A semiconductor device is provided. The semiconductor device includes a substrate, a channel layer, a barrier layer, a compound semiconductor layer, a dopant holding layer, a source/drain pair, and a gate. The channel layer is disposed over the substrate. The barrier layer is disposed over the channel layer. The compound semiconductor layer and the dopant holding layer are disposed over the barrier layer. The source/drain pair are disposed over the substrate and on both sides of the compound semiconductor layer. The gate is disposed over the compound semiconductor layer.