Study on the Feasibility of LTPS TFTs for Light Sensing Application

• Main Authors: Chia-Pin Cheng, 鄭枷彬
• Other Authors: Ya-Hsiang Tai
• Format: Others
• Language: en_US
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/07568578777051280982

碩士 === 國立交通大學 === 顯示科技研究所 === 96 === Low temperature polycrystalline silicon (LTPS) thin-film transistors (TFTs) have attracted much attention in the application on the integrated peripheral circuits of display electronics such as active matrix liquid crystal displays (AMLCDs) and active matrix organic light emitting diodes (AMOLEDs) due to its better current driving compared with amorphous silicon (a-Si) TFTs. Various attempts have been reported to integrate display circuits to peripheral area of the glass substrate. In addition to the peripheral area integration, circuit integration to pixel is considered to be required to realize so-called high-value added display or sheet computer having input function, especially in mobile equipments. Integration of LTPS optical sensor is considered to have a potential to be a key technology for various kinds of advanced functions such as ambient light sensors, image scanners, touch panel, etc. An ambient light-sensing function, which is one of several high-value added functions, can contribute to low power consumption and improve visibility by detecting ambient light around the display panel and controlling the brightness of the display panel. In this thesis, we present a detailed experimental study of the LTPS TFTs behavior under halogen lamp illumination and identify the different TFT operating regimes. We also propose a light-sensing circuit using the identical LTPS TFTs fabrication processes without any extra cost. The proposed circuit, which has a source follower, can sense the photo leakage current under different illumination intensities and convert the current to analog voltage signal and then digital one. Through the measurement of the proposed circuit under light variation from 0 to 31320 lx, we confirmed that the proposed light-sensing circuit can perform sensing and readout operations accurately. However, we also consider the device variations such as threshold voltage (Vth) shift and OFF current variation and propose the calibration methods to reduce the illumination intensity error from 4700 lx to 1200 lx and compensate the Vth shift variation.