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 (AMLC Ds) 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 co mposed of a peak at the junction region and a gradual parr at channel regio n; 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 s ide can be explained by the diffusion and tunnel current increase caused by the variation of the quasi Fermi level by photogenerated carriers in the r everse bias source junction and the larger electron mobility than the hole, respectively. The results of this study indicate the importance of the sou rce junction for the TFT off-current, in contrast to studies in the past wh ich put forth that the off-current is limited by the generation-recombinati on current at the drain junction. Our results indicate the importance of fr ont-side illumination by the reflected-light illumination from the high bri ghtness backlight of AMLCD displays.