Anomalous turn-on voltage degradation during hot-carrier stress in polycrystalline silicon thin-film transistors

In this letter, we present experimental data showing that hot-carrier stress in laser annealed polycrystalline silicon thin-film transistors provokes an anomalous turn-on voltage variation. Although under various hot-carrier stress intensities the maximum transconductance degradation shows the same power-time dependent law, turn-on voltage can exhibit different behaviors. This observation lead to the conclusion that turn-on voltage depends on two different degradation mechanisms: injection of hot carriers into the gate oxide and degradation of grain boundaries. We show that these two mechanisms may be distinguished since they obey different power-time dependent laws as a function of stress duration.