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DC and radio frequency ͑rf͒ characteristics of pseudomorphic high electron mobility transistors ͑HEMTs͒ are investigated before and after silicon nitride ͑Si 3 N 4 ͒ 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 Si 3 N 4 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.The GaAs-based high electron mobility transistor ͑HEMT͒ using a pseudomorphic AlGaAs/InGaAs/GaAs material system has proven to be one of the optimum device choices for various millimeter wave applications and high-speed active circuits. 1,2 For an InGaAs/GaAs-based pseudomorphic HEMT ͑pHEMT͒, the device characteristics are greatly enhanced by introducing the InGaAs pseudomorphic channel; the electron mobility, the conduction band discontinuity, and the two-dimensional electron gas carrier density are all enhanced.Similar to other semiconductor devices, the surface effects critically influence device performances of the pHEMTs 3 because the channel layers for the two-dimensional ͑2-D͒ electron transport can be strongly affected by the surface region exposed to air during or after the process. The exposed surface area leads to various problems such as degradation in electrical performance and physical damage via oxidation, moisture, and pollution by dust and chemical materials. Hence, passivation is important for operation stability and reliable device performance. Silicon nitride ͑Si 3 N 4 ͒ is one of the dominant materials for passivation due to its superior characteristics for GaAs monolithic microwave integrated circuit fabrication. However, the effects of Si 3 N 4 passivation on the instability of dc parameters are observed in various types of field effect transistors ͑FETs͒. 4-6 Although various arguments have been made on this phenomenon, the role of Si 3 N 4 passivation is most likely related to the surface states induced by many possible causes, such as ion bombardment during the film deposition, 7 high stress states, 8 and Si-NH bonding states. 9 In this work, the effects of Si 3 N 4 passivation on the dc and radio frequency ͑rf͒ characteristics of 0.1 m depletion mode pHEMTs were examined, and the roles of the surface states in the instability of the electrical parameters due to the passivation films were analyzed by performing comparative studies on the experimental and the devic...

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