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 presented. 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 during stressing: High drain voltage stress (HDVS) and low drain voltage stress (LDVS). Each of these modes gives different results when applied to our TFTs. During HDVS condition, two regimes are observed. First, hot-hole injection into the oxide occurs synchronically with interface-state generation. At a second stage, this mechanism saturates and electron injection through the polySi-SiO2 barrier takes place with less interface states generated. In contrast, during LDVS conditions no saturation of interface-state generation is observed and two regimes of transconductance degradation appear. The distribution in the gap of the stress-induced interface states is calculated by a known method. Finally, on- and off-state current stress was studied. Off-stressing affects mainly the gate oxide and is not accompanied by measurable impact ionization phenomena and thus no considerable interface-state generation. (C) 1999 Elsevier Science Ltd. All rights reserved.