**4. Conclusions**

In summary, recessed gate AlGaN/GaN MIS-HEMTs with double AlGaN barrier designs are investigated and discussed. A double hump of the *g*m–*V*<sup>G</sup> characteristic can be observed in the recessed gate AlGaN/GaN MIS-HEMTs with double AlGaN barrier designs. A physical model is proposed to explain the double channel characteristics, which is mainly due to the formation of the top channel under a high *V*G bias. Once the gate voltage is applied at a high enough level, the top channel is formed, leading to an increase in drain current due to the current contribution from the top channel. Furthermore, by lowering the Al% in the bottom AlGaN barrier, the devices show a more positive *V*TH with the same recessed depth, indicating that a double AlGaN barrier design in recessed gate MIS-HEMTs can be an alternative strategy to achieve an enhancement mode characteristic.

**Author Contributions:** Data curation, T.-L.W., S.-W.T., and H.-J.J.; writing—original draft preparation, T.-L.W.; writing—review and editing, T.-L.W., S.-W.T., and H.-J.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** 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. It is also supported in part by the Ministry of Science and Technology, Taiwan, under Grant MOST-108-3017-F-009-003, MOST-106-2218-E-009-025-MY3 and by the Young Scholar Fellowship Program under Grant MOST 108-2636-E-009-006.

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