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This brief compares the quality of ultrathin SiON dielectrics of nMOSFETs grown on strained-Si layers with both thin and thick SiGe strain-relaxed buffers (SRBs). The gate leakage in the strained-Si samples is found to be temperature-dependent, suggesting that the leakage is dominated by trap-assisted tunneling. The aggravated temperature-dependent gate leakage, along with the issue of self-heating, is demonstrated to have devastating consequences on the long-term lifetime reliability of the devices. However, the thin SRB samples exhibit a 20 improvement in lifetime, compared with the thick SRB samples, due to the lower thermal resistance of the former. This brief demonstrates the importance of reducing the device thermal resistance as power-dissipation levels continue to rise if lifetime-reliability requirements are to be met.