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This letter studies the influence of the passivation thickness on the device characteristics of InAlGaN/GaN high-electron-mobility transistors with a gate length between sub-30 and 70 nm. As the Al2O3 passivation thickness increases, the current collapse in 80-μs pulsed-I -V measurements decreases from 30% to 13%, while dc characteristics are almost unchanged with the exception of increasing drain-induced barrier lowering. The thicker passivation increases the fringing gate capacitance, which can be about 30% of the total gate capacitance in the devices with a gate length below 35 nm. This capacitance results in a significant drop of current-gain cutoff frequency (fT), and its effect is more important in the shorter gate length devices.