A semiconductor device includes a substrate, a channel layer, a barrier layer, a gate, a strained layer and a passivation layer. The channel layer is disposed on the substrate. The barrier layer is disposed on the channel layer. The gate is disposed on the barrier layer. The strained layer is disposed on the barrier layer. The passivation layer covers the gate and the strained layer. The material of the passivation layer differs from that of the strained layer.