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This paper elucidates the dc, pulse I-V, microwave, flicker noise, and power properties of AlGaAs/InGaAs pseudomorphic high electron mobility transistors (pHEMTs) after various ex situ sulfur pretreatments. The pHEMTs were pretreated with NH4OH, (NH4)2SX, and P2S5/(NH4)2SX solutions before SiO2 passivation to reduce the GaAs native oxide-related surface states. Stable phosphorus oxides and sulfur bound to the Ga and As species can be efficiently obtained using P2S5/(NH4)2SX pretreatment; therefore, the leakage current in pHEMT was reduced following this process. Atomic force microscopy measurements indicated that the phosphorus oxides formed by P2S5/(NH4)2SX treatment also provided a better surface roughness than obtained following traditional (NH4)2SX-only pretreatment, reducing mobility degradation after sulfur pretreatment. Based on the dc and 1 mus pulse I-V measurement results, P2S5/(NH4)2SX-treated pHEMT exhibited very similar Ids trends, especially at high currents; however, NH4OH, (NH4)2SX treatments clearly reduced the current upon pulse measurement because of the presence of surface traps. Hence, this novel pretreatment method has great potential for highly linear microwave power transistor applications.