**4. Conclusions**

In this paper, for the first time we propose the grooved-anode planar Gunn diode to greatly enhance the RF output power at a high operation frequency. We present an explicit numerical study into its working principle and output characteristics based on a simulation method. The grooved-anode diode is equal to two shorter diodes in parallel connection, as the grooved anode divides the long channel into two shorter channels. In the symmetric grooved-anode diode, the RF output power is almost doubled in the grooved-anode diode as compared with the single-channel diode. The 1.0-1.0-μm grooved-anode diode shows the best output characteristics. It operates at a fundamental frequency of 172.81 GHz and the corresponding DC-to-AC conversion e fficiency is about 3.13%. It produces over 5.48 mW of power, nearly twice as high as that of the 1.0-μm single-channel diode. This novel grooved-anode diode realizes the enhancement of the frequency and RF output power simultaneously, by simply etching a rectangular grooved anode onto the semiconductor layer at one lithographic step, which provides good design ideas in improving the output characteristic of the terahertz sources and other power devices. In the asymmetric 1.0-2.0-μm grooved-anode diode, two frequencies are obtained, and the second harmonic is enhanced as compared with the fundamental wave. The harmonic-enhanced Gunn diode shows its potentials as a mixer or frequency multiplier. Furthermore, it will provide a fast conversion between two di fferent frequencies without connecting with other terahertz oscillators. We have demonstrated that the proposed GaN heterostructure grooved-anode planar Gunn diode is an excellent candidate as a solid-state terahertz device.

**Author Contributions:** Conceptualization, Y.W. and L.-A.L.; Methodology, J.-P.A.; Software, Y.W.; Validation, Y.W., L.-A.L.; Formal Analysis, Y.W.; Investigation, Y.H.; Resources, J.-P.A.; Data Curation, Y.W.; Writing-Original Draft Preparation, Y.W.; Writing-Review & Editing, L.-A.L.; Visualization, L.-A.L.; Supervision, J.-P.A.; Project Administration, Y.H.; Funding Acquisition, Y.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was partially supported by the National Key Research and Development Program (Grant No. 2017 YFB043000), the National Natural Science Foundation of China (Grant No. 61274092) and the China Postdoctoral Science Foundation Grant (2019M653554).

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