A semiconductor device includes an enhancement mode high electron mobility transistor (HEMT) with an active region and an isolation region. The HEMT includes a substrate, a group III-V body layer, a group III-V barrier layer, recesses, a passivation layer and an etch mask layer. The group III-V body layer is disposed on the substrate. The group III-V barrier layer is disposed on the group III-V body layer in the active region and the isolation region. The recesses are disposed in the group III-V barrier layer in the active region and the isolation region, respectively. The passivation layer disposed in the recesses of the active region and the isolation region. The etch mask layer disposed between the passivation layer and the group III-V barrier layer in the active region, where the etch mask layer is spaced apart from bottoms of the recesses in the active region and the isolation region.