Self-Aligned Metal–Semiconductor–Metal Varactors Based on the AlGaN/GaN Heterostructure

The self-aligned metal-semiconductor-metal (SA-MSM) structure was utilized to fabricate varactors on top of the AlGaN/GaN heterostructures. The SA-MSM structure minimizes the access resistance associated with 2-dimensional electron gas (2DEG), resulting in the significantly improved cutoff frequency. Conventional MSM varactors have essentially the same cutoff frequency, regardless of the device width. On the other hand, the SA-MSM varactors require the optimum device width to obtain the maximum cutoff frequency which has been achieved with the device width of $20~\mu \text{m}$ in this study. The fabricated SA-MSM varactors exhibit the extrapolated cutoff frequency of 1.96 THz and a capacitance switching ratio of 2.51.