Effect of grain size on the mobility and transfer characteristics of polysilicon thin-film transistors

The effect of the grain size on the effective carrier mobility (μeff) and transfer characteristics of a polycrystalline silicon thin-film transistor (poly-Si TFT) have been theoretically investigated by developing an analytical model. The dependence of μeff is studied as a function of doping concentrations and gate voltage for different values of grain size. It is observed that at low as well as at high doping concentrations, the effective carrier mobility (μeff) increases with increase in grain size, whereas the observed dip at the intermediate doping concentration is getting confirmed. The effect of the grain size on transfer characteristics of poly-Si TFT in its linear region is also presented. It is found that at low gate voltages, μeff and ID increase rapidly with the increase in VG for all values of grain sizes due to the grain boundary barrier lowering effect. At high gate voltage the grain boundary barrier lowering effect becomes insignificant and causes the saturation of μeff and ID. The model was found to account correctly for the experimentally observed mobility variation and yield a reasonably good agreement.