Space charge limited degradation of bipolar oxides at low electric fields

P-type MOS capacitors fabricated in two bipolar processes were examined for ionizing radiation-induced threshold voltage shifts as a function of total dose, dose rate, temperature and bias. Hydrogen passivation of acceptor im purities near the Si surface was observed through decreases in the Si capac itance. The reduction in net electrically active dopants shifts the thresho ld voltage negative with total dose. The relative contribution of dopant pa ssivation to the radiation-induced threshold voltage shift is most signific ant for irradiations performed under zero bias above 100 degrees C. For zer o bias, dopant passivation and densities of radiation-induced interface tra ps and net positive oxide trapped charge all exhibit true dose rate and tim e dependent effects. A positive gate bias during irradiation eliminates the dose rate dependence. High dose rate irradiation at elevated temperatures enhances oxide degradation while simultaneously accelerating the annealing of damage. The enhancement in interface trap formation is greater than that of net positive oxide trapped charge and occurs over a greater range of te mperatures. The temperature dependence of dopant passivation indicates that hydrogen transport through the oxides is accelerated with temperature. The se results strongly suggest that metastably trapped charge in the oxide bul k reduces high dose rate degradation at room temperature by inhibiting the transport of holes and H+ ions.