A heterojunction semiconductor device having a semiconductor body is provided. The semiconductor body includes a first semiconductor region comprising aluminum gallium nitride, a second semiconductor region comprising gallium nitride and forming a heterojunction with the first semiconductor region, an n-type third semiconductor region, a p-type fourth semiconductor region forming a first rectifying junction with the third semiconductor region, and an n-type seventh semiconductor region adjoining the heterojunction formed between the first semiconductor region and the second semiconductor region. The first rectifying junction forms a rectifying junction of a transistor structure which is in ohmic contact with the seventh semiconductor region. Further, a method for producing such a heterojunction semiconductor device is provided.