High- ${Q}$ GaN Varactors for mm-Wave Applications: A Physics-Based Simulation Study

Varactors, with its capacitance tunable by the applied voltage, are enabling components for reconfigurable RF systems. The performance of a varactor could be characterized by its Q-factor and the capacitance tuning ratio. In this article, we seek to develop a new varactor with very high Q-factor and large tuning ratio, enabling reconfigurable RF systems with large bandwidth. There were two main innovations in varactor diode design. One was the use of the fin structure, in which the electrodes were placed on two opposite sidewalls of the mesa structure instead of one on top and the other at the bottom or next to the mesa. This design eliminates the extrinsic series resistance in the conventional varactor diode designs, leading to substantially increased Q-factor. The other was the use of a multichannel structure. The structure was composed of multiple AlGaN/GaN heterojunctions with high-mobility 2-D electron gas. The high-mobility electron gas helped lowering the intrinsic series resistance, thereby further improving the Q-factor.