Mechanism of ohmic contact formation in AlGaN/GaN high electron mobility transistors

We tested various ohmic contact metallization schemes on AlGaN/GaN HEMTs to achieve low contact resistance, good surface morphology and proper line edge definition. Mo, Ni and Pt intermediate layers in these schemes replaced the intermediate Ti layer interposed between Al and Au in Ti/Al/Ti/Au contacts. We recorded almost similar values of the contact resistance lying in the range of 0.3–0.5 Ω mm on all the metallization stacks, but a significant difference was observed in their surface morphology and line edge definition. The formation of particular intermetallic compounds due to different intermediate layers was found to be responsible for this difference. Ti/Al/Ti/Au/WSiN contacts possessed excellent surface morphology. Mo based contact not only delivered good surface morphology and low contact resistance but also proper line edge definition. We found that the formation of the Al–Mo phase and GaMo 3 compound was the key factor responsible for its remarkable performance.