Reduction of carbon proximity effects by including AIGaN back barriers in HEMTs on free-standing GaN

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, d_UID, 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 d_UID, the proximity effects of the carbon-doped GaN buffer were evident. A power-added efficiency (PAE) of 37% and continuous-wave power output (P_out) 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 Al_0.3Ga_0.7N back barrier, PAE and P_out improved to 50% and 6.7 W/mm, respectively, at a drain bias of 40 V.