In this work the processes of thermal-bias charge instability in the buried oxide of SIMOX SOI full depleted inversion mode (IM) n-MOSFET's and accumulation mode (AM) p-MOSFET's are studied. High-temperature kink effect in IM back n-MOSFET is first considered. Observed effects of thermal-bias instability and high-temperature kink effect are explained by thermally activated positive charge transport through buried oxide with following electron injection from the silicon film into the buried oxide.