The radiation effect on the n-channel polycrystalline silicon (polysilicon) thin-film transistors has been investigated. It is found that for an unhydrogenated device, the irradiation of Co60 with a total dose of 1 Mrads caused a negative threshold-voltage (Vth) shift and a slight subthreshold-swing (S) degradation, while for a hydrogenated n-channel device, the same irradiation results in a positive Vth shift and a serious S degradation. It is also found that the radiation hardness of the hydrogenated devices can be improved somewhat by a simple irradiation-then-hydrogenation treatment.

This paper presents a comprehensive study on the characteristics of n-and p-channel polycrystalline-silicon (polysilicon) thin-film transistors (TFTs) fabricated on fluorine-implanted-then-crystallized amorphous silicon films.Amorphous silicon films of two thicknesses were implanted with different energies and various dosages of fluorine, and studied using transmission electron microscopy (TEM) and secondary-ion mass spectrometry (SIMS). The electrical characteristics of TFTs fabricated on the films were correlated with the results of TEM and SIMS. It was found that fieldeffect mobilities of both n-and p-channel devices were improved by the fluorine implantation thanks to the enhanced grain size and the fluorine passivation effect. For the p-channel device, the fluorine implantation did not improve the subthreshold swing and even degraded it after hydrogenation. This result was thought to be caused by the fluorine-induced negative charges in oxides. However, a thin active layer and a deep implantation reduced this degradation.In 1980 Kanrins et al.1 were the first to suggest that fluorine be used to passivate grain boundaries of the polysilicon TFT. The concept was not realized until 1991, howev-

The low pressure NH3 -annealing and the H2. plasma hydrogenation were jointly used to improve the characteristics of polysilicon thin-film transistors (TFT's). It was found that the TFT's after applying the above treatments achieved better subthreshold swings, threshold voltages, field effect mobilities, off currents, and reliability. It is believed that the improvement was due to the gate oxynitride formation and the &-plasma had a better passivation effect on the oxynitride.

A bstruct-The electrical characteristics of lop-gate thin-film transistors (TFT's) fabricated on the nitrogen-implanted polysilicon of the doses ranging from 2 x 101'-2 x loL4 ions/cm2 were investigated in this work. The experimental results showed that nitrogen implanted into polysilicon followed by an 850" C 1 h annealing step had some passivation effect and this effect was much enhanced by a following Hz-plasma treatment. The threshold voltages, subthreshold swings, ON-OFF current ratios, and field effect mobilities of both n-channel and p-channel TFT's were all improved. Moreover, the hot-carrier reliability was also improved. A donor effect of the nitrogen in polysilicon was also found which affected the overall passivation effect on the p-channel TFT's.