Radiation-Induced Surface States in MOS Devices

The fact that ionizing radiation causes an increase in both oxide charge and fast surface-state density in MOS devices is well known. The relative role of each damage mechanism in total threshold voltage change with radiation, however, is not always apparent. For example, n-channel MOS transistors using a silicon dioxide gate insulator often show a reversal in the direction of the threshold shift after 10$sup 5$ to 10$sup 6$ rads(Si) total dose. This reversal could be due to a reduced net oxide charge or an increase in the interface-state density, or some combination of both mechanisms. This paper describes a series of radiation experiments on MOS capacitors which define, in greater detail, the nature of radiation-induced surface states, their dependence on radiation dose and bias, and their dependence on the MOS fabrication process. (auth)