Wafers separated by implantation of oxygen (SIMOX) and annealed at high temperature In argon or nitrogen ambient are compared in terms or film, oxide, interface. and device properties. This evaluation is conducted by correlating pseudo-metal-oxide-semiconductor field effect transistor (Psi-MOSFET) data in plain, unprocessed wafers with the performance of n- and p-channel transistors. In situ wafer characterization with the Psi-MOSFET technique reveals a pileup of nitrogen near the buried Si-SiO2, interface, where the electron mobility is degraded. MOSFET characteristics are even more dramatically affected by annealing in nitrogen : large shifts of front- and back-channel threshold voltages, Enhanced leakage currents, and doping modification. Nitrogen-induced donors are suspected to be responsible for doping compensation in n-channels (leading to an unexpected full-depletion mode of operation) and overdoping in p-channels (reinforcing the partial depletion). By contrast to argon ambient. which does not have any major drawback, nitrogen ambient appears to be unsuitable for annealing of SIMOX wafers.