Ultralow Ohmic Contact in Recess-Free Ultrathin Barrier AlGaN/GaN Heterostructures Across a Wide Temperature Range

“Ohmic-before-passivation” process was implemented on ultrathin-barrier AlGaN (<6>$(R_{c})$. In this process, alloyed Ti/AI/Ni/Au ohmic metal was formed first, followed by $\text{AlN}/\text{SiN}_{x}$ passivation contributed to restore two-dimensional electron gas (2DEG) in the access region. Due to the sharp change in the concentration of 2DEG at the metal edge, a reduced transfer length consisted with lower $R_{c}$ are achieved compared to that of ohmic contact on AlGaN (~20 nm)/GaN heterostructure with pre-ohmic recess process. Temperature-dependent current voltage measurements demonstrate that the carrier transport mechanism is dominated by thermionic field emission above 200 K and by field emission below 200 K. The “ohmic-before-passivation” process enables the relative stability of ohmic contacts between 50 K and 475 K and significantly improves the direct current characteristics of GaN-metal-insulator-semiconductor-high electron mobility transistor, offering a promising means for scaling down and enabling the utilization of low-voltage GaN-based power devices in extreme environmental conditions.