A band offset diagram for the heterojunction InyGa1-yPzAs1-z/AlxGa1-xAs based on the transitivity rule and our measured band offset for In0.5Ga0.5P/GaAs is given. A carrier leakage analysis is developed and explains the experimental observations in 670nm visible InyGa1-yPzAs1-z/AlxGa1-xAs double heterostructure (DH) lasers. The analysis based on the performance of this laser verifies that our band offset is more accurate than previous values. In contrast to GaAs/AlxGa1-xAs, InGaPAs/InP and InGaP/AlGaInP DH lasers, we found that the leakage of holes, rather than of electrons, is responsible for the high threshold current density of this type of laser.