A method for forming a high electron mobility transistor (HEMT) includes forming a buffer layer on a transparent substrate. The method further includes forming a barrier layer on the buffer layer. A channel region is formed in the buffer layer adjacent to the interface between the buffer layer and the barrier layer. The method further includes forming a dielectric layer on the barrier layer. The method further includes forming source/drain electrodes through the dielectric layer and the barrier layer and disposed on the buffer layer. The method further includes forming a shielding layer conformally covering the dielectric layer and the source/drain electrodes. The method further includes performing a thermal process on the source/drain electrodes.