A III-nitride based field effect transistor obtains improved performance characteristics through manipulation of the relationship between the in-plane lattice constant of the interface of material layers. A high mobility two dimensional electron gas generated at the interface of the III-nitride materials permits high current conduction with low ON resistance, and is controllable through the manipulation of spontaneous polarization fields obtained according to the characteristics of the III-nitride material. The field effect transistor produced can be made to be a nominally on device where the in-plane lattice constants of the material forming the interface match. A nominally off device may be produced where one of the material layers has an in-plane lattice constant that is larger than that of the other layer material. The layer materials are preferably InAlGaN/GaN layers that are particularly tailored to the characteristics of the present invention.