Surface passivation in InP-based High Electron Mobility Transistors (HEMTs) plays an important role in reducing or eliminating their surface effects which limit both direct-current (DC) and radio-frequency (RF) performances. In the present work, effect of surface passivation was studied using an ultra-thin 20 nm PECVD Si3N4 layer. In DC performance, after passivation, its maximum transconductance (g(m, MAX)) is increased up to 1200 mS/mm. It is found that, by scaling the thickness of Si3N4 layer, the increase in Cgd after passivation can be effectively limited verified by small-signal modeling. As a result, S-parameter measurements demonstrate an increase in extracted f(max) up to 450 GHz after passivation. The results show that, by using an ultra-thin Si3N4 surface passivation, its RF performance can be improved in InP-based HEMTs. (C) 2016 Elsevier Ltd. All rights reserved.