The characteristics of the defects created by Ar+-ion implantation into SIMOX substrates, to suppress the parasitic bipolar effect in a floating-body n-metal-oxide-semiconductor field effect transistor/SIMOX (n-MOSFET/SIMOX) were investigated. Secondary-ion mass spectroscopy showed that implanted Ar atoms gathered in the top Si layer by annealing and their concentration decreased with increasing annealing temperature. No secondary defects were observed in the top Si layer, by cross-sectional transmission electron microscopy, when Ar+ ions were implanted near the interface between the top Si layer and the buried oxide (BOX) layer. The implanted Ar atoms did not affect boron atoms, but they deactivated phosphorous atoms electrically. This deactivation is thought to form an argon-phosphorous complex. Electron spin resonance measurements showed that the defects created in SIMOX substrates were paramagnetic defects called the P-b centers at the top-Si/BOX interface. Therefore, it is concluded that an Ar atom itself does not form a recombination center and that the P-b centers created by Ar+ implantation act as recombination centers preventing parasitic bipolar action in an n-MOSFET/SIMOX.