In　this　paper　the　vanishing　Debye　length　limit　of　the　bipolar　time-dependent　drift-diffusion-Poisson　equations　modelling　insulated　semiconductor　devices　with　p-n　junctions　(i.e.,　with　a　fixed　bipolar　background　charge)　is　studied.　For　sign-changing　and　smooth　doping　profile　with　＇good＇　boundary　conditions　the　quasineutral　limit　(zero-Debye-length　limit)　is　performed　rigorously　by　using　the　multiple　scaling　asymptotic　expansions　of　a　singular　perturbation　analysis　and　the　carefully　performed　classical　energy　methods.　The　key　point　in　the　proof　is　to　introduce　a　＇density＇　transform　and　two　L-weighted　Liapunov-type　functionals　first,　and　then　to　establish　the　entropy　production　integration　inequality,　which　yields　the　uniform　estimate　with　respect　to　the　scaled　Debye　length.　The　basic　point　of　the　idea　involved　here　is　to　control　strong　nonlinear　oscillation　by　the　interaction　between　the　entropy　and　the　entropy　dissipation.　(c)　2006　Elsevier　Inc.　All　rights　reserved.