Lifetime Optimization of Optical Sensing System With Highly Reliable a-Si:H TFT-Based Optical Sensor and Driver Circuit in Large AMLCDs

Abstract

This paper presents a lifetime optimization of optical sensing system that integrates highly reliable optical sensors and driver circuits that use hydrogenated amorphous silicon thin-film transistors (a-Si:H TFTs) to realize binary detection of optical signal with a multi-position sensing function for use in large active-matrix liquid crystal display (AMLCD). The optical sensor suppresses the effect of variations in ambient white light using a white-light photocurrent gating (WPCG) structure and increases the reliability of two photo TFTs by halving the drain-to-source voltage (VDS) stress. The driver circuit switches two driving TFTs between their two modes to recover the threshold voltage (VTH) shifts during a long sensing period or generate an output waveform with a large pulse width for driving the optical sensor. Experimental results indicate that the degradation of the photocurrent of the photo TFT is suppressed from 3.71 nA to 1.03 nA, representing an improvement of 72.2% relative to the original WPCG structure. The minimum difference between the output voltages of the optical sensing systems with and without an optical signal under illumination by ambient white light from 500 to 10000 lux is 18.58 V. According to an accelerated lifetime test, the feasibility of using the optical sensing system in large AMLCDs is verified.

Keywords

• Driver circuit
• hydrogenated amorphous silicon thin-film transistors (a-Si:H TFTs)
• optical sensor