High electron mobility transistor and fabrication method thereof

A high electron mobility transistor, comprising:

• a semiconductor channel layer and a semiconductor barrier layer, disposed on a substrate;
• a source electrode, a gate electrode and a drain electrode, disposed on the semiconductor channel layer;
• a patterned dielectric layer, disposed on the semiconductor barrier layer and located between the gate electrode and the drain electrode;
• a first field plate, extended continuously from a side of the patterned dielectric layer to a top surface of the patterned dielectric layer, and having a step in height;
• a first dielectric layer, disposed between the semiconductor barrier layer and the patterned dielectric layer;
• and
• a second dielectric layer, covering the patterned dielectric layer, wherein the dielectric constant of the patterned dielectric layer is higher than the dielectric constant of the first dielectric layer and the dielectric constant of the second dielectric layer.