Radiation in MOS/SOS devices

The electrical properties of thin silicon films deposited on sapphire differ from those of bulk silicon not only because of crystalline imperfections but also because of the proximity of any point within a film to the silicon/sapphire interface. This interface often has a strong influence on SOS device characteristics, especially on the leakage current of irradiated n-channel MOS/SOS transistors. An important aspect of this program has been to define and develop experimental device structures and measurement techniques which would allow a detailed study of silicon/sapphire interfaces. The structures found most suitable for this purpose were junction field-effect transistors (JFET's) and metal-insulator-semiconductor capacitors using the sapphire substrate as gate insulator (back-gate MIS capacitors). Their design and use is described in this report. It is shown that they complement each other in establishing the pre-irradiation and post-irradiation properties of the silicon-on-sapphire films and silicon/sapphire interfaces. The parameters measured included the impurity concentration and mobility as a function of depth into the silicon films, the pre-irradiation silicon/sapphire interface charge, as well as the radiation induced interface charge and changes in the JFET channel conductance as a function of radiation dose.