Characterization of bias induced metastability of amorphous silicon thin film transistor based passive pixel sensor switch and its impact on biomedical x-ray imaging application

Abstract

Active Matrix Flat Panel Imagers (AMFPIs) based on amorphous silicon (a-Si:H) thin film transistor (TFT) array is the most promising technology for large area biomedical x-ray imaging. a-Si:H TFT exhibits a metastable shift in its characteristics when subject to prolonged gate bias that results in a change in its threshold voltage (V_Τ) and a corresponding change in ON resistance (R_ON). If not properly accounted for, the V_Τ shift can be a major constraint in imaging applications as it contributes to the fixed pattern noise in the imager. In this work, we investigated the timedependent shift in V_Τ (ΔV_Τ) of a-Si:H TFTs stressed with the same bipolar pulsed bias used for static (chest radiography, mammography, and static protein crystallography) and real time imaging (low dose fluoroscopy at 15, 30 and 60 frames/second, and dynamic protein crystallography). We used the well known power law model of time dependent ΔVT to estimate the change in RON over time. Our calculation showed that RON can be decreased ~ 0.03 % per frame and ~ 5 % over 10,000 hours at 30 frames/second. We verified the theoretical results with measurement data. The implication of TFT metastability on the performance (NPS, and DQE) of biomedical imagers is discussed.