EFFECTS OF THE IN-SITU DRAIN DOPING ON HOT-CARRIER DEGRADATION IN POLYSILICON THIN-FILM TRANSISTORS

Hot carrier effects owing to bias stress are studied in two types of p olysilicon TFTs: classical in-situ doped drain TFTs and lightly in-sit u doped drain TFTs. Two types of bias stress are applied: a negative b ias stress corresponding to a negative gate to drain voltage and a pos itive bias stress corresponding to a positive gate to drain voltage. T he positive bias stress induces hot-hole injection into the gate oxide leading to a decrease of the leakage (off state) current and of the t hreshold voltage, and to an increase of the transconductance in classi cal in-situ doped drain TFTs. On the other hand, due to a lower local electric field in the drain junction, no significant change is observe d in LDD TFTs. However, by stressing TETs with a negative bias stress, a very large hot-carrier degradation occurs in lightly in-situ doped drain TFTs due to hot electron injection into the gate oxide. Copyrigh t (C) 1996 Elsevier Science Ltd.