Instability mechanism of amorphous silicon-silicon nitride thin-film transistors (TFTs) is examined. By investigating double-layer insulator TFTs, it is demonstrated that the instability is caused by an electrical charge stored at the interface between amorphous silicon and silicon nitride. The amount of stored charge at the interface (Q) does not depend on either drain voltage or drain current. Study on TFTs with several insulator thicknesses has shown that Q strongly depends on the band bending in the amorphous silicon that is related to the gate electric field (E) through the gate insulator. The Q-E relationship is found to be a more general expression of the dependence of threshold voltage shift on gate voltage, and is incorporated into a formula suitable for examining the interface quality.

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The threshold voltage shift of amorphous silicon thin film transistors (TFT's) under pulse operation is discussed. The stress time, stress voltage, duty ratio and frequency dependence of the shift have been measured. A positive voltage stress causes a constant shift, when the frequency is in the range from DC to over 100 kHz. On the other hand, the shift under a negative pulse stress depends on its repetition frequency and its pulse width and can be described by an equivalent circuit model. Based on these data, a more reliable estimate of the long-term reliability of an amorphous silicon TFT panel has been realized.

A new a-Si TFT(Thin Film '&insistor) with an A1 gate electrode and an A1 0 / SiN double-layered gate insulater has been developed, and this TFT has been successfully applied to the 10.4-inch diagonal multicolor display panel. A1 is a low resistivity metal and it is also possible to form Alz O 3 by anodic oxidation.These features contribute greatly to decreasing the number of defects in the panel and are indispensable for manufacturing large -size display. The Al. which is used as a gate electrode, can also be used as a gate bus-line metal. As a result, the gate bus -line resistance of the panel can be reduced to about 2 KQ and this is quite effective to improve the image quality of the panel.

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