The role of the gate insulator in the defect pool model for hydrogenated amorphous silicon thin film transistor characteristics

Abstract

We demonstrate that differing transistor characteristics in the most important material systems can be explained by the defect pool model applied to the defects near the interface of hydrogenated amorphous silicon thin film transistors. Gate dielectrics used include plasma deposited silicon nitrides, plasma deposited silicon oxides, and thermally grown silicon oxides. The most important property of the gate dielectric is not the chemical composition but the fixed charge. In particular, as-deposited plasma deposited silicon oxide transistors can be made with similar properties to plasma deposited silicon nitride transistors or thermal silicon oxide transistors, by varying the fixed charge. After correcting the effects of the fixed charge variation, some differences still exist between the interface qualities. We introduce the parameter Ndb(min), i.e., the minimum density of dangling bonds (cm−2), which is a measure of interface quality independent of the fixed charge of the gate insulator. We propose that the variations in Ndb(min) are due to differences in disorder, perhaps caused by interface strain, leading to slight variations (∼5 meV) of the valence-band tail slope.