High-quality AlGaN/GaN heterostructures have been grown on sapphire substrates by plasma-assisted molecular-beam epitaxy. Polarization effects are exploited to achieve a two-dimensional electron-gas sheet density of 8.8×1012 cm−2 and greater on intentionally undoped material with a measured room-temperature mobility as high as 1478 cm2/V s. Transistors were then fabricated from this material, yielding a unity current gain frequency of 50 GHz and a unity power gain frequency of 97 GHz. By increasing the buffer layer thickness, output powers of 1.88 W/mm at 4 GHz with an efficiency of 34% were achieved. These results prove that the polarization effects in the nitrides are as enormous as theory predicts. The key to the improved mobility and operation of the devices of the all-molecular-beam-epitaxy-grown material, the AlN nucleation layer, will be discussed.

Two-dimensional electron gases in AlGaN/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 AlN 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 cm2/V s at 300 K and 3182 cm2/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 ft of 27 GHz and a fmax 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 (Al)GaN layers. The flipping of the (Al)GaN polarity on sapphire is achieved through the use of the AlN nucleation layer.

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