Bias dependent charge trapping in MOSFETs during 1 and 6Â MeV electron irradiation

To study irradiation-induced charge trapping in SiO2 and around the SiO2-Si interface, depletion n-MOSFETs (metal-oxide-semiconductor field effect transistor) were used. The devices were gate biased during 1 and 6 MeV pulsed electron irradiation. The ID-VDS (drain current versus drain voltage) and ID-VGS (drain current versus gate voltage) characteristics were measured before and after irradiation. The shift in threshold voltage ΔVT (difference in threshold voltage VT before and after irradiation) exhibited trends depending on the applied gate bias during 1 MeV electron irradiation. This behavior can be associated to the contribution of irradiation-induced negative charge ΔNIT buildup around the SiO2-Si interface to ΔVT, which is sensitive to the electron tunneling from the substrates. However, only weak gate bias dependence was observed in 6 MeV electron irradiated devices. Independent of the energy loss and applied bias, the positive oxide trapped charge ΔNOT is marginal and can be associated to thin and good quality of SiO2. These results are explained using screening of free and acceptor states by the applied bias during irradiation, thereby reducing the total irradiation-induced charges.