Extreme-Bandgap Al _0.62 Ga _0.38 N Thin Channel HEMT With AlN/GaN Digital-Alloy and Regrown GaN Ohmic Contacts

We report an extreme-bandgap (EBG) AlGaN HEMT employing an undoped AlN barrier over a digital-alloy (DA) channel formed by AlN/GaN short-period superlattice (SPSL) and metalorganic chemical vapor deposition (MOCVD) regrown n++-GaN contacts on sapphire. The coupling of carrier wave functions through thin AlN barriers in DA leads to the formation of minibands in the transverse direction, due to which the DA channel acts as an Al0.62Ga0.38N layer. In lateral direction, regrown n++-GaN is in direct contact with GaN layers of DA, thus removing severe interface barriers existing in most known EBG HEMTs, showing a low contact resistance of RC= 6.5 Ω·mm. With a peak drain current of 333 mA/mm, the DA-channel HEMT achieved an on-resistance RON= 28 Ω·mm at 6 μm source-drain spacing, a subthreshold swing (SS) of 77 mV/dec, an ON/OFF ratio >107, negligible gate hysteresis, and a critical breakdown field EC ≈ 2.3 MV/cm without field-plating. These results establish DA-channel as a promising new design for EBG HEMT with insulating AlN barriers for high-power and high-voltage electronics.