The present disclosure relates to a transistor device having a donor bi-layer configured to provide low-resistance to source and drain contacts while maintaining a high-mobility two-dimensional electron gas within a channel layer, and an associated method of formation. In some embodiments, the transistor device has a channel layer disposed over a substrate and a donor bi-layer disposed over the channel layer. The donor bi-layer includes a mobility-enhancing layer of AlzGa(1-z)N disposed over the channel layer and having a first molar fraction z in a first range, and a resistance-reducing layer of AlxGa(1-x)N disposed on and in contact with the mobility-enhancing layer of AlzGa(1-z)N and having a second molar fraction x in a second range less than the first range. Source and drain contacts are over the resistance-reducing layer of AlxGa(1-x)N. The donor bi-layer has a conduction band energy that monotonically decreases from top to bottom surfaces of the donor bi-layer.