Metal-Semiconductor–Metal Varactors Based on InAlN/GaN Heterostructure With Cutoff Frequency of 308 GHz

Metal-semiconductor-metal (MSM) varactor diodes based on InAlN/GaN high electron mobility transistor structures were fabricated and characterized. Among the MSM varactors with gate lengths ranging from 90 to 270 nm and gate distances of 2 and 4 μm, the highest performance was obtained from the MSM varactor with gate length of 90 nm and gate spacing of 2 μm. The capacitance switching ratio (C
_max
/C
_min
) of 2.31 together with a high cutoff frequency (f
_o
) of 308 GHz was demonstrated. The calculated figures of merit (FOMs), which were defined to be f
_o
· C
_max
/C
_min
for the comparison of devices performance, were 523 and 769 GHz for devices with gate spacing of 4 and 2 μm, respectively. The FOMs were not dependent on gate length of the devices, but highly dependent on gate spacing.