Mismatch drift failure of long channel n-MOSFETs caused by substrate hot-electron effect

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In this paper, we report the case of burn-in failure due to mismatch of two structurally paired long channel N-MOSFETs (W/L equals 15 micrometers /10 micrometers ). The mismatch was attributed to substrate hot-electron induced positive threshold voltage shift under certain stress conditions. The rate of threshold voltage shift was found to be sensitive to device geometry, bias condition and stress temperature. In contrast to the channel hot electron effect, long channel devices showed large shift and the short channel device (L equals 0.8 micrometers ) was found to be stable under the same stress conditions. In addition to gate and drain bias, a positive source voltage is required to cause the shift. A large shift was observed when gate, drain and source were all biased positively. The shift was also found to increase with the stress temperature. From the MEDICI simulations, it is revealed that the rate of shift is correlated to the vertical electrical filed int eh channel deletion region with a large shift for a high vertical electrical filed. The enhanced degradation at elevated temperature suggests that the thermally generated electrons in the substrate is the source of hot electrons.