The　breakdown　voltage　(BVDS)　variation　of　400-V　silicon-on-insulator　(SOI)　n-channel　laterally　diffused　metal-oxide-semiconductor　field-effect　transistors　(NLDMOSFETs)　is　examined　after　exposure　to　a　total　ionizing　dose　(TID).　The　results　for　OFF　bias　and　ON　bias　are　explored.For　the　OFF　bias,　BVDS　increases　first　and　then　degrades　with　accumulated　dose.　For　the　ON　bias,　the　irradiated　device　shows　soft　breakdown　after　irradiation.　The　physical　mechanisms　for　these　variations　are　analyzed　and　confirmed　by　technology　computer-aided　design　(TCAD)　simulation.