Modes of operation and radiation sensitivity of ultrathin SOI transistors

The electrical characteristics of the fully depleted SOI transistor depend to a great extent on the properties and mode of operation of the back interface. The threshold voltage of the surface MOS device can change significantly as the back interface is driven from accumulation to depletion. Elimination of the kink effect, enhancement of the inversion-layer mobility, and an improved subthreshold slope all require that the back interface be held in depletion. Shifts in MOS transistor behavior after irradiation are typically observed in threshold voltages, subthreshold slopes, transconductances, and leakage currents associated with fixed charge accumulation and fast interface state generation at silicon-oxide interfaces. The presence of an additional exposed back interface in thin, fully depleted SOI transistors suggests that charge and interface-state variations caused by radiation-induced damage at the back interface will cause additional changes in device behavior. Hence, shifts in the behavior of fully depleted SOI transistors may be greater than in those made in thick SOI films where the top surface is shielded by the neutral region of the transistor body from changes in the back interface charge