The fluorine implantation on polysilicon was found to improve the characteristics of polysilicon thin-film transistors (TFT's). The fluorine passivates the trap states within the polysilicon channel. As compared with the HZ-plasma passivation, the fluorine implantation passivates more uniformly both the band tail-states and midgap deep-state, while the HZ-plasma treatment is more effective to passivate deep states than tail states. A fluorine-implanted device can be further improved its performance if an Hz-plasma treatment is applied. In contrast to the H2-plasma passivation, the fluorine passivation improves the device hot-carrier immunity. Combining the fluorine passivation and H2-plasma passivation, a high performance TFT with a high hot-carrier immunity can be obtained. MOS has a better irradiation and hot-carrier resistance [ 121. Recently, H. Kitajima et al. had found that F+ implantation is

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.

An analytical model for the above-threshold characteristics of long-channel, small-grain and thin channel polysilicon thin film transistors (TFT's) is presented. This model is constructed by considering the barrier potential and the carrier trapping effect at grain boundaries of the channel. A band tail state located at E,-0.15 eV is taken into account to simulate the I-V characteristics. Based on the model, the theoretically simulated results show good agreement with the experimental data of the plasma-passivated and unpassivated TFT devices in a wide range of the gate, drain biases and the temperature. The correlation of the transconductance to the gate bias is also investigated. It is found that the decrease of grain-boundary barrier potential with the gate voltage enhances the transconductance, while this enhancement effect becomes insignificant and causes the decrease of the transconductance at the high gate bias. NOMENCLATURE Gate oxide capacitance, F/cm2. Activation energy of the drain current, eV. Conduction band edge of the polysilicon, eV. Fermi-level with respect to E,, eV. Grain-boundary tail state level, eV. Parallel electric field in channel, V/cm Boltzmann's constant. Device channel length, cm. Grain size of the polysilicon channel, cm. Total carrier density in channel, ~m-~. NA Acceptor concentration, ~m-~. N C Effective density of states in the conduction band, ~m-~.

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