A high electron mobility transistor includes a semiconductor channel layer and a semiconductor barrier layer disposed on a substrate in sequence. A source electrode and a drain electrode are disposed on the semiconductor channel layer. A semiconductor cap layer is disposed on the semiconductor barrier layer. A first dielectric layer is disposed over the source electrode, the semiconductor cap layer and the drain electrode. A first via passes through the first dielectric layer and is extended downward onto the semiconductor cap layer. A gate electrode is disposed on the first dielectric layer and in contact with the first via. A first field plate is disposed in the first dielectric layer. A second field plate is disposed on the first dielectric layer and in contact with the first field plate.