Two-dimensional electron gases in AlCaN/GaN heterostructures grown by plasma induced molecular beam epitaxy have been formed by polarization induced interface charge effects. Growth of the "normal" structure (AlGaN on GaN) has formed a two-dimensional electron gas confined in the GaN when grown on sapphire with an AIN nucleation layer, SIC (Si face), or on GaN nucleated by organometallic vapor phase epitaxy on sapphire. Hall mobilities in GaN for normal interfaces are as high as 1238 cm(2)/V s at 300 K and 3182 cm(2)/V s at 77 K. Direct current results from field effect transistors fabricated from this material yield a maximum transconductance of 210 mS/mm and a current density of 710 mA/mm. Microwave measurements gave a f(t) of 27 GHz and a f(max) of 37 GHz. When nucleating GaN directly on sapphire, an "inverted" structure (GaN on AlGaN) was used to create a two-dimensional electron gas. The difference in the normal and inverted structures is the polarity of the (AI)GaN layers. The flipping of the (AI)GaN polarity on Sapphire is achieved through the use of the AlN nucleation layer.