Abstract: The fundamental state，excited state，and two.dimensional surface state in A1GaN/GaN HFET are investigated through self-consistent solution of Schrbdinger equation and Poisson equation,from which the trap location and active energy are determined.It is found that the barrier width is thinned by the surface defects，which enhances the hot electron tunneling between channel and surface tremendously. From new trap model constituted of emission from surface defect and hot electron tunneling，the transient current and G.R noise in AlGaN/GaN HFET are explained satisfactorily.At last a new way to alleviate traps and improve HFET performance through the changing of material growth and technological process is discussed.

Key words: A1GaN/GaN HFET, trap, two-dimensional surface state, surface defect, hot electron tunneling