Hot-carrier phenomena in high temperature processed undoped-hydrogenated n-channel polysilicon thin film transistors (TFTs)

A study on hot-carrier phenomena in high temperature processed undoped and hydrogenated n-channel polysilicon thin film transistors (TFTs) is presente d. First, stress conditions are determined by photon emission measurements during impact ionization condition. Next four stress regimes are performed. We distinguish two modes of stress conditions according to drain voltage d uring stressing: High drain voltage stress (HDVS) and low drain voltage str ess (LDVS). Each of these modes gives different results when applied to our TFTs. During HDVS condition, two regimes are observed. First, hot-hole inj ection into the oxide occurs synchronically with interface-state generation . At a second stage, this mechanism saturates and electron injection throug h the polySi-SiO2 barrier takes place with less interface states generated. In contrast, during LDVS conditions no saturation of interface-state gener ation is observed and two regimes of transconductance degradation appear. T he distribution in the gap of the stress-induced interface states is calcul ated by a known method. Finally, on- and off-state current stress was studi ed. Off-stressing affects mainly the gate oxide and is not accompanied by m easurable impact ionization phenomena and thus no considerable interface-st ate generation. (C) 1999 Elsevier Science Ltd. All rights reserved.