The behavior of the failure hot spots in GaN based HEMT devices was studied. When VGS > Vth, the drain current ID was mainly the drain-source conduction current IDS, and the transport mechanism was drift. When VGS < Vth, ID was mainly the reverse gate leakage current IGD, and the transport mechanism was Fowler Nordheim tunneling related to conductive dislocations. By analyzing the effects of different VGS and VDS on the hot spot distribution, it was shown that the transverse electric field formed by the depletion channel near the drain side of the gate boundary and the vertical electric field in the barrier layer were the main reasons for the hot spots of IDS and IGD current, respectively. The low-light spectrum of hot spots was measured. According to the critical electric field and the maximum photon energy, the average free path of the electron was about 60 nm.