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

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 NH
₄
OH, (NH
₄
)
₂
S
_X
, and P
₂
S
₅
/(NH
₄
)
₂
S
_X
solutions before SiO
₂
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 P
₂
S
₅
/(NH
₄
)
₂
S
_X
pretreatment; therefore, the leakage current in pHEMT was reduced following this process. Atomic force microscopy measurements indicated that the phosphorus oxides formed by P
₂
S
₅
/(NH
₄
)
₂
S
_X
treatment also provided a better surface roughness than obtained following traditional (NH
₄
)2S
_X
-only pretreatment, reducing mobility degradation after sulfur pretreatment. Based on the dc and 1 mus pulse I-V measurement results, P
₂
S
₅
/(NH
₄
)
₂
S
_X
-treated pHEMT exhibited very similar I
_ds
trends, especially at high currents; however, NH
₄
OH, (NH
₄
)
₂
S
_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.