Unpassivated AlGaN-GaNHEMTs with minimal RF dispersion grown by plasma-assisted MBE on semi-insulating 6H-SiC substrates

High electron mobility transistors (HEMTs) are fabricated from AlGaN-GaN heterostructures grown by plasma-assisted molecular beam epitaxy (MBE) on semi-insulating 6H-SiC substrates. At a sheet charge density of 1.3 x 10(13) cm(-2), we have repeatedly obtained electron mobilities in excess of 1350 cm(2)/Vs. HEMT devices with a gate length of 1 mum, a gate width of 200 mum, and a source-drain spacing of 5 mum show a maximum drain current of 1.1 A/mm and a peak transconductance of 125 mS/mm. For unpassivated HEMTs, we measured a saturated power output of 8.2-W/mm continuous wave (cw) at 2 GHz with an associated gain of 11.2 dB and a power-added efficiency of 41%. The achievement of high-power operation without a surface passivation layer suggests that free surface may not be the dominant source of radio-frequency (RF) dispersion in these MBE-grown structures. This data may help discriminate between possible, physical mechanisms of RF dispersion in AlGaN-GaN HEMTs grown by different techniques.