High-frequency AlGaN/GaN polarization-induced high electron mobility transistors grown by plasma-assisted molecular-beam epitaxy

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 ${\text{8.8×10}}^{12} {\mathrm{cm}}^{\mathrm{-2}}$ 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.