Precise　ac　electrical　measurements　as　well　as　dc　I-V　plots　at　forward　bias　have　been　used　to　characterize　multi-quantum-well　(MQW)　laser　diodes.　Step　offsets　of　apparent　conductance,　apparent　capacitance,　junction　voltage,　series　resistance,　and　ideality　factor　at　lasing　threshold　were　observed.　To　compare　the　electrical　characteristics　of　different　diodes　numerical　simulations　of　the　IdV/dI-I　curve　of　the　double-heterostructure　(DH)　laser　were　performed.　We　find　that　the　conventional　model　used　to　explain　this　curve　is　not　very　satisfactory.　Our　simulations　demonstrate　that　the　junction　voltage,　series　resistance,　and　ideality　factor　for　both　DH　and　MQW　lasers　exhibit　the　same　sudden　changes　at　threshold.　Perhaps　the　electrical　behavior　near　threshold　for　all　laser　diodes　is　very　similar.(C)　2007　American　Institute　of　Physics.