Effects of surface passivation and the interfacial layer on the reliability characteristics of Al/HfYOx/GaAs metal-oxide-semiconductor capacitor structures are reported. Stress-induced leakage current mechanism, critical for understanding the degradation and breakdown in Al/HfYOx/GaAs capacitors, has been studied in detail. While the devices fabricated with (NH4)(2)S-passivated GaAs substrates show both the soft and hard breakdown failure modes, capacitors with ultrathin interfacial layer (Ge or Si) show only hard breakdown. It is shown that the degradation dynamics follows more closely the logistic power-law relationship rather than the conventional power-law model, frequently used to describe leakage current conduction in high-k gate dielectrics. (C) 2011 Elsevier B.V. All rights reserved.