Owing to the great influence of surface passivation on DC and RF performance of InP-based HEMTs, the DC and RF performance of InAlAs/InGaAs InP HEMTs were studied before and after passivation, using an ultra-thin 15 nm atomic layer deposition Al2O3 layer. Increase in C-gs and C-gd was significantly limited by scaling the thickness of the Al2O3 layer. For verification, an analytical small-signal equivalent circuit model was developed. A significant increase in maximum transconductance (g(m)) up to 1150 mS/mm, drain current (I-DS) up to 820 mA/mm and f(max) up to 369.7 GHz was observed, after passivation. Good agreement was obtained between the measured and the simulated results. This shows that the RF performance of InP-based HEMTs can be improved by using an ultra-thin ALD-Al2O3 surface passivation.