Investigation of trap processes in polycrystalline silicon thin film transistors by ac measurement

Abstract

We report a method for characterizing trap generation–recombination processes in polycrystalline silicon thin film transistors. A small ac voltage was superimposed on a dc gate voltage and the ac current was measured at the source. A theoretical model was developed, whereby n-channel thin film transistors were analyzed. A resonant peak in the imaginary part of the ac current and a corresponding step in the real part were found at frequency of 147 Hz. Our data indicate that the ac response is dominated by a single trap level. By combining the ac current measurement with a low frequency capacitance measurement, we have determined the trap energy, capture cross section, and trap density to be, respectively, 0.91 eV above the valence band, 3.1×10−21 cm2, and 5.7×1015 cm−3.