The effects of static surface states and bulk traps on output characteristics have been studied. The effects of surface charge and bulk traps on current collapse, saturation current and knee voltage are investigated, and their relationships have been determined. The results show that the increase of the surface charge can exhaust the two-dimensional electron gas, and reduce the current collapse effect and saturation current, inducing the abnormal shift of the knee voltage. At the same time, reducing the bulk traps can alleviate the current collapse effect and increase the saturation current with the slight change of the knee voltage. At low lattice temperature, the hot electron effect and quantum tunneling effect play an important role in the current collapse. By using the hydrodynamics model, possible physical mechanisms are discussed, and an approach is proposed to reduce the effects of the static surface states and bulk traps on the output characteristics.