The present disclosure relates to a semiconductor device and a manufacturing method thereof. The semiconductor device includes: a substrate, a doped group III-V layer, a gate conductor, a field plate, a first passivation layer, and a second passivation layer. The doped group III-V layer is disposed on the substrate. The gate conductor is disposed on the doped group III-V layer. The field plate is disposed on the gate conductor. The first passivation layer is located between the field plate and the gate conductor. The second passivation layer is located between the field plate and the first passivation layer.