DC and radio frequency (rf)! characteristics of pseudomorphic high electron mobility transistors (HEMTs)! are investigated before and after silicon nitride (Si3N4) passivation. After the passivation, we observed significant degradation of cut off frequency and noise performance of the HEMTs. We also observed clear increases in the drain-source saturation current at a gate voltage of 0 V and in the extrinsic transconductance at a drain voltage of 1 V from 325 and 264 to 365 mA/ mm and 304 mS/mm, respectively, with no significant variation in pinchoff voltage. We propose that the observed variations in the dc characteristics are due to the positively charged surface states after deposition of the Si3N4 passivation film. Also, the degradation of rf and noise performance is associated with the increase of gate-source capacitance. Hydrodynamic device model simulations were performed based on the proposed mechanisms for the change in electrical behavior, and the calculated results show good agreement with the experimental results. (c) 2005 The Electrochemical Society. All rights reserved.