The present disclosure relates to a semiconductor device and its manufacturing method, and the semiconductor device includes a substrate, a channel layer, a gate electrode, a first electrode, a second electrode, and a metal plate. The channel layer is disposed on the substrate, and the gate electrode is disposed on the channel layer. The first electrode and the second electrode are disposed on the channel layer, at two opposite sides of the gate electrode respectively. The metal plate is disposed over the channel layer, between the first electrode and the gate electrode. The metal plate includes a first extending portion and a second extending portion, wherein the second extending portion extends towards the substrate without contacting the channel layer, and the first extending portion extends toward and directly contacts the first electrode or the second electrode.