Effects of trap-state density reduction by plasma hydrogenation in low-temperature polysilicon TFT

The reduction of trap-state densities by plasma hydrogenation in n-channel polysilicon thin-film transistors (poly-TFTs) fabricated using a maximum temperature of 600 degrees C has been studied. Hydrogenated devices have a mobility of approximately 40 cm/sup 2//V*5, a threshold voltage of approximately 2 V, an inverse subthreshold of approximately 0.55 V/decade, and a maximum on/off current ratio of 5*10/sup 8/. The effective channel length decreases by approximately 0.85 mu m after a short hydrogenation which may be attributed to the activation of donors at trap states near the source/drain junctions. Trap-state densities decrease from 1.6*10/sup 12/ to 3.5*10/sup 11/ cm/sup -2/ after hydrogenation, concomitant with the reduction of threshold voltage. Using the gate lengths at which the trap-state densities deviate from the long-channel values as markets for the leading edge of passivation, the apparent hydrogen diffusivity is found to be 1.2*10/sup -11/ cm/sup 2//s at 350 degrees C in the TFT structure.<>