Analysis of the off current in nanocrystalline silicon bottom-gate thin-film transistors

The off current in bottom-gate nanocrystalline silicon (nc-Si) thin-film transistor (TFT) is shown to be determined by the conductivity of the channel layer and by the quality of the interface with the passivation nitride. Indeed, the presence of fixed charges at the nc-Si/passivation nitride interface serves to increase the band bending, leading to an increase in the off current by about two orders of magnitude. In contrast, when the nc-Si channel layer is capped with hydrogenated amorphous silicon (a-Si:H), the off current decreases and is determined by the bulk conductivity of nc-Si, as the a-Si:H makes a less defective interface with the passivation nitride. The general effect of the gate and passivation nitride interfaces on band bending and transfer characteristics of the TFT is analyzed by numerical simulations. We find that the band bending due to fixed charges at the gate nitride interface is modulated by a negative gate voltage, while that due to fixed charges at the passivation nitride interface is not, leading to a high off current. (C) 2008 American Institute of Physics.