The　limit　of　vanishing　Debye　length　in　a　bipolar　drift-diffusion　model　for　semiconductors　with　p-n　junctions　is　studied　in　one　space　dimension.　For　general　sign-changing　doping　profiles,　the　quasi-neutral　limit　(zero-Debye-length　limit)　is　proved　by　using　the　asymptotic　expansion　methods　of　singular　perturbation　theory　and　the　classical　energy　methods.　An　exact　approximating　solution　with　the　1st　order　term　expansion　is　given,　which　takes　into　account　the　effects　of　initial　and　boundary　layers.　Then　the　structural　stability　of　this　approximate　solution　is　established.　©　2009　IEEE.