A semiconductor structure having a buffer layer, a pseudomorphic, impurity doped, back-barrier layer disposed on the buffer layer, a channel layer disposed on the back-barrier layer, the channel layer lattice matched to the buffer layer, and a top barrier layer disposed on the channel layer. A Group III-Nitride transition layer is disposed between the buffer layer and the pseudomorphic back-barrier layer. The buffer layer and the pseudomorphic back-barrier layer are both Group III-Nitride materials. The Group III-Nitride material of the buffer layer is different from the Group III-Nitride material in the back-barrier layer. The back-barrier layer has a wider bandgap of than the buffer layer bandgap. The composition of the Group III-Nitride material in the transition layer varies from the composition of the Group III-Nitride material in the buffer layer to the composition of the Group III-Nitride material in the pseudomorphic back-barrier layer as a function of distance from the buffer layer.