Instability measurements in amorphous hydrogenated silicon using capacitance-voltage techniques

Abstract

In the field of flat panel displays, the current leading technology is the active matrix liquid-crystal display; this uses an amorphous hydrogen silicon (a-Si:H) based thin-film transistors (TFTs) as the switching element in each pixel. However, under gate bias a-Si:H TFTs suffer from instability, as is evidenced by a shift in the gate threshold voltage. The shift in the gate threshold voltage is generally measured from the gate transfer characteristics, after subjecting the TFT to prolonged gate bias. However, a major drawback of this measurement method is that it cannot distinguish whether the shift is caused by the change in the midgap states in the a-Si:H channel or by charge trapping in the gate insulator. In view of this, we have developed a capacitance-voltage method to measure the shift in threshold voltage. We employ metal-insulator-semiconductor structures to investigate the threshold voltage shift as they are simpler to fabricate than TFTs.