In this paper, we report the results of consecutive irradiation and negative bias temperature (NBT) stress experiments performed on p-channel power vertical double-diffused metal-oxide semiconductor transistors. The purpose is to examine the effects of a specific kind of stress in devices previously subjected to the other kind of stress, as well as to assess if possible the behavior of devices subjected to simultaneous irradiation and NBT stressing. It is shown that irradiation of previously NBT stressed devices leads to a further increase of negative threshold voltage shift due to additional build-up of both oxide trapped charge and interface traps. NBT stress effects in previously irradiated devices may, however, depend on gate bias applied during irradiation and on the total dose received: in the cases of low-dose irradiation or irradiation without gate bias, the subsequent NBT stress seems to lead to further device degradation, whereas in the cases of devices previously irradiated to high doses or with gate bias applied during irradiation, NBT stress seems to have a positive role since it practically anneals a part of radiation-induced degradation.Index Terms-Interface traps, irradiation effects, negative bias temperature instability (NBTI), oxide traps, power devices, thermal annealing, threshold voltage shift, VDMOSFET.
In this study, which is aimed at assessing a possible relationship between the recoverable and permanent components of negative bias temperature instability (NBTI) degradation, we investigate NBTI in commercial IRF9520 p-channel VDMOSFETs (vertical double diffused MOSFETs) stressed under particular pulsed bias conditions by varying the pulse on-time while keeping the off-time constant and vice versa. The stress-induced threshold voltage shifts are found to be practically independent of duty cycle when the pulse on-time is kept short or the offtime is kept long, and are found to start increasing with duty cycle only when the on-time is increased or the off-time is decreased beyond specific values. These results, which are discussed in terms of dynamic recovery effects and the mechanisms leading to NBTI degradation, point to the existence of an important correlation between the recoverable and permanent components of degradation.
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