Diamond field effect transistor and process for producing the same

A diamond semiconductor element is disclosed which has a gate electrode, a source electrode and a drain electrode formed on a diamond single-crystal thin-film at a distance from each other, characterized in that said source electrode includes a first area close to said diamond single-crystal thin-film and a second area except the first area, where a first distance between a first end face of said first area close to said gate electrode and said gate electrode is no more than a second distance between a second end face of said second area close to said gate electrode and said gate electrode. Further, said drain electrode includes a third area close to said diamond single-crystal thin-film and a fourth area except the third area, where a third distance between a third end face of said third area close to said gate electrode and said gate electrode is no more than a fourth distance between a fourth end face of said fourth area close to said gate electrode and said gate electrode.