Monolithic image sensors based on Thin Film on CMOS (TFC) Technology are becoming more and more attractive as an alternative solution to conventional active pixel sensors (APS). Imager with high sensitivity, high dynamic coupled with low dark current values (10-100 pA/cm2 @ 104 V/cm) have been developed. However, issues such as light-induced degradation and image lag hinder the commercial development of a-Si:H based image sensors. The problem of image lag is caused by residual current due to the release of trapped charges after the switch off of the illumination.
In this paper, we present a comprehensive study of the transient behavior of the photocurrent in a-Si:H photodiodes deposited on glass, as well as in corresponding diodes implemented in a TFC image sensor when illumination is switched off or periodically varied. The influence of the pixel architecture for two different cases is also analyzed: One setup reproduces the typical 3 transisor APS pixel architecture behavior, in which the bias voltage of the diode varies with the photogenerated charge while the second setup keeps a constant bias voltage applied to the diode by using a charge integrator.
The influence of the light-induced defect creation on the performance of the sensors is also presented and discussed.