The effects of positive and negative bias stress on hydrogenated amorphouse silicon thin films transistors(a-Si:H TFTs) with gate nitride were investigated. Positive bias stress creates dangling bond states at low energy in a-Si:H band gap and negative bias stress creates dangling bond states at higher energy. The creation of defects were studied by measuring the threshold voltage shift as a function of time and temperature. The kinetics of these defects are explained by diffusive hydrogen diffusion. Also, the annealing behaviors of these defects created by bias stress were studied. It is found that the annealing temperature of the created defects by bias stress depends on the sign of the gate bias voltage. The created defects by hole accumulation can be more easily removed by thermal annealing compared with the dangling bonds generated by a positive gate bias.
The performances of a-Si:H TFTs are limited by bulk a-Si:H series resistance and source/drain contact resistance. The effects of the series resistance on the threshold voltage and the transconductance, which are evaluated from transfer characteristics, were investigated. The series resistance mainly affects the TFT performance at low drain voltage. Changing the TFT characteristics which are caused by the temperature variation, white light illumination, rapid quenching, and annealing above the deposition temperature, we studied the effects of the changes of the series resistance on the threshold voltage and transconductance. This simple method may be used to analyze and to characterize a-Si:H TFTs.
