Effect of localized traps on the anomalous behavior of the transconductance in nanocrystalline TFTs

It has been reported that nanocrystalline and microcrystalline devices show an anomalous behavior in the transconductance where several rates of increase of the transconductance with applied gate voltage, not present in amorphous TFTs are observed. In this paper we show that the anomalous effect of the transconductance is observed for an acceptor tail states activation energy similar to the normal values for hydrogenated silicon amorphous devices, (a-Si:H), provided that some conditions are met regarding the density of trapped charge in tail and deep states and the density of free charge in the material, which does not necessarily suggest a behavior in between amorphous and polycrystalline. The effect appears if the density of deep tail states, is smaller (higher) than the typical values in a-Si:H. The localized state distribution present in a nanocrystalline TFT prepared by hot wire deposition technique is estimated by comparison of experimental and simulated transconductance curves. In our case a lower density of deep states is obtained, which corresponds with their better light and bias stability.