Sun Yat-sen University-Carnegie Mellon University Joint Institute of Engineering, Sun Yat-sen University, Guangzhou 510275, People's Republic of China

Amorphous silicon flat panel X-ray detector was introduced a decade ago and has established itself as a ubiquitous platform for several digital radiography modalities including chest radiography, mammography, and fluoroscopy. The fundamental building block of a flat panel detector is called a pixel. In all current pixel architectures, either passive or active, sensing, storage, and readout are unanimously kept separate. This will inevitably compromise resolution by increasing pixel size. To address this issue, we hereby propose a “smart” pixel architecture where the aforementioned three components are combined in a single dual-gate photo thin-film transistor (TFT). In another word, the dual-gate photo TFT itself functions as a sensor, a storage capacitor, and a switch at the same time. Additionally, by harnessing the amplification effect of thin-film transistor, we for the first time created a single-transistor active pixel sensor. A compact physical model was developed to describe the photosensing behavior and signal amplification of the device.

To further prove the device concept, we had fabricated a single pixel device with a W/L ratio of 250um/10um using a simple five-mask photolithography process, where 70nm transparent ITO was used as the top photo gate, and 200nm amorphous silicon was used as the absorbing channel layer. The preliminary results demonstrated that the photocurrent had been increased by four orders of magnitude due to light-induced threshold voltage shift in the sub-threshold region. The device sensitivity could be simply tuned by photo gate bias to specifically target low-level light detection. The dependence of threshold voltage on light illumination indicated that a dynamic range of at least 80dB could be achieved.

The "smart" pixel technology holds tremendous promise for achieving high-resolution and low-dose X-ray imaging, which may potentially lower cancer risk imposed by radiation, especially among pediatric patients.

Kai Wang is an assistant professor at SYSU-CMU Joint Institute of Engineering (JIE) since December 2013. He received a B.Sc. degree in Electrical Engineering from Huazhong University of Science & Technology, Wuhan, China in 2000, and a M.A. Sc. degree in Microelectronics & Solid State Electronics from Dalian University of Technology, Dalian, China in 2003. He earned his Ph. D. degree in Electrical & Computer Engineering from the University of Waterloo, Waterloo, Ontario, Canada in 2008 and was thereafter cross-appointed as a NSERC postdoctoral research fellow by Thunder Bay Health Science Centre，where he had been conducting research on biomedical imaging until he joined Apple Inc. in Cupertino, California in August 2011. At Apple, he was involved in touch sensor development for various Apple products.

He has a broad spectrum of research interests including biomedical imaging, touch and display, biosensors, optoelectronics, and etc. He has published over 30 journal papers/conference proceedings and coauthored 8 patents. He also currently serves as an Associate Editor of IEEE/OSA Journal of Display Technology and is an active member of IEEE, SPIE and SID.