The　mixed　layer　problem　and　vanishing　Debye　length　limit　(space　charge　neutral　limit)　of　the　bipolar　time-dependent　drift-diffusion　model　for　semiconductors　with　p-n　junctions　are　studied　in　one　space　dimension.　For　the　general　sign-changing　doping　profile　and　the　general　initial　data,　the　quasi-neutral　limit　is　proven　rigorously　by　constructing　a　more　accurate　approximate　solution　by　taking　into　account　the　effects　of　the　initial　layer,　the　boundary　layer,　and　an　extra　mixed　layer　mixing　of　the　fast　time　and　fast　space　scales,　and　by　using　an　elaborate　energy　method.