Effects of consecutive irradiation and bias temperature stress in p-channel power vertical double-diffused metal oxide semiconductor transistors

The mechanisms responsible for the effects of consecutive irradiation and negative bias temperature (NBT) stress in p-channel power vertical double-diffused MOS (VDMOS) transistors are presented in this paper. The investigation was performed in order to clarify the mechanisms responsible for the effects of specific kind of stress in devices previously subjected to the other kind of stress. In addition, it may help in assessing the behaviour of devices subjected to simultaneous irradiation and NBT stressing. It is shown that irradiation of previously NBT stressed devices leads to additional build-up of oxide trapped charge and interface traps, while NBT stress effects in previously irradiated devices 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 leads to slight further device degradation. On the other hand, in the cases of devices previously irradiated to high doses or with gate bias applied during irradiation, NBT stress may have a positive role, as it actually anneals a part of radiation-induced degradation.