Regrown n++-GaN ohmic contacts to high-Al content AlxGa1−xN (x > 0.6) digital alloy channels using AlN/GaN short-period superlattice

In this paper, we report a novel ohmic contact formation scheme for extreme bandgap AlxGa1−xN (x > 0.6) channel high electron mobility transistors with undoped barrier layers. Our approach consists of using a new low-temperature pulsed metal-organic chemical vapor deposition doping scheme for the n++-GaN regrown contacts and an AlxGa1−xN digital alloy (DA) channel layer comprising short-period superlattices of AlN and GaN. Pulsed growth and doping yield a sheet resistivity that is a factor of 3–5 lower than that of conventional doped n++-GaN layers grown under identical conditions. Moreover, the regrown n++-GaN layer has no hetero-barrier with the GaN layers of the DA channel, which leads to linear ohmic contacts and a record low contact resistance Rc ∼ 6.5 Ω-mm to the Al0.62Ga0.38N DA channel layer of a HEMT with an AlN barrier layer.