Characterization of instability in amorphous silicon thin-film transistors

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.