AC measurement for characterizing the trap processes in polysilicon TFTs

Abstract

ABSTRACT We report a new method for characterizing trap generation-recombination processes in Polysilicon TFTs. A small AC voltage was superimposed on a DC gate voltage and the ac current was measured at the source. A theoretical model has been developed, whereby n-channel TFTs have been analysed. A resonant peak in the imaginary part of the ac current and a corresponding step in the real part were found at the frequency of 147Hz. Our data indicate that the ac response is dominated by a single trap level. By combining the ac current measurement with low frequency capacitance measurement, we have determined the trap energy, capture cross section, trap density to be respectively 0.35eV above midgap, 3.1 210 -21 cm , 5.7 x 10 15 cm -3 . Keywords: Polysilicon TFTs, ac measurement, generation-recombination process, capture cross section, density of states. 1. INTRODUCTION Poly-Si TFTs have many applications as they can be fabricated on cheap substrates like glass or plastic 1 . The transient properties of poly-Si TFTs are of paramount importance for the optimum design of electronic circuits. As reported in previous papers