Low-frequency noise spectroscopy of polycrystalline silicon thin-film transistors

Polycrystalline silicon thin-film transistor (polysilicon TFT's) characteristics are evaluated by using a low-frequency noise technique. The drain current fluctuation caused by trapping and detrapping processes at the grain boundary traps is measured as the current spectral density. Therefore, the properties of the grain boundary traps can be directly evaluated by this technique. The experimental data show a transition from 1/f behavior to a Lorentzian noise. The 1/f noise is explained with an existing model developed for monocrystalline silicon based on fluctuations of the inversion charge near the silicon-oxide interface. The Lorentzian spectrum is explained by fluctuations of the grain boundary interface charge with a model based on a Gaussian distribution of the potential barriers over the grain boundary plane. Quantitative analysis of the 1/f noise and the Lorentzian noise yield the oxide trap density and the energy distribution of the grain boundary traps within the forbidden gap.