comprising:

providing a substrate;

forming a mesa structure on the substrate, wherein the mesa structure comprises a channel layer, a barrier layer disposed on the channel layer, two opposite first edges extending along a first direction, and two opposite second edges extending along a second direction, the first direction and the second direction are perpendicular;

forming a passivation layer covering the substrate and the mesa structure;

forming at least a first opening, wherein the first opening penetrates through the passivation layer and exposes the barrier layer, and in a top view, the first opening exposes the two opposite first edges of the mesa structure without exposing the two opposite second edges of the mesa structure;

forming a plurality of second openings, wherein the plurality of second openings are connected to a bottom surface of the first opening and penetrate through the barrier layer and a portion of the channel layer; and

forming a metal layer in the first opening and the plurality of second openings thereby forming a contact structure on the mesa structure; and

forming a gate structure on the mesa structure, wherein in the top view, the gate structure is at a side of the contact structure along the first direction, and extends lengthwise along the second direction.