Bias dependence of instability mechanisms in amorphous silicon thin-film transistors

Powell, M. J.; van Berkel, C.; French, I. D.; Nicholls, D. H.
October 1987
Applied Physics Letters;10/19/1987, Vol. 51 Issue 16, p1242
Academic Journal
We have measured the bias dependence of the threshold voltage shift in a series of amorphous silicon-silicon nitride thin-film transistors, where the composition of the nitride is varied. There are two distinct instability mechanisms: a slow increase in the density of metastable fast states and charge trapping in slow states. State creation dominates at low fields and charge trapping dominates at higher fields. The state creation is found to be independent of the nitride composition, whereas the charge trapping depends strongly on the nitride composition. This is taken as good evidence that state creation takes place in the hydrogenated amorphous silicon (a-Si:H) layer, whereas the charge trapping takes place in the a-SiN:H. The metastable states are suggested to be Si dangling bonds in the a-Si:H, and the state creation process similar to the Staebler–Wronski effect. The confirmation of state creation in a thin-film transistor means that states can be created simply by populating conduction-band states in the undoped material. The slow states are also thought to be Si dangling bonds, but located in the silicon nitride matrix.


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