Study of Au-Based and Au-Free Ohmic Contacts in AlGaN/GaN HEMTs by Recessed Patterns

In this work, the effects of various recess depths and patterns in the ohmic contact of AlGaN/GaN HEMTs on device performance are investigated. In the studied device with a 22-nm Al0.23Ga0.77N barrier layer, as the recess depth and recess pattern are 5.5 nm and 1/3/5 μm, the Au-based (Ti/Al/Ni/Au) contact resistance of 0.89 Ωmm is achieved, which is 40% lower than that without recess. Compared to Au-based non-recessed ohmic contact AlGaN/GaN HEMTs (Au-based NROC AlGaN/GaN HEMTs), Au-based recessed ohmic contact AlGaN/GaN HEMTs (Au-based ROC AlGaN/GaN HEMTs) show a lower on-resistance and higher maximum drain current. Subsequently, the dynamic characteristics of AlGaN/GaN HEMTs were also observed. Through the pulse I-V measurement, both Au-based ROC and NROC AlGaN/GaN HEMTs exhibit severe current collapse. To improve the severe current collapse, Au-free ohmic contacts (Ti/Al/Ti) with low annealing temperature of 550 °C were adopted. The result shows that devices with Au-free ohmic contacts exhibit a significant improvement in current collapse.