Channel-length impact on radiation-induced threshold-voltage shift in N-MOSFET's devices at low gamma rays radiation doses

SiO2 gate dielectric and Si/SiO2 interface are two important components, which will shape the future of the MOSFET and IC technologies for ionizing radiation environment applications. This study discusses their size effect on irradiated NMOS device response. N-channel MOSFETs of different gate sizes were first irradiated with a Co-60 gamma-ray source at several total doses (low doses). Then they were characterized using both voltage- and frequency-charge pumping (CP) techniques. On the one hand, all the transistors reveal two radiation-induced oxide-trap formation mechanisms, caused by low radiation total doses. Initially, there is a positive charge build-up in the oxide layer, followed by diminution of net positive charge (turn around effect). The interface traps exhibit a linear increase with radiation dose. On the other hand, the response of irradiated transistors is shown to depend on their gate lengths. However, transistors with shorter gate lengths show more negative threshold-voltage shifts (Delta-V(th)) during irradiation than transistors with longer gate lengths. We found that this behavior is caused by oxide traps. They induce a real difference in Delta-V(ot) shift in transistors of different channel length. The Delta-V(it) shift remains relatively unchanged. (Author)