Au‐Free Ohmic Contact for GaN High‐Electron‐Mobility Transistors

Herein, the fabrication of Au‐free ohmic contacts for mm‐wave GaN heterojunction field‐effect transistors is investigated. To find an optimum metal stack and annealing recipe, different metallization and rapid thermal annealing temperature/duration combinations are tested. They are compared with the well‐known Ti/Al/Ni/Au ohmic contact scheme optimized for AlGaN/GaN epitaxial structures. Herein, a Ta/Al/Ta metal stack is initially fabricated and analyzed. Subsequently, further developments for improving the contact resistance and surface roughness of Ta‐based ohmic contacts are carried out. The best achieved Au‐free contact resistance is ≈0.28 ± 0.18 Ω mm for Ta/Al/W metallization after annealing at 600 °C. The root mean square (RMS) surface roughness and edge definition in this contact are improved significantly to 7.5 nm RMS and about 30 nm edge accuracy compared with 16 nm RMS and 160 nm in Au‐based contacts.