Hot Carrier Induced Degradation in the Low Temperature Processed Polycrystalline Silicon Thin Film Transistors Using the Dynamic Stress

Abstract

The dynamic stress on low-temperature processed polycrystalline silicon thin-film transistors (poly-Si TFTs) is studied under two different stress conditions. As compared to static stress, the enhanced degradation in poly-Si TFT can be observed in dynamic stress. The enhanced degradation in dynamic stress (Vgs=0-20 V, Vds=22 V) is due to (1) the impact ionization effect in the ON state (Vgs=20 V, Vds=22 V), (2) the drain avalanche hot carrier effect in the OFF state (Vgs=0 V, Vds=22 V), and (3) the transient current stressing effect (at the switching period). However, in the stress condition of Vgs=-20 V to 20 V, Vds=0 V, both the source and drain regions are equally damaged. As the falling time becomes shorter, the transient current will increase to cause more device degradation near drain. It is also found that the degradation is more serious in short channel device than that in long channel device. As the stress frequency increases, the degradation will be enhanced. Moreover, the reduced degradation under high stress temperature is due to reduced hot carrier effect under high temperature stressing.