Mj. Murphy et al., High-frequency AlGaN/GaN polarization-induced high electron mobility transistors grown by plasma-assisted molecular-beam epitaxy, APPL PHYS L, 75(23), 1999, pp. 3653-3655
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].