High Breakdown-Voltage GaN-Based HEMTs on Silicon With Ti/Al/Ni/Ti Ohmic Contacts

We propose the Ti/Al/Ni/Ti ohmic contacts to improve the breakdown voltage (V
_BD
) of GaN-based high electron mobility transistors (HEMTs). Using the same photolithography process, the first Ti/Al metal stack and the second Ni/Ti metal stack were achieved by electron beam (EB) evaporation and magnetron sputter, respectively. Attributed to the better particle filling on the sidewall by magnetron sputtering, the second Ni/Ti metal stack completely wraps the first Ti/Al metal stack and extends to both sides after a lift-off process. Even after high temperature annealing (HTA), the Ti/Al/Ni/Ti metal stack can keep smooth edge acuity and surface morphology, resulting in a uniform electric-field distribution near the ohmic contacts. Compared with Ti/Al/Ni/Au ohmic contacts, HEMTs with Ti/Al/Ni/Ti ohmic contacts have higher V
_BD
and can improve the uniformity as well as repeatability of V
_BD
.