Impact of gate material on low-frequency noise of nMOSFETs with 1.5 nm SiON gate dielectric: Testing the limits of the number fluctuations theory

It is shown that the gate material has a strong impact on the low-frequency (LF) 1/f noise of silicon nMOSFETs with a 1.5 nm SiON gate dielectric. Highest noise is observed for transistors with an n-type polysilicon gate, compared with their counterparts having a metal (TaN) or a fully nickel-silicided polysilicon gate (NiSi). The differences are particularly pronounced in strong inversion (high gate voltage V _GS). The observations cannot be explained readily in the frame of the standard correlated-mobility fluctuations theory. They point rather to the impact of the charges/traps at the gate-dielectric interface, which are better screened in case of a metal gate. At the moment, one can only speculate on the origin of the LF fluctuations, giving rise to the higher noise in strong inversion. One hypothesis is that the image charge at the gate induced by a filled oxide trap contributes to excess scattering in the channel.