High-electron-mobility transistor (HEMT) structures were regrown by molecular beam epitaxy on GaN-on-SiC templates and free-standing (FS) GaN substrates with very low threading dislocation density (TDD). To ensure a high buffer breakdown voltage, the thickness of the unintentionally doped (UID) GaN buffer layer, dUID, was reduced to 200 nm for the HEMTs regrown on FS GaN. A reduction in TDD entailed an increase in the three-terminal breakdown voltage for passivated HEMTs. With a low dUID, the proximity effects of the carbon-doped GaN buffer were evident. A power-added efficiency (PAE) of 37% and continuous-wave power output (Pout) of 4.2 W/mm were measured at 4 GHz with a drain bias of 40 V for a HEMT on FS GaN without a back barrier. By including a 5 nm Al0.3Ga0.7N back barrier, PAE and Pout improved to 50% and 6.7 W/mm, respectively, at a drain bias of 40 V. © 2013 Institution of Engineering and Technology.
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