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Extraction of interfacial trap density Nit in extremely reduced gate oxides with equivalent oxide thickness (EOT) below 1 nm by conventional charge pumping is virtually impossible due to the high gate leakage current through the very thin oxide. However, interface quality assessment in subnano EOT devices is essential for the reliability and performance improvement of future logic devices. In this paper, an accurate approach to determine the interfacial trap density in a 5.8-Å EOT device is performed by an advanced charge pumping technique employing ring-oscillator-connected devices. A consistency comparison of this technique to the conventional charge pumping is done by a frequency sweep on the 10.1-Å EOT device. Clear charge pumping currents are obtained on the 5.8-Å EOT oxide, and further analysis by varying the applied frequency and amplitude is performed. The interface trap density in the 5.8-Å EOT device is found to be higher than that in the 10.1-Å EOT device due to the physically reduced interfacial layer in the thinner EOT device. Moreover, direct tunneling-based calculation gives the charge injection distance as about 2Å inside the oxide. Stress-induced defect generation is investigated by applying dc stress between charge pumping and Idrain - Vgate measurements. The 5.8-Å EOT device shows higher initial Nit but lower stress-induced Nit as compared with the 10.1-Å EOT device. The bulk trap Not generated after stress is higher in the 5.8- Å EOT device due to the higher initial bulk trap density.