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Trap spectroscopy by charge injection and sensing method was applied to the In0.53Ga0.47As-Al2O3 system, yielding the spatial and energetic distribution of the traps inside the Al2O3 layer. The trap density inside the atomic-layer-deposited (ALD) Al2O3 layer was found to be significantly reduced by (NH4)2S treatment of the InGaAs surface prior to the Al2O3 deposition. Indium concentration inside the Al2O3 layer was found to be reduced once the InGaAs surface is (NH4)2S treated prior to the Al2O3 deposition as measured by time-of-flight secondary ion mass spectroscopy, indicating indium as a possible origin of the oxide traps. The results suggest a new mechanism for the sulfur action at the InGaAs surface, which might be responsible for the transistor performance improvements observed after ( NH4)2S passivation. This mechanism involves sulfur as an indium diffusion/segregation barrier stabilizing the InGaAs surface during the ALD Al2O3 deposition, lowering the oxide trap density. This, in turn, improves the electron mobility through a reduction in the Coulomb scattering of the carriers due to border traps and improves the device drive current.