▎ 摘　　要

The shift of Dirac point in graphene devices is of great importance, influencing the reliability and stability. Previous studies show the Dirac point shifts slightly to be more positive when the drain bias increases. Here, an abnormal shift of Dirac point is observed in monolayer graphene field effect transistors by investigating the transfer curves under various drain biases. The voltage of Dirac point shifts positively at first and then decreases rapidly when the channel electric field exceeds some threshold. The negative Dirac point shift is attributed to holes injection into oxide layer and captured by the oxide traps under high channel electric field. This can also be demonstrated through a simple probability model and the graphene Raman spectra before and after the DCmeasurement.