The decrease of the Richardson constant by more than 3 orders of magnitude in the indirect energy‐gap range of composition of the GaAs/AlxGa1−xAs interface has been interpreted in the past in several ways. We present here a phenomenological model based on envelope wave functions that can describe the transmission coefficient involving two mechanisms:zero‐phonontransitionsdue to &Ggr;‐Xmixing andphononassistedtransitions. The model shows reasonable agreement with a wide range of experimental data. We conclude that a general relation for the thermionic current across heterojunctions must include phonon effects when the Al mole fraction exceeds 0.45. We also find that calculated transmission coefficients are very different from the step function used in the classical theory.