Low-Resistance and Low-Thermal-Budget Ohmic Contact by Introducing Periodic Microstructures for AlGaN/AlN/GaN HEMTs

In this study, we proposed a low-resistance and low-thermal-budget ohmic contact by introducing periodic microstructures for AlGaN/AlN/GaN high-electron-mobility transistors (HEMTs). Insertion of AlN spacer layers is effective in improving electron mobility but can degrade contact resistance, in general. An ohmic contact with periodic microstructures was fabricated through low-damage patterned recess etching and metallization via low-temperature annealing (600 °C). The optimization of structural parameters of periodic microstructures allowed a low contact resistance of 0.29 $\Omega \cdot $ mm, which is less than half of that of the conventional recessed ohmic contact prepared in this study. A simplified equivalent circuit model reproduced the experimental results and predicted that enhanced metal/channel direct contact areas contributed to the reduction of contact resistance. Pulsed current–voltage measurements revealed that an AlGaN/AlN/GaN HEMT with periodic microstructures showed considerably suppressed current collapse compared with that of a selective-area growth (SAG) ohmic contact, indicating that the introduction of trap states was suppressed in periodic-structured devices owing to low-thermal-budget processes. Under high off-stress conditions, periodic-structured devices showed the highest drain current at knee voltage compared to the devices with a conventional planar-recessed ohmic contact and an SAG ohmic contact. Therefore, ohmic contact with periodic microstructures is one of the best techniques to achieve low contact resistance with low-thermal-budget processes.