New insights into fully-depleted SOI transistor response after total-dose irradiation

In this work we explore the effects of total-dose ionizing irradiation on fully-depleted SOI transistors. Closed-geometry and standard transistors fabricated in two fully-depleted processes were irradiated with 10-keV x rays. Our results show that increases in radiation-induced leakage current are caused by positive charge trapping in the buried oxide inverting the back-channel interface. At moderate levels of trapped charge, the back-channel interface is slightly inverted causing a small leakage current to flow. This leakage current may be amplified to considerably higher levels by impact ionization. At high levels of trapped charge, the back-channel interface is fully inverted and the gate bias has little effect on leakage current. Large increases in leakage currents can be obtained with or without impact ionization occurring in the channel region. For these transistors, the worst-case bias configuration was determined to be the "ON" bias configuration for both the cases where radiation-induced transistor response was dominated by charge buildup in the buried oxide and in the trench sidewall isolation. These results have important implications on hardness assurance.