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 substra tes by plasma-assisted molecular-beam epitaxy. Polarization effects are exp loited to achieve a two-dimensional electron-gas sheet density of 8.8 x 10( 12) cm(-2) and greater on intentionally undoped material with a measured ro om-temperature mobility as high as 1478 cm(2)/V s. Transistors were then fa bricated 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 la yer 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 n itrides are as enormous as theory predicts. The key to the improved mobilit y and operation of the devices of the all-molecular-beam-epitaxy-grown mate rial, the AlN nucleation layer, will be discussed. (C) 1999 American Instit ute of Physics. [S0003-6951(99)01249-8].