An integrated circuit die has a layer of first semiconductor material comprising a Group III element and nitrogen and having a first bandgap. A first transistor structure on a first region of the die has: a quantum well (QW) structure that includes at least a portion of the first semiconductor material and a second semiconductor material having a second bandgap smaller than the first bandgap, a first source and a first drain in contact with the QW structure, and a gate structure in contact with the QW structure between the first source and the first drain. A second transistor structure on a second region of the die has a second source and a second drain in contact with a semiconductor body, and a second gate structure in contact with the semiconductor body between the second source and the second drain. The semiconductor body comprises a Group III element and nitrogen.