Characterization of Photo Leakage Current of Amorphous Silicon Thin-Film Transistors

The photo leakage current of amorphous silicon thin-film transistors (a-Si TFTs) for switching elements in active-matrix liquid crystal displays (AMLCDs) is studied to achieve high-image-quality LCDs. The position dependence of photo leakage current generation in the a-Si:H TFT is evaluated using a slit light from the channel side. The generated photo leakage current is composed of a peak at the junction region and a gradual parr at channel region; both of which are larger at the soul ce electrode side than at the drain electrode side. This large photo leakage current at the source electrode side can be explained by the diffusion and tunnel current increase caused by the variation of the quasi Fermi level by photogenerated carriers in the reverse bias source junction and the larger electron mobility than the hole, respectively. The results of this study indicate the importance of the source junction for the TFT off-current, in contrast to studies in the past which put forth that the off-current is limited by the generation-recombination current at the drain junction. Our results indicate the importance of front-side illumination by the reflected-light illumination from the high brightness backlight of AMLCD displays.