**4. Conclusions**

In this study, the copper metallization technique for GaN HEMT devices operating at millimeter-wave frequencies has been realized. Thick copper metallization contributes to the reduction of *RS* and *RD* and alleviates skin effect under high-frequency operation, which in turn improves the DC and RF characteristics of the devices. Experimental verifications revealed that the device with 2 × 15 μm gate width exhibited a record-high maximum output power density of 8.2 W/mm, which is the highest among the state-of-the-art published results at the Ka band. Such superior results have proven the feasibility of integrating thick copper metallization in the device process and make it promising for next-generation wireless communication system applications.

**Author Contributions:** Conceptualization and methodology, Y.C.L., H.-T.H., and E.Y.C.; data curation, S.H.C., P.H.L., K.H.L., and T.J.H.; software and validation, T.J.H.; writing—original draft preparation, Y.C.L.; writing—review and editing, H.-T.H., and E.Y.C.; supervision, project administration and funding acquisition, E.Y.C., and H.-T.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Ministry of Science and Technology, Taiwan, under Grants 107-2221-E-009-093-MY2 and 108-2911-I-009-502. This work was financially supported by the "Center for the Semiconductor Technology Research" from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan. Also supported in part by the Ministry of Science and Technology, Taiwan, under Grant MOST-108-3017-F-009-003.

**Conflicts of Interest:** The authors declare no conflict of interest.
