CONDUCTION AND LOW-FREQUENCY NOISE IN HIGH-TEMPERATURE PROCESSED POLYCRYSTALLINE SILICON THIN-FILM TRANSISTORS

The performance of n- and p-channel high-temperature processed polycry stalline silicon thin-film transistors (polysilicon TFTs) has been inv estigated by conduction and low-frequency noise measurements. The poly silicon films were dopes by boron or phosphorus ion implantation at co ncentrations of about 6 X 10(16) and 3 X 10(17) cm(-3), respectively, and hydrogenated by ion implantation of hydrogen. Undoped and nonhydro genated polysilicon films were also used for comparison. Channel lengt h reduction due to dopant diffusion from the source and drain contacts wa found to affect the transistor conduction and its associated noise . Low-frequency noise measurements indicate that the noise power spect ral density of the drain current is mainly due to carrier number with correlated mobility fluctuation. The experimental data reveal the pres ence of exponential band tails in both n- and p-channel hydrogenated u ndoped polysilicon TFTs. In nonhydrogenated boron doped n-channel devi ces, high density of band tails and deep levels are present. Hydrogena tion removes the deep levels and passivates significantly the band tai ls. In both hydrogenated and nonhydrogenated phosphorus doped p-channe l devices, the density of traps is very high resulting in pinning of t he Fermi level. The results indicate the necessity for improvement of the doping technology. (C) 1998 American Institute of Physics. [S0021- 8979(98)06703-6].