INSTABILITY MECHANISMS IN AMORPHOUS-SILICON THIN-FILM TRANSISTORS AND THE ROLE OF THE DEFECT POOL

The threshold voltage shift in amorphous silicon thin film transistors at moderate applied bias is determined by changes in the density of dangling bond states in the a-Si:H. Positive bias-stress creates dangling bond states at a low energy (D(e) states), in both oxide and nitride transistors. Negative bias-stress creates dangling bond states at higher energy (D(h) states) in oxide transistors, but mainly reduces the density of D(e) states in nitride transistors. These results are explained using a defect pool model for the dangling bond states. The difference for oxide and nitride transistors is due to a different zero bias Fermi energy position at the interface. For nitride transistors, charge trapping at higher bias, followed by thermal annealing leads to a new zero bias thermal equilibrium density of states. Transistor characteristics can be optimised in this way.