An Efficient Trapped-Charge Calculation in amorphous Silicon for Device Simulation

Abstract

We present an efficient analytical model for calculating the trapped-charge density as a function of Fermi energy based on two exponential regions for density-of-states distribution in hydrogenated amorphous silicon. in this efficient model, the trappedcharge density is calculated without numerical integration and without curve fitting as a function of Fermi energy. Comparisons between the analytical and the numerical models have been made and excellent agreement has been obtained. Such a model is useful as an aid to study the impact on the performance of amorphous-silicon devices such as thin-film transistors. SUMMARY Amorphous silicon (a-Si) thin-film transistors (TFT) are finding applications for Iargearea integrated circuits in flat liquid-crystal displays, solid state imager, electronic copiers, printers, and scanners. Device simulation [l] has been used to investigate the performance of a-Si TFT. The major difference between a-Si and single crystalline silicon is the large localized states in mobility gap. To simulate device performance, one needs to accurately include the trapped charge into Poisson’s equation. In general, trapped charge is calculated by integrating the product of the Fermi-Dirac occupation function and the localized density of states (DOS). This calculation requires a numerical integration and will cause problem in two- or three-dimensional device simulation due to the large amount computer time for numerical integration. In this paper, we presents an efficient analytical model for calculating trapped-charge density as a function of Fermi energy based on two exponential regions for DOS distribution in hydrogenated amorphous silicon. If we change the magnitude and slope of DOS distribution, there is no need to modify this model. The localized states may be characterized by an two exponential regions for the tail states and the deep states which has been confirmed by experimental data published in [2]. Analytic expressions for the density of trap states can be cypressed by