The role of fixed and switching traps in long-term fading of implanted and unimplanted gate oxide RADFETs

Radiation-sensing field effect transistors (RADFETs) have become widely used as radiation monitors in space missions. However, partial recovery (fading) of the dosimetric parameter (threshold voltage shift) may introduce severe complexities in measured dose analysis. The evolution of charge trap density during irradiation and postirradiation annealing of implanted and unimplanted RADFETs with different gate oxide thickness was studied in detail. It was found that a surprisingly high increase of border traps and their subsequent annealing in positively biased 400 nm implanted RADFETs were the origin of substantial fading, whereas the same devices irradiated with zero bias exhibited much lower fading. Additional differences between implanted and unimplanted RADFETs irradiated and annealed with and without bias are discussed.