We report detailed measurements of current (I) vs voltage (V) on double heterojunctions (AlxGa1−xAs&sngbnd;GaAs&sngbnd;AlyGa1−yAs) at very high injection levels which reveal nearly discontinuous ∂2V/∂I2at currents ∼ 30% below lasing threshold. This behavior persists unchanged after the laser mirrors are destroyed. A similar breakpoint in ∂2V/∂I2is found on nonlasing double heterostructures and correlates with the onset of intense infrared emission.