Characterization of hot-electron-stressed MOSFET's by low-temperature measurements of the drain tunnel current

It is shown that measurements of the forward and reverse current in the gated drain-substrate diode of a MOSFET in accumulation, before and after stress, provide a sensitive means of characterizing hot-electron-induced degradation. Measurements on this diode under forward and reverse bias at temperatures between 300 and 77 K reveal an anomalous current that is weakly temperature-dependent. Computer simulations suggest that under reverse bias this excess current is due to band-to-band tunneling, while under forward bias it is due to tunneling through interface states. Both currents are modified after hot-electron (HE) injection in the gate-drain overlap region. Under reverse bias, the current is most sensitive to oxide trapped charges, while under forward bias, it is also sensitive to the generated interface states. The measurements confirm the high concentration of trapped electrons and acceptor interface states above the drain in NMOSFETs, after an HE stress. Characteristic peaks in the I-V curves are related to the energetic and spatial distribution of these interface states. In PMOSFETs, no degradation is detected above the drain after a HE stress.<
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