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 impurities near the Si surface was observed through decreases in the Si capacitance. The reduction in net electrically active dopants shifts the threshold voltage negative with total dose. The relative contribution of dopant passivation to the radiation-induced threshold voltage shift is most significant for irradiations performed under zero bias above 100/spl deg/C. For zero bias, dopant passivation and densities of radiation-induced interface traps and net positive oxide trapped charge all exhibit true dose rate and time 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 temperatures. The temperature dependence of dopant passivation indicates that hydrogen transport through the oxides is accelerated with temperature. These results strongly suggest that metastably trapped charge in the oxide bulk reduces high dose rate degradation at room temperature by inhibiting the transport of holes and H/sup +/ ions.