The initial substrate nitridation effects on the crystal structure of GaN epitaxial layers grown on GaAs substrates by gas source molecular beam epitaxy using RF-radical N2 as a nitrogen source were investigated. The crystal structure of GaN grown on (100) GaAs substrates was critically influenced by substrate nitridation time, that is, zincblende GaN was grown on the substrate without nitridation, and wurtzite GaN was grown on the nitrided substrate (longer than 120 s). The substrate misorientation effects on surface morphology and X-ray full width at half-maximum of zincblende GaN layers were also studied. A featureless smooth surface was obtained for a layer grown under the high V/III beam flux ratio condition.

Two dimensional (2D) layer by layer growth conditions for GaN epita,xial layers by gas source molecular beam epitaxy using a 13.56 MHz RF-radical nitrogen source were systematically investigated for the first time. It was clarified that the lower growth rate @<0.1 pm/h) enhanced the 2D growth. Furthermore, the introduction of annealed thin Gal.I buffer layer (60 A) drastically enhanced the 2D growth for the layers grown,on (0001) Al2O3, (001) MgO and (001) GaAs substrates.

This paper reports on the excellent performance of V -band monolithic high electron-mobility transistor (HEMT) oscillators, and discusses oscillation characteristics on drain bias. With regard to output characteristics, double-hetero (DH) HEMT (especially with a high-density Si-planar doped layer) are superior to single-hetero (SH) HEMT's. A monolithic microwave integrated circuit (MMIC) oscillator has been developed with a planar doped DH HEMT and has achieved the peak output power of 11.1 dBm at a 55.9-GHz oscillation frequency. Phase noise of 085 dBc/Hz at 100-kHz offset and 0103 dBc/Hz at 1-MHz offset have been achieved at a drain voltage of 5.5 V and a gate voltage of 0 V. These characteristics have been achieved without any buffer amplifiers or dielectric resonators. This study has revealed that the phase noise decreases as drain voltage increases. This phenomenon is caused by lower pushing figure and lower noise level at a low-frequency range obtained under a high drain voltage. It is because the depletion layer in the channel is extendedto the drain electrode with increase of drain voltage, resulting in the small fluctuation of the gate-to-source capacitance. We also investigate low-frequency noise spectra of AlGaAs/InGaAs/GaAs DH HEMT's with different bias conditions. The low-frequency noise decreases for more than 3 V of the drain voltage. A unique mechanism is proposed to explain this phase noise reduction at high drain voltage.

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