Characterization of soft breakdown in thin oxide NMOSFETs based on the analysis of the substrate current

We have investigated the properties of soft breakdown (SBD) in thin oxide (4.5 nm) nMOSFETs with measurements of the gate and substrate leakage currents using the carrier separation technique. We have observed that, at lower gate voltages, the level of the substrate current exhibits a plateau. We propose that the observed plateau is due to the Shockley-Hall-Read (SHR) generation of hole-electron pairs in the space charge region and at the Si-SiO2 interface. At higher voltages, the substrate current steeply increases with voltage, due to a tunneling mechanism, trap-assisted or due to a localized effective thinning of the oxide, from the substrate valence band to the gate conduction band, which becomes possible for gate voltages higher than the threshold voltage. The proposed interpretation is consistent with the results of measurements performed at different operating conditions, in the presence of light and in the case of substrate reverse bias. The presented results are also useful for characterizing the performance of MOSFETs after SBD.