Surface Effects of Radiation on MOS Transistors

Recent investigations have shown the metal-oxide-silicon field effect transistor (MOSFET) to be quite sensitive to radiation-induced surface effects.(1) The object of the present work was to obtain further experimental evidence toward identification of mechanisms responsible for device degradation. In particular, the time dependence of effects following both pulsed and steady-state irradiation was studied, since information is thereby provided on the type and concentration of radiation-dependent charge trapping states which affect device conductivity. Both intermediate and semipermanent conductivity changes following exposure to ionizing radiation were studied for three types of MOSFET´s. These included n-channel enhancement, p-channel enhancement, and thin-film n-channel depletion units. Intermediate effects (referring to those whose time constants are typically 50 to 500 milliseconds following a radiation pulse of the order of 1 microsecond pulse width) were studied by means of observing drain current changes as a function of successive irradiations at various bias points. Both drain current changes and changes in transconductance were observed in the study of semi-permanent effects resulting from each irradiation.