Electrical Characteristics of Passivated Pseudomorphic HEMTs With $\hbox {P}_{\hbox {2}}\hbox {S}_{\hbox {5}}/(\hbox {NH}_{\hbox {4}})_{\hbox {2}}\hbox {S}_{\hbox {X}}$

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)₂S_X, and P₂S₅/(NH₄)₂S_X 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)₂S_X 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)₂S_X treatment also provided a better surface roughness than obtained following traditional (NH₄)2S_X-only pretreatment, reducing mobility degradation after sulfur pretreatment. Based on the dc and 1 mus pulse I-V measurement results, P₂S₅/(NH₄)₂S_X-treated pHEMT exhibited very similar I_ds trends, especially at high currents; however, NH₄OH, (NH₄)₂S_X 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.