Submicron AlGaN/GaN HEMTs with very high drain current density grown by plasma-assisted MBE on 6H-SiC

High electron mobility transistors (HEMTs) were fabricated from AlGaN/-GaN layers grown by plasma-assisted molecular beam epitaxy on semi-insulating 6H-SiC substrates. Room-temperature Hall effect measurements yielded a polarization-induced 2DEG sheet charge of 1.3/spl middot/10/sup 13/ cm/sup -2/ and a low-field mobility of 1300 cm²/V/spl middot/s. Submicron gates were defined with electron beam lithography using an optimized two-layer resist scheme. HEMT devices repeatedly yielded drain current densities up to 1798 mA/mm, and a maximum transconductance of 193 mS/mm. This is the highest drain current density in any AlGaN-GaN HEMT structure delivering significant microwave power reported thus far. Small-signal testing of 50-μm wide devices revealed a current gain cutoff frequency f_T of 52 GHz, and a maximum frequency of oscillation f_max of 109 GHz. Output power densities of 5 W/mm at 2 GHz, and 4.9 W/mm at 7 GHz were recorded from 200-μm wide unpassivated HEMTs with a load-pull setup under optimum matching conditions in class A device operation."