Using a Si3N4 layer as passivation layer, effects of surface passivation on device performances have been investigated. After passivation, devices exhibited better pinch-off characteristics and lower gate leakage current. For a device with a gate-length of 0.25 mum, the I-dss increased from 791 to 812.2 mA/mm and the peak extrinsic transconductance increased from 207.2 to 220.9 mS/mm, The f(T) and f(MAX) values decreased from 53 and 102.5 to 45.9 and 90.5 GHz, respectively, due to the increase of parasitic capacitances. Microwave noise measurements showed that devices exhibited 0.2-0.25 dB increase in minimum noise figure (NFmin) after passivation. (C) 2002 Elsevier Science Ltd. All rights reserved.