A method for forming a high electron mobility transistor includes the steps of providing a substrate, forming a channel layer, a barrier layer, and a first passivation layer sequentially on the substrate, forming a plurality of trenches through at least a portion of the first passivation layer, forming a second passivation layer on the first passivation layer and covering along sidewalls and bottom surfaces of the trenches, and forming a conductive plate structure on the second passivation layer and filling the trenches.