Abstract: When the channel temperature of GaN high electron mobility transistor (GaN HEMT) is too high, the performance of the device is degraded. To solve this problem, a new structure is proposed, the channel temperature is reduced by optimizing the electric field. The barrier layer under the new structure gate is called the mixed barrier layer, which consists of two layers, the upper layer is AlN and the lower layer is heavily doped AlGaN. In addition, the field plate is also introduced to change the channel electric field. For better heat dissipation, the passivation layer material of the new structure is AlN instead of Si₃N₄, because it has a higher thermal conductivity. Compared with the traditional structure, the channel temperature distribution of the new structure is more uniform, and the channel temperature peak drops by 47K. In addition, the breakdown voltage of the new structure is increased by 50%, and the output characteristics are also improved.