The semiconductor device has a barrier layer formed on a channel layer, an n type diffusion preventing layer formed on the barrier layer and containing aluminum, and a source electrode and a drain electrode formed on the diffusion preventing layer. The semiconductor device further has a p type cap layer formed on the diffusion preventing layer sandwiched between the source electrode and the drain electrode and a gate electrode formed on the cap layer. The diffusion preventing layer has an aluminum composition ratio greater than the aluminum composition ratio of the barrier layer.