Extremely low on-resistance enhancement-mode GaN-based HFET using Ge-doped regrowth technique

In this paper, we present a normally-off GaN-based transistor with extremely low on-state resistance fabricated by using Ge-doped n++GaN layer for ohmic contact. We developed a new GaN regrowth technique using Ge, which achieved extremely high doping level of 1 × 1020 cm-3, and thereby the lowest specific contact resistance of 1.5 × 10-6 Ω·cm2. Selectively deposited NiO gate using Atomic Layer Deposition (ALD) technique contributed to shorten the spacing between source and drain, making normally-off characteristics even with the 30% Al mole fraction of AlGaN. The fabricated device showed the record-breaking Ron of 0.95 Ω·mm with maximum drain current (Id,MAX) and transconductance (gm) of 1.1 A/mm and 490 mS/mm, respectively. It is noted that the obtained Vth was 0.55 V. An on/off current ratio of 5 × 106 is also achieved.