Determination of interface state distribution in polysilicon thin film transistors from low-frequency noise measurements: Application to analysis of electrical properties

Abstract

Low-frequency noise is studied in n-channel furnace solid phase crystallized and in laser solid phase crystallized (LSPC) polysilicon TFTs biased from weak to strong inversion. Noise analysis is supported by theoretical results of the additivity of generation-recombination spectra to explain trapping/detrapping processes following the tunneling theory of carriers into the gate oxide. In the weak inversion region the normalized drain current spectral density is inversely proportional to the number of trapped carriers. The corresponding distribution of the trap states [density of states (DOS)] in the polysilicon band gap at the interface is determined. In the strong inversion region the normalized drain current spectral density is proportional to the effective oxide trap number/(freecarriernumber)2 ratio. For LSPC devices the DOS is one decade lower and the corresponding effective oxide trap number is one decade higher. The results are attributed to the effects of the laser annealing on the active layer crystal quality. Quantitative dependence of DOS with static conduction parameters in relation with interface quality is then analyzed.