Effects of silicon-nitride passivation on the electrical behavior of 0.1-μm pseudomorphic high-electron-mobility transistors

We examine the effects of surface state formation due to silicon-nitride passivation on the electrical characteristics of GaAs-based 0.1-μm pseudomorphic high-electron-mobility transistors (pHEMTs). In this study, DC and noise characteristic are investigated before and after the passivation of the pHEMTs. After the passivation, we observe significant degradation of noise performance in the frequency range of 55 - 62 GHz. We also observe 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 pinch-off voltage. We propose that the observed variations in the DC and the noise characteristics are due to the positively charged surface state after deposition of the silicon nitride passivation film. Hydrodynamic device model simulations were performed based upon the proposed mechanisms for the change in electrical behavior, and the calculated results show good agreement with the experimental results.