High electron mobility transistor and method for forming the same

A HEMT structure includes a compound semiconductor substrate, a gate electrode, a source electrode, a drain electrode, a first metal pillar, a second metal pillar, a dielectric layer, and a metal layer. The gate electrode is disposed on the compound semiconductor substrate. The source electrode is disposed on the compound semiconductor substrate at a first side of the gate electrode. The drain electrode is disposed on the compound semiconductor substrate at a second side of the gate electrode. The first metal pillar is disposed on the source electrode. The second metal pillar is disposed on the drain electrode. The dielectric layer is disposed on the compound semiconductor substrate. The dielectric layer surrounds the gate electrode, the first metal pillar, and the second metal pillar. The metal layer is disposed on the dielectric layer. The metal layer straddles the gate electrode, the first metal pillar, and the second metal pillar.