV and so on is continuing to grow, so that the development of mobile displays with high resolution and high image quality is inevitable. Since most of people would like to use mobile electronic products outdoors under the sunlight, the readability in ambient illumination is a critical issue for mobile displays. To meet the requirement of superior readability under sunlight, the brightness of backlight becomes higher and higher. However, poly-Si TFTs operated in the high illumination environment exhibit substantial photo leakage current and degraded sub-threshold-swing (S.S), leading to the errors of gray level and difficulty in pixel design.
In addition, the application of circuit integration using poly-Si TFT continuously grow up as device characteristics improve further. Enlarging the grains in poly-silicon layers is an effective approach for improving TFT performance. Several poly-Si re-crystallization methods based on laterally grown grains have been proposed to enlarge the grains and control the location of the grain boundaries [1.19-1.20]. In poly-Si TFT devices, however, the status of defect states at grain boundaries plays a crucial role for electrical characteristics, as shown in Fig. 1.2 and Fig. 1.3. The stability of poly-Si TFT is one of the important issues for poly-Si technology.
Recently, the researches about the stabilities of conventional excimer laser-crystallized (ELC) poly-Si TFTs have been reported. The creation of trap states at poly-Si/gate dielectric interface or the charge trapping in the gate insulator is responsible for the degradation in electrical characteristics of poly-Si TFTs. Since TFT devices in driving circuits are frequently subjected to high-frequency voltage pulses, the degradation behavior under dynamic stress is a critical issue for integrated peripheral circuits.
AMOLED displays have been widely developed due to their high brightness, high
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efficiency, fast response time and wide viewing angle. Although poly-Si TFT is considered as the main backplane technology for AMOLED, the device variation is still a most critical issue to implant AMOLED panel with good image quality. The conventional 2T1C pixel shown in Fig. 1.4 directly suffers from the non-uniformity of brightness among pixels. For the demand of large size AMOLED panel, this problem will become more and more serious. To solve this issue, several methods have been proposed to compensate for the variation in poly-Si TFT characteristics. The pixel circuit of AMOLED displays can be divided into two catalogs, including voltage driving and current driving methods [1-21-1.22]. The current driving method can provide an excellent uniformity of brightness. However, it need a long time to driving panel for high resolution displays. The voltage driving method can compensate for the variation of the threshold voltage and is easy to integrate poly-Si TFT drivers on the glass substrate. Therefore, the voltage driving method would be considered as a great potential solution for eliminate the variation of poly-Si devices.
In addition, recently, a-Si:H TFT attract much attention to be taken as pixel element for large size and low cost panels in AMOLED display because of its good uniformity and simple fabrication process [1.23-1.24]. However, the threshold voltage shift of a-Si:H TFT over time under operation is another critical issue to degrade the image quality on AMOLED panels [1.25]. Moreover, since OLED is placed on the source node of a-Si:H TFT in the conventional pixel circuit and IOLED is determined by VGS of the driving TFT, the threshold voltage shift in OLED raise the source voltage of the driving TFT to decrease the driving current..
In this thesis, the electrical characteristics of poly-Si under illumination are studied in detail. In order to solve the photo leakage current problem, a metal shielding layer structure for poly-Si TFT is used for top gate poly-Si TFT structure. From the SEM
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image and transfer curves of poly-Si TFT with metal shielding layer, the elimination of photo leakage current and increase in S.S are confirmed. However, VTH of poly-Si TFT with metal shielding layer would shift with the drain bias. Therefore, a simple method to improve the electrical characteristics of poly-Si TFT under illumination and maintain the original key parameters is proposed. In addition, the behavior of S.S under illumination is also discussed based on the result of proposed test devices. An energy band diagram is proposed to explain the degradation of S.S under light exposure. Furthermore, the mechanism of conventional ELC poly-Si TFT under dynamic stress is investigated by voltage-capacitance measurement. We also proposed a model to describe the degradation of laser-crystallized laterally grown poly-Si under dynamic stress. For the application of poly-Si TFT in AMOLED, a 5T1C design is proposed to eliminate the VTH variation of poly-Si TFT and the voltage drop of supply power in one time operation. Additionally, a simple pixel circuit composed of 4T1C with source follower type compensation is presented to release the issue of degradation in a-Si TFT and OLED.