Conduction and low-frequency noise in high temperature processed polycrystalline silicon thin film transistors

The performance of n- and p-channel high-temperature processed polycrystalline silicon thin-film transistors (polysilicon TFTs) has been investigated by conduction and low-frequency noise measurements. The polysilicon films were doped by boron or phosphorus ion implantation at concentrations of about 6×1016 and 3×1017 cm−3, respectively, and hydrogenated by ion implantation of hydrogen. Undoped and nonhydrogenated polysilicon films were also used for comparison. Channel length reduction due to dopant diffusion from the source and drain contacts was found to affect the transistor conduction and its associated noise. Low-frequency noise measurements indicate that the noise power spectral density of the drain current is mainly due to carrier number with correlated mobility fluctuation. The experimental data reveal the presence of exponential band tails in both n- and p-channel hydrogenated undoped polysilicon TFTs. In nonhydrogenated boron doped n-channel devices, high density of band tails and deep levels are present. Hydrogenation removes the deep levels and passivates significantly the band tails. In both hydrogenated and nonhydrogenated phosphorus doped p-channel devices, the density of traps is very high resulting in pinning of the Fermi level. The results indicate the necessity for improvement of the doping technology.