Study on the Pixel Circuits of Active Matrix Touch Panel and Light Sensor

• Main Authors: Chou, Lu-Sheng, 周祿盛
• Format: Dissertation
• Language: English
• Published: 2013

博士 國立交通大學 光電工程研究所 101 This dissertation studies the issues about how to develop touch panel or light sensor integrated in flat panel displays using thin film transistors. The possible problems in the practical applications of the sensors and the sensing circuits are discussed and their respect solutions are proposed. For the touch sensing, the principle of RC time delay on the scan bus, which makes the display design difficult, is applied to invent the new touch sensing circuit. A pair of consecutive scan buses is used to drive the proposed touch sensing circuit. When human touch causes the capacitance increase, the turn-on pulses of the scan bus signals overlap. In this overlapping time, the proposed circuit outputs an ON-level current as a sensing signal when it is touched. Compared to other touch sensing technologies, the touch signal of the proposed circuit is obvious and easy to be read out. Therefore, the cost of the readout IC can be reduced. Meanwhile, the large signal provides the immunity against the device variation. On the other hand, if the pixel is not activated or touched, the output current is at the OFF-level, which can save the power consumption. In this study, the different circuit configurations using RC delay are discussed. The circuits are further implemented by amorphous silicon (a-Si) and a-IGZO TFTs to prove that the proposed circuit is universal to different kinds of TFTs. Furthermore, the circuit can be adapted in both structures of out-cell and in-cell. For the various conditions of using the circuit, we propose a general design procedure, which can be helpful to apply the circuit in flat panel displays more quickly and effectively. In addition to the demonstration of a 2 inch touch panel to check the validity of the circuit function, we further discuss the feasibility and possible issues in applying the proposed method to large panels by simulation. The results show that the circuit is applicable even for the 42-inch panel. In the aspect of the light sensing, we respectively discuss the sensing for the backlight and the front light. The conventional a-Si TFT has little photo response to backlight illumination because the blockage of the metal gate, and it only has photo response in the nA order in the OFF region under front illumination. We think it is not suitable to be a photo sensor. In this study, we introduce an a-Si TFT with asymmetric structure, called gap-type a-Si TFT, which has a gap as a sensing region between bottom gate and one of the source and drain electrodes. The gap-type TFTs have obvious photo sensitivity in ON region not only under backlight illumination but under front illumination. The large sensing current can improve the signal-to-noise ratio. The photo sensitivity of the gap-type TFTs are examined to look for the best operation condition. After that, we analyze the error factors for the sensing in real cases, including device uniformity, temperature effect, and reliability under illumination. Especially, the a-Si suffers from a serious current degradation under continuing illumination, which is well known as Staebler-Wronski effect. If the influence of the effect cannot be offset, it will be difficult to use a-Si for light sensing. In this study, we analyze the current degradation behavior of the gap-type a-Si TFT under illumination. The calibration method and the light sensing circuits integrated in flat panel display are proposed. Using the proposed touch sensing and light sensing pixel circuits, a flat display can be embedded with multiple functions with no need of extra devices. In this way, the panel can be made in a thinner form and lower cost. A smart display with good image quality and interactive function can thus be implemented.