Alternately Controlled Optical Pixel Sensor System Using Amorphous Silicon Thin-Film Transistors

Abstract

This paper presents an optical pixel sensor system that uses hydrogenated amorphous silicon (a-Si:H) photo thin-film transistors (TFTs) for innovating the user interface of displays with optical input functions. The proposed optical pixel sensor applies photo TFTs that are combined with one primary color filter (red, green, or blue) to determine the input signal to the optical sensor. Other photo TFTs covered with filters of other colors are utilized to provide compensating photocurrents for achieving a robust optical input function with a high signal-to-noise ratio under intense ambient white light. To improve the lifetime of the sensor and the degradation of photo TFTs under constant drain-to-source voltage (VDS) bias stress, an alternately controlled sensing structure is proposed to reduce the effective stress time of the photo TFTs. The optical characteristics and the degradation of a-Si:H photo TFTs under VDS stress with different duty ratios are investigated to verify the effect of reduced stress time on photo TFTs. Measurements further reveal that the proposed optical sensor achieves a significant initial difference in output voltages under high-intensity ambient white light of 13 230 lx, and that the difference remains high after 408 h of long-term operation at 70 °C, demonstrating the feasibility of the alternately controlled sensing structure and the long-term reliability of the sensor.