Study of the drain leakage current in bottom-gated nanocrystalline silicon thin-film transistors by conduction and low-frequency noise measurements

The drain leakage current in n-channel bottom-gated nanocrystalline silicon (nc-Si) thin-film transistors is investigated systematically by conduction and low-frequency noise measurements. The presented results indicate that the leakage current, controlled by the reverse biased drain junction, is due to Poole-Frenkel emission at low electric fields and band-to-band tunneling at large electric fields. The leakage current is correlated with single-energy traps and deep grain boundary trap levels with a uniform energy distribution in the band gap of the nc-Si. Analysis of the leakage current noise spectra indicates that the grain boundary trap density of 8.5 X 10(12) cm(-2) in the upper part of the nc-Si film is reduced to 2.1 x 10(12) cm(-2) in the lower part of the film, which is attributed. to a contamination of the nc-Si bulk by oxygen.